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Biological Agriculture and Horticulture, 1987, Vol. 4, pp.309-357 0144-8765/87 85 ' 1987 A B Academic Publishers.
C. Arden-Clarke' and R.D. Hodges
'The Political Ecology Research Group, 34 Cowley Road, Oxford OX4 1 HZ, U.K. Department of Biochemistry, Physiology and Soil Science, Wye College {University of London), Wye, Ashford, Kent, TN25 5AH, U.K.
Soil erosion is a world-wide phenomenon having a serious effect upon many agricultural soils. Until recently it was considered that erosion in Britain was limited to only a few susceptible soils in specific areas. However, the increasing monitoring of soils over the past 20 years has shown that water induced erosion is now a widespread and growing threat to farmland in many parts of Britain. This increase in the incidence of 'erosion can be directly related to changes that have taken place in agricultural practice in recent decades, with traditional mixed farming systems based on complex rotations being replaced by increasingly intensive and specialised cropping systems.
After a brief introduction to the world situation, the review concentrates upon an assessment of soil erosion in Britain; concluding that the threat to the soil is almost entirely due to water and not wind erosion. A detailed assessment of the evidence describing the extent and rates of water induced erosion is followed by a description of the factors contributing to soil erosion. These factors can be divided into physical factors, rainfall, soil type and land form, which are relatively stable characteristics; and agricultural management factors, which have been undergoing steady change in recent years. The most important factor contributing to erosion is probably a reduction in soil organic matter levels, resulting in deterioration of soil structure. Other management factors, which often interact with soil organic matter, are: continuous arable production, converting grassland to arable, increasing cereal acreage, use of heavier machinery, tramlining, working up and down slope, the shift to autumn- as opposed to spring-sown cereals, removal of field boundaries, and untimeliness of cultivation.
Soil erosion is also discussed in a wider sense in relation to economic, environmental and resource implications and, in particular, it is considered in relation to different systems of farming. It is clear that the soil degradation and erosion being recorded in Britain is largely a direct result of' several decades of increasingly intensive conventional farming; organic farming, which is inherently a soil-enhancing system, does not result in damage to soils.
The review ends with a series of recommendations for research which will help to combat the problem of soil erosion in Britain.
*This paper is a modified version of: R. D. Hodges & C. Arden-Clarke (1986). Soil Erosion in Britain. Levels of Soil Damage and Their Relationship to Farming Practices. The Soil Association; Bristol.
Introduction
Soil Erosion in Britain
Types of erosion
Water erosion
Historical perspective
The recent past
The present
Factors Contributing to Soil Erosion
Introduction
Physical factors
Rainfall characteristics
Soil type
Landform and slope
Management factors
Soil organic matter
Compaction and tramlining
Up and downslope cultivation
Removal of field boundaries
Increasing production of winter cereals Untimeliness of cultivation
Soil Erosion in Conventional and Organic Farming Systems
The Wider Economic, Environmental and Resource Implications of Soil Degradation
Prospects and Conclusions
Summary and Recommendations
Acknowledgements
References
Throughout recorded history, humanity's impact on the soil has almost always been a detrimental one, with the result that many ancient civilisations declined as a direct consequence of soil deterioration; they either disappeared or were forced to move to new land (Carter & Dale, 1974). More obvious examples of civilizations which, however flourishing they were originally, declined after only a few hundred years mainly because of environmental and soil deterioration, are the Fertile Crescent of the Middle East, Ancient Greece and Ancient Rome. On the other hand, civilizations such as those of China, Egypt and the Indus Valley survived for thousands of years because, for various reasons and in various ways, they were able to maintain the structure and fertility of their soils (Lowdermilk, 1953; Carter & Dale, 1974; Hicks, 1975). For example, soil fertility in Egypt was naturally renewed annually by the silt brought down by the Nile floods; a process which man was unable to interrupt until the recent building of the Aswan High Dam. About 139 million tonnes of silt per year are now trapped behind the dam.
Soil deterioration and destruction can occur via a numbed of mechanisms, examples of all of which are well known from recent history and the causes of which are well understood; and it is these mechanisms, either singly or in combination, which have resulted in the downfall of earlier civilizations. The primary factors or mechanisms which directly damage the soil are (Hodges, 1983):
1. Desertification. The spread of deserts in arid parts of the world, resulting from the increasing human impact upon what are very fragile ecosystems.
2. Waterlogging, salinisation and alkalisation. Problems arising from careless irrigation techniques in semi-arid but potentially fertile lands.
3. Deforestation. The loss of plant protective cover from soils, particularly on slopes, frequently results in breakdown and erosion.
4. Urbanisation. The spread of urban and industrial development usually results in permanent loss to food production of the soil involved.
5. Erosion. The loss of unprotected topsoil resulting from the effects of wind and water.
Unfortunately, these damaging mechanisms have not been restricted to past times nor to relatively small, ancient civilizations. They are still widespread throughout the world and, because our civilization is now worldwide and the impact of population growth and intensive modern agriculture is increasing, their damaging effect upon the world's soils is proliferating.
Probably the most important of the above mechanisms is soil erosion, because it is the most widespread and damaging of them all. To quote Morgan (1980):
"On a global scale soil erosion is associated with misuse of the land where the soil is inadequately protected by a plant cover. It occurs with continuous arable cropping, overgrazing and intensive recreational use. It results in thinning of the top soil, loss of plant nutrients, reduced crop yield and lower grassland productivity, so that farming costs are increased and incomes are reduced. Eroded soil is washed into rivers, resulting in pollution and, through sedimentation, reduced channel capacity and flooding. Soil erosion thus brings about a deterioration in land quality and, ultimately, abandonment of the land."
Soil erosion is a widespread phenomenon throughout most of the world and it is particularly prevalent in the Americas, Africa, Asia and Australia (Eckholm, 1976, Lal, 1984). In America, probably the country with the best documented record of soil erosion (Howard, 1981; Lyles, 1981; Council for Environmental Quality, 1982; Office of Technology Assessment, 1982; Cook, 1985) the position is almost certainly as bad now as it was in the "dust-bowl" era of the 1930's. The U.S. Soil Conservation Service considers that the maximum annual erosion losses that soils can sustain without significant harm to productivity (the so-called tolerance or T-values) are 0.4 tonnes per hectare for shallow soils and 2.02 tonnes per hectare for deep soils. Nationally erosion may exceed 2 tonnes per hectare on more than 46 million hectares out of the 170 million hectares of cropland, with the highest levels usually occurring where row-crops are grown. Many corn farms lose from 4 to 8 tonnes per hectare per year; on gentle slopes this may increase to an average of 8 tonnes per hectare and on steeper slopes this latter figure may increase two to four-fold. Many parts of Canada also are suffering from increased erosion as a result of more intensive agricultural practices (Rennie, 1979).
In Australia, soil degradation in general and soil erosion in particular are widely prevalent (Conacher & Conacher, 1982 & 1983). The Indian subcontinent suffers extensive erosion in many areas (D'Silva, 1977; Brown, 1978; Haigh, 1982), with about 600 million tonnes of fertile topsoil being lost every year. Many African soils also are being damaged by erosion, with particularly severe examples occurring in Madagascar, Nigeria and Ethiopia (Olembo, 1977; Brown, 1978; Christiansson, 1981; Aneke, 1985). Erosion in China appears to be worsening (Smil, 1985). In the U.S.S.R. the position is serious, with estimates of 0.5-1.5 million hectares of crop land being lost annually because of erosion (Harrison Reed, 1986a). Morgan (1982) quotes rates of erosion under different conditions in selected countries from four continents.
The position in Europe is in many ways little better, with many parts of southern Europe bordering the Mediterranean being particularly badly affected (Fournier, 1972; Harrison Reed, 1986a). In many areas of countries such as Spain, Italy and Greece, processes of deforestation followed by unsuitable agricultural practices, often beginning many hundreds of years ago, have resulted in soil breakdown and loss by erosion. A combination of topography, unsuitable land use, and climate have meant a total loss of soil or the ability only to produce hardy perennial crops such as vines and olives; and the process is still continuing.
In comparison to this the more temperate areas of Europe--Germany, most of France, the Low Countries, Britain, etc.--have largely been considered not to be affected by erosion problems; a combination of soil types, climate, etc. not giving rise to the conditions which initiate soil erosion. For example, Hyams (1952) stated that the soils of Atlantic Europe are the "perfect artificial soil", and that they had developed a high degree of fertility and stability during centuries of transformation from the original forest soils. Again, regions with temperate maritime climates have probably the lowest rates of natural erosion in the world (Saunders & Young, 1983) and water-induced soil erosion has never been considered a major agricultural hazard in lowland Britain. However, recent evidence has clearly shown that soils in many areas of this part of Europe are suffering from a degradation of structure (e.g. A.A.C., 1970; Boels et al., 1982), and this is resulting, particularly in sensitive soils under pressure from intensive agriculture, in an increasing incidence of soil erosion (Fournier, 1972; Pauwels et al., 1980; Schwing & Vogt, 1980; Hodges, 1984; Kromarek, 1984).
This account attempts to comprehensively review recent trends and present conditions with regard to soil degradation and soil erosion in Britain, and to discuss the findings in relation to the techniques of agricultural production (called here conventional agriculture) that have been developed over the past 30-40 years. It is the first part of a series considering the comparative environmental effects of conventional and organic/biological agriculture.
Soil erosion in Britain has been classified under three headings (Evans & Cook, 1987). These are:
1. Erosion of upland pastures and moors. Erosion in upland Britain results from a combination of the impacts of wind, water and animals on the pastures or peat moors. In many upland areas this problem is widespread but overall it is a relatively minor aspect of the national erosion situation. It has been reviewed by Evans & Cook (1987).
2. Wind erosion of lowland soils. Wind tends to erode peaty and sandy, especially fine sandy, soils, and such soils are generally at risk when they are exposed with a fine, smooth surface in spring and early summer. Until recently wind was considered to be the most important erosive force affecting lowland soils (see, for example, Davies et a/., 1982), even though Britain's wet maritime climate is likely to restrict its impact only to highly susceptible areas. There are some such susceptible areas confined to erodible soils under intensive arable cropping systems on the eastern side of the country. These are: some 200,000 ha of loamy sandy textured soils in East Anglia, Nottinghamshire and the Vale of York; minor areas on coastal sands in the east of Scotland; and 60,000 ha of peats mainly in the Fens and on Humberside (Davies, 1983a). In general, however, wind erosion in Britain is only locally important and tends to damage crops (by abrasion with blown materials) rather than have significant long-term consequences for soil fertility (Davies, 1983a). Soils in lowland England and Wales at risk from wind erosion comprise only about 5% of the total land area (Evans & Cook, 1987), and consequently this aspect of the problem of erosion is fairly restricted in its potential impact. Because of this, even though it may be quite severe in intensity where it does occur, wind erosion will not be considered in detail in this review.
3. Water erosion of lowland soils. Evidence is accumulating that water erosion has been in the past, and is now, the most important aspect of the overall soil erosion problem. Consequently the remainder of this account will concentrate very largely upon an assessment of water-induced soil erosion in Britain and the relationships between its incidence and the type of agricultural techniques practiced.
Soil erosion is a natural, geological process which takes place slowly over long periods of time, gradually altering the natural topography of` the landscape. In natural ecosystems rates of erosion are more or less balanced by rates of soil formation from the underlying rock strata. In many natural ecosystems, well established climax vegetation provides a stable protection for the underlying soil, resulting in only minimal erosion. Sometimes, soil production exceeds soil loss resulting, over long periods of time, in the development of deep soils.
However, human activities, particularly agriculture (Hudson, 1981) tend to remove the protective vegetation and undermine the stability of the soil and its associated ecosystem; and therefore rates of soil erosion tend to increase. This increase over the natural base-line levels can be termed "accelerated erosion" (Harrison Reed, 1985; Boardman,1987)and, in general, the degree of increase is closely related to the "intensiveness" of the agricultural system; the greater the impact of the system upon the soil the more susceptible the latter may become to erosion .Harrison Reed(1986a,1987)separates"cultural"erosion, the slow loss of soil over longer periods of time, from the more rapid soil destruction often associated with post-war agricultural change. Hereafter in this review the term soil erosion will be used to mean accelerated soil erosion resulting from human agricultural activities.
Erosion has been going on in Britain for a long time, ever since the original agricultural clearances, but generally only at minimal levels (Harrison Reed, 1986b); although there may have been periods of higher intensity erosion from time to time (Evans, 1985a). Spiers & Frost (1987) have reviewed the historical evidence for erosion in the U.K. Evans & Cook (1987) consider that there is little documentary evidence of erosion in the past in Britain; and that there are three possible reasons for this:
a. There was no erosion. (This is very unlikely).
b. The evidence exists but has not been unearthed by historians.
c. Erosion was not noteworthy. (This is the most likely reason).
On the other hand Morgan (1980) believes that there is ample documentary evidence for specific erosion events in both historical, and geographical sources. However, he does not quote any examples before the 18th. century.
A review of the evidence available from assessments of deposition of colluvium and alluvium suggests that, since the land was originally cleared, definite erosion has probably been limited to periods when the land has been under pressure (Evans & Cook, 1987). For example, in the Bronze Age, Iron Age and the Romano-British Period there is evidence of much erosion of valley sides, particularly in the chalk lands which were more heavily populated at those times (Evans, 1981a; Boardman & Robinson, 1985). Other evidence from the landscape such as taluds--step-like structures on slopes formed behind hedges, etc. and often perhaps 500 to 900 years old--indicates erosion occurring in the historical past (Harrison Reed, 1985), but either only very slowly over considerable periods of time or in one or more short periods of severe erosion.
In the recent past, that is about the last 100 years up to roughly 20 years ago, reports of soil erosion tended to be sporadic and often only recorded unusual or severe events (e.g. Brade-Birks, 1944; Oakley, 1945; Stamp, 1962). This could be attributable either to a relatively low incidence of obvious erosion (apart from that which occurred in accepted sensitive areas) or to a failure to recognise and record what would have been a regular phenomenon. The former seems to be the most likely option here since, at least prior to the 1940's, the agricultural system tended to be oriented towards the conservation of the soil and its fertility; thus Boardman & Robinson (1985) have described the results of recent changes in areas of the South Downs from permanent pastureland to cereal production. Speirs & Frost (1987) conclude that the incidence of erosion throughout the first half of this century was low and not a cause for concern.
The Extent of Erosion. In the context of this discussion, the present will refer to the past 20 years or so; and the question that needs to be considered is--has the incidence and/or intensity of water-induced soil erosion in Britain increased during this period? Depending upon the assessment of the evidence the answer to this question may be positive, negative or no change; but if it is positive then a further question needs to be considered--what are the causes of any increases that have occurred?
Until quite recently some authors have considered that erosion is not a problem. For example, Davies et al. (1982) give little consideration to the subject in their recent textbook on soil management. Similarly, Cooke (1979), in his 1978 Presidential Address to the British Society for Soil Science on the subject of priorities in British soil science, did not mention erosion either as a present problem or as a need for future research. Again, the Ministry of Agriculture/A.D.A.S. maintains that the extent of erosion in England and Wales is significant only in localised areas characterized by sandy and light loamy soils in undulating landscapes (Davies, 1986) and that, in general, there is no cause for concern (Needham, 1985). Speirs & Frost (1987) consider this to be a "complacent view". On the other hand, where specific attention has been concentrated on the subject and where significant research has been performed, it has become clear that soil erosion has been increasing over the past two decades or so; thus Evans & Cook (1987) have stated: "As awareness of soil erosion has spread, so more work is being done, and more erosion is being reported". Possibly the most obvious signs of erosion occur after soil cultivation when lighter coloured patches appear on slopes and hillsides--the result of loss of topsoil and subsequent ploughing of the subsoil on to the surface (Hodges, 1984; Morgan, 1986).
Of those workers who have been involved in research on erosion, the most cautious conclusions have come from Morgan. In spite of statements such as: "Sufficient evidence exists to show that soil erosion in Britain is not insignificant" and "Every year there is ample visual evidence of soil erosion in the U.K.", until recently Morgan has generally concluded that erosion is a local-scale problem particularly associated with sandy and sandy loam soils under continuous cereal production or market gardening (Morgan, 1980, 1981, 1982, 1983). Most recently, however, Morgan (1985, 1986) has spoken out about the increasing incidence and severity of the problem. On the other hand, other involved workers, such as Boardman, Evans and Harrison Reed, have been more explicit over a longer period in stating that erosion has been increasing, mainly under the impact of agricultural intensification, and that it will continue to increase unless specific steps are taken to control it (Boardman, 1985a, 1985b, 1987; Evans, 1981a, 1985a; Evans & Cook, 1987; Harrison Reed, 1985, 1987; Speirs & Frost, 1985).
Probably the first systematic attempt to assess soil erosion was that of Harrison Reed (1974, 1979, 1987) who, from 1965 onwards, surveyed areas of the West Midlands where a combination of soil type, topography, rainfall pattern and agricultural practices combined to enhance the likelihood of erosion. One detailed aspect of this study revealed that 27, 17 and 38 % of the total arable area in three parishes just west of Wolverhampton was affected by both water and wind erosion, with some fields eroding every year. He concludes, in part, that the survey:
"revealed the widespread incidence of accelerated soil erosion by rainfall run-off` and to a lesser extent wind. In parts of the region an interaction of physical factors with land use and management practices combine to produce a serious erosion hazard. On more than 1000 sites where water erosion was recorded during this period soil compaction and downslope cultivation lines were identified as major contributory factors in over 95% of cases. Hedgerow removal, which increases the overall length of slopes, was seen to be another important factor." (Harrison Reed, 1987).
In 1976 Harrison Reed commenced field trials at Hilton, East Shropshire, where a sloping site was subdivided into eight 25 sq. m plots and one 300 sq. m plot to measure rainfall run-off and sediment yield under a simple management technique, a hand-held rotovator (Harrison Reed, 1974). The plots were then allowed to compact by the natural compaction of raindrop impact and splash. Average annual soil losses were in the range of 15-17 tonnes/ha/year depending upon slope angle, rising to over 40 t/ha/yr. during the adverse seasons of 1976 and 1983. These values far exceeded the accepted tolerance (T) value of 2 t/ha/yr. (see p.341 below) in spite of the fact that the plots had not been compacted by tractor wheelings. Further studies to measure rainfall run-off` from tractor wheelings begun in 1984 on the same site (Harrison Reed, 1986b) pointed initially to very high yields of sediment from single wheelings 50 m long, ranging from 50-85 kg of sediment after rainfall of 8.5-11.6 mm during 24 hours. These tentative findings substantiated evidence from observations in numerous fields in the West Midlands where erosion initiated in tractor wheelings was found to range from 15 to over 100 t/ha/yr. (Harrison Reed, 1986a). It has been suggested that if similar studies were conducted elsewhere in the country they might reveal that soil erosion is a much more serious problem than is currently accepted (Harrison Reed, 1987).
During the 1970's another detailed study was undertaken by Morgan at Silsoe College with the purpose of providing data on rates of erosion on agricultural land and to develop methods of assessing the risk of erosion at both national and regional levels (Morgan, 1977, 1980, 1981). Most of the study took place in the Silsoe area of Bedfordshire where the object was to measure the rate of erosion by rainsplash, overland flow and fill flow in the field under a variety of soil and land-use conditions. As noted above, Morgan concluded that erosion is a localised problem wherever sandy, sandy loam and chalky soils coincide with farming conditions where the land is bare or under sparse crop conditions for much of the year. Under such conditions mean annual soil loss by overland flow alone can exceed 10 tonnes per hectare (Morgan, 1981, 1986).
The work of Evans, mainly in association with the Soil Survey of England & Wales (SSEW), has been reviewed by Evans & Cook (1987). In a paper published in 1971, following his own personal observations, Evans made a call for work to be done on soil erosion. Subsequently, he reported specific erosive events of a spectacular nature (Evans & Morgan, 1974; Evans & Nortcliff, 1978), and all this led him to a wider assessment of erosion (Evans, 1981):
"Thus, in 1977 I knew of about 100 eroded fields and this information had been collected since 1968. Erosion was common in 1977 and so I decided to see how widespread it was. This led to a search through many air photo collections as well as contacting many individuals and organizations. By March 1978, I knew of about 600 eroded fields (Evans, 1980a); this increased to about 1100 fields by May 1981 and since this date I have discovered a further 200 fields. This figure of about 1300 eroded fields had been obtained without systematically searching localities for eroded fields, except for six strips of air photos taken in 1979 in East Anglia and the West Midlands. Hence, many of the fields were located by chance."
This increasing awareness of widespread erosion led to an erosion monitoring project jointly sponsored by the Soil Survey and the Ministry of Agriculture, starting in 1982 and running for five years. Aerial photographs are taken annually along 18 flight-lines over areas where the soils are likely to be susceptible to erosion, and the photographs are analysed for signs of erosion. Subsequently, a survey of field sites is made on the ground after harvesting of the crops. The results so far from this survey have been summarised in recent Soil Survey annual reports and in Evans & Cook (1987).
In 1982, 297 fields at 15 selected localities were visited by Soil Survey scientists--148 of these fields (49.8%) were eroded and recorded erosion rates varied from 0.1-36.8 cubic metres/hectare/year (Soil Survey, 1983). Furthermore, evidence suggested erosion rates as high as 20-30 cubic metres/hectare/year (equivalent to a lowering of the soil surface by 2-3 mm/year) could be missed by the aerial photography techniques used to identify eroding fields. In 1983, 16 localities were surveyed and on average about twice as many fields within the sample localities were eroded in this year compared to 1982: in Norfolk this factor was eight (Soil Survey, 1984). In 1984 17 localities were surveyed late in the growing season (which may have reduced the likelihood of identifying eroded fields) and the number of eroded fields was only one third of that in 1983 and two thirds of that in 1982 (Soil Survey, 1985). This was probably related to low winter and spring rainfall rather than any reduction in the credibility of the soil. Evans & Cook (1987) conclude that water erosion occurs widely throughout England and Wales, and is more prevalent than was previously thought. During the period of Evans' ad hoc survey and the Soil Survey/Ministry of Agriculture survey erosion was especially noteworthy in 1969, 1977, 1979, 1983 and 1985.
Studies of the increasing incidence of erosion in and around the South Downs have been carried out by Boardman (Boardman, 1983, 1984; Boardman & Robinson, 1985; Boardman & Stammers, 1984)~. These upland chalk areas with a high incidence of long slopes and hillsides, and frequently covered by fairly thin soils, were traditionally put down to permanent pasture and used for grazing sheep. Over the past 40 years the balance has changed until now about 80~o of the Downs are under arable crops. During the past decade, there has been a considerable increase in the cultivation of autumn-sown cereals, and such crops are vulnerable to the erosive effects of autumn and winter rains. Erosion has never been considered a major hazard to contemporary farming on the Downs but, after scattered occurrences of erosion in the 1970's, particularly in the wet autumn of 1976, erosion on an unprecedented scale occurred in the autumn of 1982 as the result of a particularly wet season. In 29 square km of arable land between Brighton and Lewes erosive events occurred at 66 sites in a variety of topographic locations, with the damage being almost entirely confined to arable crops and newly-sown grass leys (Boardman & Robinson, 1985). Boardman's work has involved the study of these and similar events in subsequent seasons. He concludes that if the recent changes in cropping practices are allowed to continue and develop, areas of the Downs could be totally denuded of their productive agricultural capacity within a few decades.
Colborne & Staines (1985, 1987) have studied some of the fine sandy and silty soils that occur in parts of Somerset and Dorset in relation to the erosion that occurred in the winter of 1982-83. They observed a close relationship between the weather, structural deterioration of the soil surface and the amount of erosion occurring. Their account of what took place in many of the fields studied is a good description of how erosion develops on exposed, sensitive soils:
"During the first 2 or 3 months, following drilling of cereal crops in the autumn, topsoils gradually slaked and capped producing a smoother surface but there was little run-off until field capacity was reached in November. By early December the combination of surface capping and a fully wetted soil allowed surface flow. At first run-off was limited in amount and merely smoothed the bottom of wheelings or created traces. Traces reached a numerical peak at the end of January and were precursors of fill development. Most rills did not develop until January and in many fields did not occur in large numbers prior to the last 3 wet days of the month which culminated in a storm on the last day. The smooth pathways established during the previous 3 months were then easily etched into rills, many of which achieved a cross-section of more than 100 sq. cm."
Although most reports of soil erosion have been concerned with the position in England and Wales, available information about the incidence of this problem in Scotland (Al-Ansari et al., 1977; Anon., 1982; Evans & Cook, 1987; Frost & Speirs, 1984; Ragg, 1973; Speirs & Frost, 1985; Watson, 1986) suggests that, at least in some areas, erosion here is very much on the increase. Indeed, Speirs & Frost (1985) conclude that though the first serious incidents occurred later than in England and Wales, the problem is now at least as bad in Scotland. They describe a trend similar to that found in England and Wales, in which erosional events related to arable land in Scotland are not only becoming more prevalent but also more severe. They conclude that this increase in erosion is related to three factors: the considerable increase in winter cereals grown; the expansion of this cereal production particularly in the higher, wetter districts with lighter, more credible soils; and changes in cultivation practices which produce finer, smoother seed-beds. During the last decade, the area of winter cereals has increased by a factor in excess of 2.6, whereas the annual incidence of soil erosion has increased roughly thirty-fold. Over a similar period, erosion on land sown to crops other than cereals has increased more than ten-fold. The factor, other than cropping pattern, which is most affecting the credibility of soils appears to be the fine, firm seed-beds which are now produced as a result of the introduction of powered cultivators in this region at the end of the 1970's. Such seed-beds, which are important in improving the efficiency of soil-acting herbicides and in increasing the efficiency of seed sowing depth and sowing rate, markedly decrease the water infiltration rate and the surface storage capacity of the topsoil. This must result in a significant increase in the incidence of erosion (Speirs & Frost, 1985).
Surveys by Watson (1986) in north-east Scotland suggest an increasing problem of moderate to severe erosion, particularly related to the recent changes in cropping and cultivation practices. Watson believes that soil erosion is now worse in some parts of Scotland than anywhere in England or Wales, citing the frequency of cases where erosion has already cut through to the underlying bedrock (see also Speirs & Frost, 1985). The lack of awareness of the problem is largely attributable to the fact that even less erosion monitoring has been performed in Scotland than in England and Wales (Watson, 1986).
Rates of Erosion. The above summary of recently-published work indicates the way in which erosion is becoming more prevalent; we need now to consider briefly rates of soil loss due to erosional events.
Many papers have described fairly spectacular events limited in space and/or time. Thus Evans & Nortcliff (1978) recorded rates of erosion as high as 195 tonnes/ha per year in some parts of a 61 ha field in Norfolk, with losses from one gully of 450 tonnes of soil. Harrison Reed (1979) recorded erosion rates of 156 tonnes/ha in one field on a farm in Shropshire in the course of rainstorms lasting two days, during which 83 mm of rain fell. Boardman (1983) recorded the loss of 181 tonnes/ha of soil from a field in Sussex in the course of a nine month period. Rates of erosion as high as this are clearly unacceptable and, in some cases, could lead to the effective destruction of a field in the course of only a few years.
Frost & Speirs (1984), in Scotland, mention instances with losses in excess of 60, 75 and 80 tonnes/ha in single erosional events. Colborne & Staines (1985, 1986) found that more than one third of their randomly-chosen arable fields lost more than 4 cubic metres/ha (approximately 6.2 tonnes/ha) over a nine month season, with two fields suffering large losses of 11 and 21 cubic metres/ha (approximately 14.3 and 27.3 tonnes/ha respectively). Figures obtained by the Soil Survey during the 1982 season (Soil Survey, 1983) record rates of erosion varying between 0.1-36.8 cubic metres/ha/year (approx. 0.1-47.8 tonnes/ha/year), with rates as high as 20-30 cubic metres/ha/year (approx. 26.0-39.0 tonnes/ha/year) possibly being missed owing to various factors such as crop cover and time of survey in relation to the occurrence of erosion. (The Soil Survey tends to express its figures in volumes (cubic metres) rather than in weights (tonnes). In order to facilitate comparisons here, approximate conversions of their figures to tonnes/ha have been made by multiplying by soil bulk density. Estimates of bulk densities of eroded soils take from Evans & Nortcliff (1978), Harrison Reed (1979) and Boardman (1983) range from 1.3-1.8g/cu.cm. This range of bulk densities suggests that, bearing in mind the great variability of this soil characteristic, an average of 1.5 g/cu cm might be taken as a mean for British soils. However, the majority of agricultural soils in this country are substantially lighter than this, and a mean 1.3 g/cu cm is probably more representative (Evans, 1985a)). It should be noted that estimates of erosion rates such as those given above are likely to understate the problem. Only some parts of the affected area can be monitored, transfer of sediment within the area is generally ignored and some eroded material remains unaccounted for as it has been removed in suspension (Boardman, 1986a). Allowing for such uncertainties, the figure of 181 tonnes/ha eroded over nine months, quoted above by Boardman (1983), would rise to 268 tonnes/ha over the same period (Boardman, 1987).
Under ideal conditions maximum tolerable rates of erosion should not exceed the rate of new soil formation. However, in practice this objective is almost invariably unattainable in agro-ecosystems. In Britain, Morgan (1980) estimates the rate of new soil formation as 0.2 tonnes/ha/year (a figure which Evans (1985a) considers rather high) and proposes tolerable rates of soil erosion, which nevertheless imply some net loss of soil, of 2.0 tonnes/ha/year. Boardman (1987) has assessed the 'natural' rate of erosion from wide areas of countryside as, on average, 0.5 tonnes/ha/year. This figure was obtained from sampling sediment loads from streams and reservoirs draining diverse catchment areas and could be considered as being typical of erosion rates in Britain, although in reality it would vary considerably. Such a rate does not represent erosion rates from wooded or permanent grass surfaces, which would be much lower. Boardman considers that 0.5 tonnes/ha/year can he suggested as the threshold for accelerated erosion.
Returning to the question that was posed at the beginning of this section--has the incidence and/or intensity of water-induced soil erosion increased during the past 20 or 30 years?--the evidence at present available, and outlined above, clearly suggests that erosion has increased over this period and that the trend is almost certainly towards further increases in the future. This conclusion is supported by the statements of those most directly involved in the investigation of this problem (some of which have already been quoted above). Thus: "Erosion of crops by water is becoming increasingly recognised as a serious problem in parts of lowland England" (Colborne & Staines, 1985).
"[The evidence] suggests that erosion is widespread throughout England and Wales, and occurs more often than is generally realised" (Evans & Cook, 1987).
"Soil erosion has increased substantially over the last 15 years" (Boardman, 1985a); and "The recently published National Soil Map shows 44% of the arable soils of England and Wales to be at risk of water and wind erosion" (Boardman, 1985b).
It can be concluded that, because of the ad hoc way in which much of the evidence has been obtained, and because until the recent SSEW/MAFF project literally no support for research (or even recognition of the problem) had come from the relevant authorities, the full extent of the present and potential problem of soil erosion is by no means understood. More extensive, organised research will almost certainly demonstrate it to be more widespread than is presently realised.
The rates of soil erosion reported in many investigations are greatly in excess of both natural soil renewal rates and that which can be considered as a tolerable rate of erosion (2 tonnes/ha/year). Some of the rates recorded at individual sites exceed the tolerable rate by almost two orders of magnitude. The period of time over which such losses can be tolerated before there is a significant impact upon the productivity of the soil will vary greatly, largely in relation to the depth of soil on a particular site. However, any regular loss of soil in excess of natural replacement rates means that the farmer is managing a depleting asset (Boardman, 1986a) and the widespread continuation of such a
process over the medium or long term can only result in significant damage to our primary asset--the soil--and to the productivity and prosperity of the agricultural industry.
The second question that was posed above--what are the causes of the increase in soil erosion?--will be considered in the next section.
There are a whole range of factors which may contribute to the actual or potential erodibility of soil and they can be divided into two main categories: physical factors and agricultural management factors. Physical factors can be categorized under (i) rainfall characteristics, (ii) soil type, and (iii) landform and slope. The only one of these factors likely to change enough on a human time-scale to affect rates of erosion is the first, rainfall characteristics. Yet it would appear that, at least since the onset of' the intensification of conventional agricultural methods (which can be taken as occurring in the immediate post-war period) no such major change has occurred in the climate. Therefore, any increases in soil erosion since then must be attributable to agricultural management practices. These physical factors are, in their primary form, outside the control of any land-user, though it should be noted that different aspects of farming systems can modify the impact of these factors so that they may result in more or less soil erosion in specific circumstances. Farm management factors all of which are directly related to recent trends towards intensification in agriculture, are: (i) having the land under continuous arable cultivation; (ii) converting grassland to arable cropland; (iii) increasing the area of land under cereals; (iv) using heavy machinery which compacts and damages the soil structure; (v) single "wheelings" and tramlining--the use of the same wheelings for successive agricultural operations; (vi) working the land up and down slope; and (vii) removing field boundaries (Boardman, 1985a; Evans, 1983; Evans & Cook, 1987). To this list could be added: (viii) untimeliness of cultivation, a factor cited in the Strutt Report (A.A.C., 1970); and (ix) the production of finer seed-beds. There is evidence that the increasingly fine tilths produced by mechanical cultivators, and favoured for efficient fertilizer and herbicide use and for improved seed sowing, are directly implicated in increased erosion (Speirs & Frost, 1985; see p.320). Evans (1983) also cited increasing the area of land under irrigation as a factor contributing to erosion. Inasmuch as increased irrigation may be part of an intensification programme it can be seen as relevant, but it is not considered further here. It should, however, be mentioned that the Soil Survey Report for 1984 (Soil Survey, 1984) noted that in this year erosion occurred most often in late spring and summer and that at least a proportion of it was probably attributable to irrigation, which was widespread in the dry summer (Soil Survey, 1985).
Insofar as management factors are concerned, techniques (i)-(iii) mediate their effects largely through consequent alterations in organic matter levels in the soil; techniques (iv) and (v) through mechanical impacts, mainly compaction; techniques (vi) and (ix) through enhancement and localization of surface water run-off; and technique (vii) by increasing slope lengths which leads to an increase in water run-off velocities, reducing shelter effects important in preventing wind erosion and reducing mechanical amelioration of any erosion which does take place. Untimeliness of cultivation tends to reinforce the mechanical impacts of techniques (iv) and (v).
Results from the Soil Survey Reports and some of the individual studies cited above have indicated that the precise cause of erosion varies from site to site. The Soil Survey (1984) was able to relate erosion to lowered soil organic matter levels and steepness of the slope in the eroded fields. Evans (1980a, 1983), Evans & Nortcliff (1978) and Colborne & Staines (1985) also identified these two factors in their studies and suggested that steeper slope convexities should be left under grass if erosion is to be avoided. Compaction and tramlining were cited by Harrison Reed (1979, 1987) as major factors in the erosion occurring in the West Midlands. Low organic matter levels were also a factor here, as was the field gradient. Boardman (1984), Evans (1980a) and Harrison Reed (1979) all identified the practices of working the land directly up and down the maximum slope as contributing significantly to the observed erosion. Evans & Nortcliff (1978) and Boardman (1983) both recommended contour ploughing to reduce erosion. Removal of field boundaries (hedgerows, often with ditches) was mentioned as an important factor in the studies of Evans & Nortcliff (1978) and Boardman (1983,1984). Untimeliness of cultivation is mentioned in the Strutt Report (1970) as an important factor in the deterioration of soil structure, but it was not specifically linked to soil erosion.
The relationship between these variables and the credibility of agricultural land will now be examined in more detail. Comparison of the effects of contrasting agricultural systems, particularly conventional and biological/ organic agriculture, on soil structure and erodibility will be reviewed thereat ten
Although the relationship between a heavy rainstorm and an erosive event in a susceptible field may seem to be clear cut, the rainfall characteristics most likely to induce erosion of arable soils in Britain remain uncertain (Boardman & Robinson, 1985). Morgan (1980) has suggested that high intensity rainfalls are important and that rain falling at intensities greater than 10 mm/hour is likely to be erosive in nature; however, he has also emphasized (Morgan, 1977) the importance of moderate rainfall and the duration and volume of rainfall received. Evans (1980a) has suggested that erosion can occur during rainfalls of relatively low magnitude if soils have already been brought to field capacity by preceding rainfall; and consequently the amount of rain which falls may be more important than the intensity of rainfall (Evans & Nortcliff`, 1978). Harrison Reed (1979) has shown that where soils are compacted and capped, erosion may be initiated by rainfall events supplying 10 mm of total rainfall at a rate as low as 1 mm/hour. Frost and Speirs (1984), in describing a single erosion event in which more than 60 tonnes/ha of soil were lost, considered the erosion not to be the result of abnormal rainfall. Thus with any particular intensity and duration of rainfall, and with all other factors such as topography and plant cover being equal, the likelihood of an erosive event occurring would seem to be directly related to the ability of the soil to "handle" the rain--that is to maintain its surface structure and to prevent slaking and capping and thus to absorb as much rain as possible.
Many British soils are inherently poorly structured. In particular, those containing less than 35°o clay, sandy loams and clay loams, which contain a high proportion of silt, are likely to be susceptible to erosion (Evans, 1980a). The proportion of water stable aggregates less than 0.5 mm in diameter in a soil is a good index of credibility--the greater the proportion of such aggregates the greater the credibility of the soil (Evans, 1980b). Soils with higher clay and organic matter contents will tend to have more stable aggregates because of the strong colloidal bonding resulting from these components. Where clay and organic matter levels are low, soil structure may be poor and thus compaction, capping and other damage may easily result. An important aspect of` the interrelationship between soil structure and erosion concerns the level of organic matter in a soil. This will be considered in detail in a subsequent section.
Although the topography of fields clearly has an affect on the incidence of erosion, the specific aspects of landform which are most important are not agreed by all workers (Boardman, 1984, 1985a; Boardman & Robinson, 1985; Colborne & Staines, 1985; Evans 1980a; Evans & Cook, 1986; Evans & Nortcliff`, 1978). Boardman (1984,1985a) suggests that slope gradient is not of prime importance in determining the occurrence and intensity of erosion; rather the size of the contributing area and the length of uninterrupted slope are probably more important controlling factors. Harrison Reed (1979) agrees with this. Evans (1980a) considers that slope length is not an important factor contributing to erosion, although other work has suggested that erosion occurs where critical lengths of slope of a certain steepness are exceeded (Evans & Nortcliff`, 1978). One factor, however, on which all agree is the importance of slopes which are convex in profile. In such situations, where there is a flattened crest on which water can be stored, succeeded by a steeper slope, significant fill and gully erosion can be initiated below the convexity once the water is released. Evans (1980a) believes that erosion can occur in the gently rolling arable landscapes of lowland England, where such convex slopes are a regular occurrence, when rainfall is sufficient to exceed the surface storage on the field crest. Much erosion may also occur in valley floors where little or none is found on adjacent slopes (Evans & Cook, 1987); the run-off from the valley sides may be insufficient to initiate erosion but where the flows of` water merge in the valley their volume and velocity may then become sufficient to incise rills.
A factor of` crucial importance in relation to erosion is the level of organic matter in a particular soil, since many studies and reviews have clearly demonstrated the positive effect of` organic matter on soil structure, stability and fertility (see, for example, Baver, 1968; Chaney & Swift, 1984; Koepf et al., 1976; Kononova, 1966 & 1975; Waksman, 1936). In most recent studies of soil erosion, reduced organic matter levels have been recognized to be a primary factor in the development of susceptibility to erosion (e.g. Boardman, 1983 & 1985a; Boardman & Robinson, 1985; Evans, 1971; Frost & Speirs, 1984; Harrison Reed, 1987). The two major influences on soil organic matter that are relevant to the present context are the proportion of grass in crop rotations and the nature of` the fertilizer inputs.
Reducing the proportion of` grass in a crop rotation, whether it takes the f arm of a complete switch to continuous arable cultivation, an increase in the arable component of the rotation or an increase in the land under cereals, invariably reduces soil organic matter levels. The organic matter content of pasture soils ranges from 5-10%, whereas that in arable soils can be as low as 1-2% (Johnston, 1973). The amount of organic matter a grass ley (in a grassland-arable rotation) maintains in the soil falls somewhere between these two (A.A.C., 1970). After the conversion of grassland to arable cultivation, there is a progressive decline in soil organic matter levels which may continue for decades (Boardman & Robinson, 1985). It is not simply a case of arable crops returning a smaller proportion of organic matter to the soil--the increased tillage operations necessary in arable cropping lead to accelerated losses of organic matter by increased oxidation consequent upon the improved aeration of tilled soils.
The effects of changes in organic matter level on the structural stability and susceptibility of` soils to erosion are now well documented. Low (1972) found that old grassland soil had a structural stability of` 73-78% (percentage of oven-dry soil particles of diameter <2mm in water-stable aggregates of` diameter >2mm) compared to 12-17% for the same soil under old arable fields. A six-course rotation, comprising three years ley and three years cereals, appeared sufficient to maintain structural stability at a good cropping level. Dettman & Emerson (1959) reported that the cohesion of soil crumbs on unmanured arable land, as measured by a modified permeability test, was only 4%, compared to 50% f or land that had been down to grass for only f our years. They concluded that a continuous arable system produces "very bad structure". Greenland et a/. (1975) were able to show that there was a critical level of organic matter content, namely 3.4% (equivalent to 2.0% organic carbon content), below which soils were liable to structural instability. Wischmeier & Smith (1978) have shown that, in some U.S. soils, a net increase of 1% in the soil organic matter content can reduce the average annual soil erosion losses by 10%. In examining a range of British soils, Chaney & Swift (1984) found highly significant correlations between soil aggregate stability and organic matter. No other soil constituent investigated had a significant relationship with aggregate stability, indicating that organic matter is mainly responsible for the stabilization of aggregates in these soils.
The details of` how organic matter contributes to the structural stability of` soils are not fully understood, largely due to the lack of` relevant research. However, the aggregation of` soil particles which determines structural stability is known to be achieved largely through the action of cohesion and cementation. Soil moisture levels and roots are also important factors as molecular cohesion tends to be at a maximum when soil is dry; the ramification of` the root systems enhances the soil porosity and can contribute to the stable binding of soil aggregates (Briggs & Courtney, 1985). Organic matter contributes to both cohesion and cementation; of the three types of soil organic matter recognised, namely fresh/undecomposed (e.g. intact root systems), partially decomposed and well-decomposed (humus) material, it is the partly decomposed organic matter which makes the most immediate contribution to structural stability, by virtue of its products of decomposition. Polysaccharides, many of` them believed to be of bacterial origin, appear to be the most active stabilising fraction (Burns & Davies, 1986), these linear polymers adsorbing to the surfaces of clay and soil particles. (The microbiology of soil structure has been reviewed recently by Burns & Davies (1986)). Though well-decomposed organic matter contributes few products of decomposition which have stabilising effects on the soil, the fact that at does contribute to structural stability is evidenced by the instability of soils in which it has dropped to low levels (A.A.C., 1970).
Addition of organic matter to the soil does more than increase the cohesion of` soil aggregates--it also increases the capacity of the soil to absorb and retain water. A more structured soil is also a more porous soil. The "infiltration capacity" of soil is critical in determining rates of erosion because it is the water that does not penetrate the soil and contributes instead to surface run-off, which is the major cause of` water erosion (Morgan, 1980). Soils with poor structural stability easily slake and cap, virtually sealing the surface to falling rain. Soils with a low infiltration capacity are effectively poorly drained, and in the absence of any slope this can lead to waterlogging, which in turn can exacerbate structural damage due to compaction (see below). A reduction in the soil's ability to retain water will tend to increase--or even create--the need for irrigation, which can itself facilitate erosion (see above).
Pasture crops serve not only to maintain organic matter levels, but also provide greater protection for the soil surface from raindrop impact, than do arable crops (Russell, 1973). This reduces splash erosion and encourages infiltration rather than surface flow of water, as the increased cover helps to prevent slaking. Slaking is the disaggregation of soil particles which is often followed by capping--the formation of an impermeable crust on the soil surface, which in turn increases run-off. Increased levels of soil organic matter are perhaps a more Boardman, 1983).
Another significant factor is the large increase in earthworm numbers seen under grass (650-1100 kg/hectare) as compared to arable crops (approx. 100 kg/hectare), as these animals have a significant beneficial influence on soil structure and infiltration capacity (Russell, 1973). The burrowing activities of earthworms generate an extensive network of` channels which increase infiltration capacity. It appears, in addition, that a combination of the influence of` earthworms and plant roots may be critical for the formation of water-stable aggregates, and that dense root systems alone do not have as great an effect (Stewart & Salih, 1981). Earthworm population densities are correlated with soil organic matter levels, soils that are poor in the latter usually supporting small populations of these animals (Edwards & Lofty, 1977). So any general lowering of organic matter levels may be seen to have a wide range of` undesirable effects on soil structure.
Other practices important in the maintenance of` soil organic matter levels critical influence in the prevention of slaking (e.g.include applications of farmyard manure (FYM), other bulky organics, retention of crop residues and green manuring. In quantitative terms these practices are usually not as important as grass leys in returning organic matter to the soil. However, Morgan (1980) states that applying FYM at a rate of 12 tonnes/ha/year (1.2kg/square meter/year) increases soil organic matter by an amount equal to that achieved by a three year fey. Composted FYM will not affect soil stability for a considerable time after incorporation, but has a long-lasting effect when it does, as its "iso-humic factor" is higher than most organic manures (Fournier, 1972). Crop residues, such as straw (usually mechanically incorporated into the top few centimetres of soil), have a somewhat lower isohumic factor, but improve soil stability more rapidly. Green manures, generally leguminous crops which also fulfil a nitrogen-fixing role, have the most prompt effect on soil stability; but it is relatively short-lived due to their very low isohumic factor (Russell, 1973).
All three types of organic matter provide some protection from raindrop impact and can reduce the velocity of surface flow. The application of FYM and the retention of crop residues both increase the infiltration capacity of the soil (Aarstad & Miller, 1978). Minimum tillage practices resulting in the retention of crop residues on or near the surface of the soil are useful in controlling both wind and water erosion (Chepil & Woodruff, 1963; Russell, 1973. See also Elliott & Papendick (1986) for a recent review on residue management). The removal or burning of` crop residues can exacerbate erosion problems. Precise data on this effect are somewhat lacking but studies in North America by Biederbeck et al. (1980)and Rasmussen et al. (1980)both noted significant declines in organic matter levels as a result of` long-term straw burning. These losses resulted in reductions in aggregate stability (increasing the credibility of the soil), increased likelihood of structural damage during cultivation and a less workable soil (i.e. a reduction in filth). There were other adverse effects on the fertility of the soil such as reductions in levels of mineralizable soil nitrogen. Both groups of` authors recommended the retention of crop residues in the future. In the U.K., M.A.F.F. studies at High Mowthorpe have also shown accelerated declines in organic matter levels where straw has been burnt or removed (M.A.F.F., 1964).
Green manures may also serve as cover crops, undersown in crops which leave a considerable proportion of the ground bare, or as a catch crop in seasons when the ground would otherwise be left bare. In recent years all these practices have declined or even disappeared from large parts of` the U.K. farming scene, so that they currently contribute much less than they formerly did to soil organic matter levels (A.A.C., 1970).
The Strutt Report sounded an early warning on the dangers of soil compaction by agricultural machinery (A.A.C., 1970). The problem has been exacerbated since then as the weight of agricultural machinery has continued to increase in tandem with the frequency of cultivations necessary in more intensive arable systems (Harrison Reed, 1979; Greenland, 1977). This section of the report deals largely with structural damage due to the wheels of tractors and agricultural machinery, rather than that due to the cultivation itself (e.g. ploughing). The latter is dealt with in the section on untimeliness of cultivation.
The potential for compaction to become a serious problem in agricultural systems in the U.K. is well illustrated by Soane's finding (Soane, 1975) that up to 90% of an arable field surface may be crossed by "wheelings" in the course of a single year. Conventional tillage practices may require as many as six or even seven separate passes over the land (Briggs & Courtney, 1985). Heavily laden wheels passing over cultivated soils can result in bulk density increases to the ploughing depth (e.g. Soane, 1975). The effects of compaction by wheels tend to be less damaging in the later stages of crop growth and at harvesting (A.A.C., 1970). However, the condition of the soil (e.g. moisture levels) is critical in determining the extent of the damage to soil structure. This will be dealt with in the section on untimeliness of cultivation and in the discussion of the interactions of various environmental effects of differing farming systems. Compaction due to wheelings tends to be restricted to the surface, whereas that due to tillage (i.e. induced by the implements rather than the power unit) tends to be localized at the subsurface (Harrison Reed, 1983). Surface compaction is far easier to remedy than subsurface compaction, which may lead to more permanent damage (Greenland, 1977). However, any activity which seals the surface can lead to run-off and soil loss, and therefore from the point of view of erosion the condition of the surface is critical (Boardman, 1986b).
Harrison Reed identified compaction as a particularly severe problem in the West Midlands, but has more recently noted that "widespread effects of soil compaction are being experienced in the intensive arable areas" (Harrison Reed, 1983). The Soil Survey report for 1983 noted that much of the erosion observed in 16 separate localities appeared to start in wheelings, which often run downslope (Soil Survey, 1984). Compacted soil surfaces are less permeable to water, so infiltration during rainfall events is decreased, surface run-off is augmented and, given sufficient continuous rain (10 mm in some parts of the West Midlands (Harrison Reed, 1979)), erosion is initiated. Tractor wheelings running in a downslope direction can be converted into deep gullies (over 1 metre deep) in the course of a single, prolonged storm event (5 mm of rainfall per hour). Soil loss may exceed 100 tonnes/ha in such events (Harrison Reed, 1983). Colborne & Staines (1985), in Somerset, showed that the more wheelings there were, the more erosion occurred. However, there are some fairly simple measures which can be taken to reduce soil erosion initiated by tractor wheelings. A winged tine mounted behind the rear wheels will break up the lug pattern (which promotes surface run-off when the wheeling is oriented downslope) and will reduce the compaction effects (Harrison Reed, 1983). Contour working of sloping fields also limits the problem and restriction of field size can reduce the scale of any erosion which does take place (Harrison Reed, 1979). Minimal cultivation techniques which reduce the number of passes necessary will also ameliorate the problem.
Tramlining is a more localized and severe form of compaction due to wheelings. It originates from the positioning of subsequent tractor wheelings on the tracks of the initial cultivations. This technique limits the area of cropland subject to compaction and is of particular importance when crops receive repeated applications of pesticides during their growth phase Compaction effects are reinforced and the soil remains bare throughout the growing season, so the risk of erosion is greatly increased. In primary tractor wheelings the infiltration capacity of the soil during heavy rainfall events can be reduced by as much as 70%--in tramlines created by several passes run-off approaches 100°o (Harrison Reed, 1983). Thus in any situation where surface compaction is an important factor inducing erosion, the most spectacular and destructive instances are likely to originate in tramlines. This is one reason why the incidence of erosion has increased in parallel with the increasing production of` winter cereals. These crops are treated with pesticides around the time of emergence and the compacted tramlines resulting from such treatments are left exposed to the weather throughout the winter.
It should be noted that the compaction can have an even more direct effect on yield than that mediated by erosion. The high mechanical resistance of` compacted soils often reduces crop yields by inhibiting the development of root systems, a problem identified in the Strutt Report (A.A.C., 1970). Experiments with vining peas have indicated that yield losses are the result of` both a reduction in plant numbers and poorer performance of individual plants (Dawkins, 1983). The severity of the effects depends on the type of crop grown, soil type and weather conditions at emergence. Yield losses of` 28-48% in spring beans, up to 50% in spring barley and 60% in oil seed rape have been recorded in compacted soils (Dawkins, 1983). In experiments on various horticultural crops it has been shown that increasing the coverage of` the planting area by wheelings from 50% to 90% reduced yields by 12-50% (Davies, 1983a). Blackwell et a/. (1985) found that the winter oat crop in wheeled clay soil had fewer plants, lower root density and a reduced yield compared to the crop in unwheeled soil. The overall yield was 27% lower on the wheeled soil.
Orientation of the furrows, drill lines and wheelings generated by cultivation operations has often been cited as an important determining factor in the occurrence and severity of water erosion. Colborne & Staines (1985) found that perfect downslope alignment of wheelings, tramlines and crop drillings resulted in high rates of erosion on gentle slops (less than 2%) in Somerset. Downslope alignment of cultivations is an important factor in erosion events observed on the South Downs (Boardman, 1984) and in the West Midlands (Harrison Reed, 1979). Evans & Nortcliff (1978) noted that the parts of the severely eroded field that they studied, which had been ploughed and drilled parallel to the contours, suffered no erosion. Erosion rates in other parts of the field were as high as 195 tonnes/ha.
Ploughing, planting and cultivating along contours can generally reduce soil losses from sloping land by as much as 50% (Morgan, 1979). Where it is not possible the cultivations should be carried out as close (i.e. as parallel) as possible to contours. Evans (1980a) remarks that only 49 out of 302 (16.2%) instances of erosion that he observed in England and Wales, were in fields worked across the slope. However, on steep or irregular slopes it is often impossible to cultivate along the contour, particularly when heavy machinery is being used. The effectiveness of "contouring" is determined by the gradient and length of the slope, the condition of the soil itself and rainfall intensity. Harrison Reed (1983) noted that in large fields with large catchment areas and local increases in slope, contouring helped to reduce run-off from low intensity storms but did little to offset run-off in high intensity storms. In some instances (e.g. short, steep slope convexities, particularly on inherently erodible soils) the only solution is to put the land under grass, permanently (Evans & Nortcliff, 1978; Boardman 1983; M.A.F.F., 1984b).
Removal of field boundaries can increase the magnitude of both wind and water erosion, but is generally a more critical factor in instances of wind erosion. Hedgerows and shelterbelts of trees, placed at right angles to erosive winds, reduce wind velocity and the wind "fetch" (distance of uninterrupted blow). Shelterbelts of trees with a width of 9 metres can give effective protection over a distance of about 12 times the height; three metre wide hedges can give protection out to about 30 times the height (Morgan, 1979). The Strutt Report is not in absolute agreement with this, suggesting that some protection is afforded over five times the height to windward of a hedge and 30 times the height to leeward, appreciable protection extending over only eight times the height to leeward (A.A.C., 1970). Wilson & Cooke (1980) observed wind erosion starting only ten metres or less from the windward edge of fields hedges in the Vale of York. They suggested that the main beneficial effect of hedgerows was to limit transport (by saltation) of eroded material beyond the leeward edge of the field. By trapping windborne particles hedgerows prevented isolated areas of erosion becoming contiguous during major sandstorms. Hedgerows can serve a similar function in water-eroded fields, as in the Merse of Berwick on Scotland, where soil has collected to depths of one or two metres against hedges and fences on the downslope side of fields (Rag", 1973). Such local retention of eroded soil does provide limited scope for remedial action and limits undesirable (and often expensive) off-farm effects such as blocking of roads and pollution of water courses (Boardman, 1984).
The value of hedgerows in limiting the soil loss inflicted by water erosion is reflected in the findings of several authors. Evans & Nortcliff (1978) noted that removal of old hedges and ditches on valley sides has resulted in a less restricted water flow, surface run-off attaining higher velocities at which soil particles can be eroded and transported. Harrison Reed (1983) recommended that "key" hedges sub-dividing long slopes should not be removed, especially at points where slope angle increases and the slope is backed by a large potential catchment area. Boardman (1984) cites a strikingly similar example of this in the South Downs where hedges have been removed on or just below the broad crests of the Downs, which form extensive catchments. In such areas even grass strips between fields can be important in controlling water flow and therefore sediment transport, whilst hedges and ditches along a valley floor can help prevent erosion there. Re-planting of specific hedges shortening slope lengths was recommended by both Evans & Nortcliff (1978) and Boardman (1983).
These observed instances of hedgerow removal exacerbating soil erosion problems were to some extent predicted in the Strutt Report. It was stated that hedgerow removal "can cause or aggravate structural problems" in a number of ways, such as increasing the wind velocity or by amalgamating several soil types in one field which creates cultivation and drainage difficulties (A.A.C., 1970). The Report concluded that there was no hedgerow removal related damage--this is no longer the case; and hedges are still being removed on the South Downs (Boardman, 1986b).
Although a whole range of factors acting together are responsible for the increased susceptibility of British soils to erosion, one factor in particular, the increasing production of winter cereals, has tended to demonstrate this susceptibility by exposing soils to adverse weather conditions during the autumn and winter (e.g. Boardman, 1984 & 1985a; Boardman & Robinson, 1985; Evans & Cook, 1987; Harrison Reed, 1987; Frost & Spiers, 1984; Speirs & Frost, 1985). Figures quoted by Evans & Cook (1987) show how the pattern of al production has changed over 14 years:
'In 1969, 795,022 ha were sown to wheat, most of this would be winter sown and 2,068,526 ha were sown to barley, then dominantly a spring sown crop. In 1983 2,484,554 ha were sown to winter wheat and barley, and only 809,029 ha to spring barley; an increase in area of winter sown cereals of 3.1 times."
With spring-sown cereals, ploughing the previous autumn leaves the soil in rough furrows which are relatively resistant to the erosive effects of the autumn and winter rains. Recent practice with autumn-sown cereals, however, tends to till the soil and sow the crop within a few weeks of having harvested the previous crop, thus exposing a smooth, fine seed-bed with minimum crop cover to the maximum period of adverse weather conditions. If at the same time, the soil structure has been weakened by other aspects of :intensive production, then the potential for significant soil loss is greatly enhanced.
Induced sub-surface compaction and the development of a dense, relatively impermeable "plough pan" is one consequence of repeated cultivations, particularly if they are undertaken with inadequate implements (e.g. a blunt ploghshare) or in unfavourable conditions (e.g. at times of high soil moisture content). Soil particles are smeared and realigned resulting in increases in bulk density and reduction in the size and number of pores and channels in the soil, with consequent marked reductions in soil permeability (Soane, 1975). Surface compaction effects can also be exacerbated in wet conditions; the Strutt Report noted that there are often too few "workable days", especially in wet seasons, when the soil is dry enough to allow cultivation without such damage (A.A.C., 1970). Any poorly drained soil will be particularly susceptible to these effects.
Untimeliness of cultivation is damaging in itself, but the deleterious effects can be substantially enhanced by other factors such as low organic matter levels. An example of how several of these management factors can combine together to cause erosion on a sensitive soil is given by Rutherford (1986). The farm he describes is concerned with intensive production of horticultural crops from a productive, brickearth soil. The constraints of continuous, intensive, horticultural production result in: the use of heavy machinery working in regular wheelings, frequently up and down slope; untimely operations whatever the soil conditions; regular irrigation of some crops; exposure of unprotected soil to water and wind for significant periods of time; and minimal return of organic matter to the soil. Brickearth soils have a high silt and a low clay content and tend to be low in organic matter when cultivated. The resulting weak structure is susceptible to damage when subject to intensive production systems and it is therefore not surprising that the farm described has suffered from significant erosion problems. The problems arising on this farm are also discussed by Boardman & Hazelden (1986).
The often complex interactions of all the factors affecting soil structure and rates of erosion are now discussed in the context of conventional and biological/organic farming systems.
The post-war trend to intensification of conventional agricultural systems in Britain has had, and continues to have, a major impact on soil management. The emphasis in farming has shifted from soil maintenance to crop nutrition leading to the modification or abandonment of many traditional techniques and the rapid development and application of new ones. The new techniques have often been applied with little regard to their effects on the agroecosystem in general and on the soil in particular. Individually these changes might not cause serious problems--acting together, and often synergistically, they may threaten the very existence of the soil. The surprisingly far-reaching nature of these effects and the complexity of their interactions have prevented anticipation of this threat. This is best illustrated by taking what is probably the most fundamental effect, namely the reduction in soil organic matter levels, and then examining its interactions with the other effects of modern agricultural practices.
The Strutt Report noted that in the U.K. "... practices to maintain the organic matter level in the soil, such as applications of F.Y.M., other bulky organics, green manuring, and the growing of grass and clover leys, have declined and disappeared from large sections of farming" (A.A.C., 1970); and the use of these practices has continued to decline in the 16 years since this report was published. Straw burning and the burning or removal of other crop residues has increased in intensive systems, largely as an aid to the simplification of tillage and planting operations. This reduces still further the returns of organic matter to the soil (see p.329). The loss of crop cover consequent upon these methods directly promotes wind and water erosion (e.g. Chepil & Woodruff 1963; Smith & Wischmeier, 1962) in addition to structural effects related to loss of organic matter. All these trends have led to a steady decline in organic matter levels, which in stable soils may not seriously affect the structure for a long time. However, in unstable soils, reduction of these levels can lead to serious damage and erosion. Other intensive cultivation practices such as: substitution of simple crop rotations or continuous growing of one crop (monocropping) for traditional, more complex rotations; more frequent tillage operations in arable crops; the use of' large machines in these and other orations; the application of' synthetically compounded (i.e. inorganic) fertilizers and pesticides; and an increase in field size as farmers specialize in a single crop and machinery size increases; all exacerbate the situation.
Monocropping of arable crops, particularly cereals, and the disappearance of the clover/grass ley is probably the most critical influence reducing soil organic matter levels. The Strutt Report (A.A.C., 1970) recommended that there should be an increase in the proportion of leys in rotations in f our out of the eight agricultural regions identified in England and Wales (the Eastern region, the East and West Midlands and the Yorkshire/Lancashire region). Application of' bulky organic manures has been dispensed with, largely for economic reasons as cheap synthetic substitutes have been developed and mixed farming practices have declined. The contribution of inorganic fertilizers to organic matter levels is restricted to that consequent upon increased crop yield, which is frequently insufficient to replenish soil levels of organic matter. Some inorganic fertilizers have even more deleterious effects on soil structure by causing disaggregation of' veil clods (Wilson & Cooke, 1980).
There has been a marked geographic redistribution of farming enterprises in Britain, with the majority of livestock enterprises concentrating in the west of the country, while most of the arable cropping is undertaken in the east. It is simply not economically viable to transport livestock wastes over distances of more than 8 miles/13 kilometres (Vine & Bateman, 1981). Green manuring practices have fallen into disuse as farmers have specialised and cheap, inorganic nitrogen fertilizers have become available. The Strutt Report concluded that, as a consequence of these changes, some soils (generally those with a high silt or fine sand fraction) were suffering from dangerously low levels of organic matter and could not be expected to continue to sustain the `farming systems imposed upon them (A.A.C., 1970).
The lowering of soil organic matter levels facilitates mechanical damage due to cultivation, especially on unstable soils. Timeliness of cultivation is a critical factor determining the extent of such damage, as the periods when cultivation can be carried out satisfactorily (i.e. with a maximum of beneficial and a minimum of' deleterious effects) are restricted by low organic matter levels. The soils has a weaker structure and is less workable, consequently timely cultivations become more necessary and yet more difficult to achieve. The lower infiltration capacity of such soils (see p.330) implies poorer subsurface drainage, so that where surface run-off does not provide an alternative (e.g. on level ground) waterlogging ensues, ruling out "timely'' cultivation. Where surface run-off` does occur, erosion is frequently the result. Once structural deterioration has set in, excessive and/or untimely tillage can trigger a positive feedback sequence that renders the soil progressively more difficult to cultivate and more prone to damage.
Specialization and monocropping practices have led to less flexibility in the timing of cultivations. Where the whole farm is under only one or two crops, cultivations over large areas have to be completed within a restricted time frame. Consequently, tillage and harvesting operations will have to be conducted when soil conditions are unsuitable and when smearing and compaction of` the soil are enhanced (A.A C., 1970). The general increase in farm size associated with intensification has also contributed to this problem.
Untimeliness reinforces induced surface and sub-surface compaction, i.e. compaction due to both the power units and the tillage implements involved in cultivation. Wheelings from tractor tyres tend to be deeper when operations are conducted in wet conditions (e.g. Boardman, 1984). Plastic deformation and smear at the surface and below, due to wheels and tillage implements respectively, is enhanced when the soil is too moist (A.A.C., 1970). Where subsurface compaction has created a plough pan, the farmer often responds by increasing the number of tillage operations or by deploying tillage implements capable of` greater mixing and disruption of the soil. This may serve to increase the deleterious mechanical effects and can also compound the problem less directly by enhancing the chemical oxidation of organic matter in the soil. The improved aeration leads to more rapid oxidation rates and greater losses of organic residues (see A.A.C., 1970 and Briggs & Courtney, 1985). The consequent lower soil organic matter levels render the soil less workable and timeliness of cultivation more critical and less achievable (see p.334). Water erosion is enhanced by the loss of structural stability, decrease in infiltration capacity and increase in run-off attendant on this positive feedback sequence of damaging effects.
Specialization and monocropping practices have led to less flexibility in the timing of cultivations. Where the whole farm is under only one or two crops, cultivations over large areas have to be completed within a restricted time frame. Consequently, tillage and harvesting operations will have to be conducted when soil conditions are unsuitable and when smearing and compaction of the soil are enhanced (A.A.C., 1970). The general increase in farm size associated with intensification has also contributed to this problem.
Untimeliness reinforces induced surface and sub-surface compaction, i.e. compaction due to both the power units and the tillage implements involved in cultivation. Wheelings from tractor tyres tend to be deeper when operations are conducted in wet conditions (e.g. Boardman, 1984). Plastic deformation and smear at the surface and below, due to wheels and tillage implements respectively, is enhanced when the soil is too moist (A.A.C., 1970). Where subsurface compaction has created a plough pan, the farmer often responds by increasing the number of tillage operations or by deploying tillage implements capable of greater mixing and disruption of the soil. This may serve to increase the deleterious mechanical effects and can also compound the problem less directly by enhancing the chemical oxidation of organic matter in the soil. The improved aeration leads to more rapid oxidation rates and greater losses of organic residues (see A.A.C., 1970 and Briggs & Courtney, 1985). The consequent lower soil organic matter levels render the soil less workable and timeliness of cultivation more critical and less achievable (see p.334). Water erosion is enhanced by the loss of structural stability, decrease in infiltration capacity and increase in run-off attendant on this positive feedback sequence of damaging effects.
Conventional agricultural methods can have profound effects on the soil microflora and fauna, which in turn influence the soil structure. Tillage operations in any agricultural system can reduce the populations of` soil organisms, either directly by injury or disturbance, or indirectly by increasing oxidation losses of the soil organic matter on which these organisms depend (Stewart & Salih, 1981; Aarstad & Miller, 1978). Obviously, where tillage operations increase, these effects will also increase. A particular case in point is that of earthworm populations, whose beneficial influence on soil structure has already been remarked upon (see p.328). With specific reference to conventional agricultural practices, it has been established that some pesticides are toxic to elements of the soil fauna. Graham-Bryce (1977) noted that some specific fungicides are toxic to earthworms and some herbicides can kill other elements of the invertebrate fauna.
Soil erodibility is enhanced in other ways by conventional agricultural practices. The large machinery used in intensive cropping systems is less capable of working along contours, so that even on relatively shallow slopes up and downslope cultivation may be unavoidable. These large machines are also less manoeuvrable, an important determining factor in the increase in field size and removal of field boundaries. There is a general tendency in conventional farming systems to let short-term economic considerations override all others and this will be dealt with in more detail below (p.340).
The effects of biological/organic agricultural methods on soil organic matter levels, soil productivity and filth, contrast strikingly with those of conventional agricultural methods. One of the primary concerns of organic farmers is to maintain high levels of soil organic matter (Hodges, 1977 & 1981; Parr et a/., 1983). That attainment of this objective is perhaps the most critical factor in controlling soil erosion, can be seen from the foregoing review of the effects of conventional agricultural.
Soil organic matter levels are maintained, or even raised, in organic farming systems by a range of techniques. Arable monocropping is never practiced, as crop rotations including grass leys (or occasionally another fallow crop such as a green manure) are an indispensable element of organic farming systems, maintaining soil fertility, enhancing nutrient cycles and controlling pests and diseases (U.S.D.A., 1980a). Short or medium term clover/grass leys are an integral part of all organic arable production in the U.K., and are generally down for at least half of the rotation (Vine & Bateman, 1981). It should be noted that rotations including a substantial proportion of grass leys are not a cure-all for soil structure problems. Some sandy soils with low organic matter levels should be permanently under grass as they are inherently erodible (A.A.C., 1970; Evans, 1985a), and leys can be sown too late to arrest erosion if structural deterioration is already at an advanced stage (e.g. Boardman, 1984).
The crop residues of arable crops are not removed or burned, as they frequently are in conventional farming systems, but are either left on the surface or incorporated into the top few centimetres of soil. However, conservation tillage systems developed in the U.S., involving minimum tillage and no-till methods, are frequently not a viable option for organic farmers as these methods usually rely on extensive use of pesticides to control pests and weeds (Parr et al., 1983). Similar emphasis is placed on the recycling of animal wastes, care being taken in their collection, storage and application. Composting is frequently practiced as this facilitates handling of the waste and improves its fertilizer value (Hodgson & Thompson, 1985). Organic wastes from non-agricultural sources (e.g. sewage sludge) are also applied on organic farms, often after blending and composting. Green manuring is commonly practiced, primarily as a nitrogen-fixing crop, but ploughing in of such crops does add some organic matter to the soil. Together, these practices make it possible to maintain relatively high soil organic matter levels.
High organic matter levels help to reduce compaction problems, both on the surface and at the sub-surface, as the soil is more stable and better drained (more porous and with a higher infiltration capacity). A consequence of more stable structure is that fewer cultivations are generally required for preparation of the seed-bed (A.A.C., 1970). The use of grass leys in rotation with arable crops ensures that the soil is not subject to cultivation every year. Rotations and the absence of tight cropping sequences also allow a more flexible approach in the timing of cultivations, and the beneficial effect of raised organic matter levels on soil structure mean that untimeliness of cultivation is generally less of a problem. There are no repeated applications of pesticides throughout the growing season in organic farming systems and this, combined with the lower wheeling densities produced by fewer cultivations, obviates tramlining effects.
Matching crops to the prevailing edaphic (soil), climatic and topographic conditions is another important characteristic of organic farming methods; short-term economic considerations are not allowed to dominate all others. Where the soil is too wet for arable cropping, an organic farmer will not drain it or otherwise attempt to convert it from grass to arable crops. This trend is currently much in evidence in many parts of the U.K. (e.g. Briggs & Courtney, 1985). Such practices can lead to serious structural problems. Where drainage is ineffective, soil structure can be damaged during periods of high soil moisture content. Recent conversion of parts of the North Kent marshes from grassland to cereal cropping provides an example. Chemical changes attendant on the lowering of the water table resulted in a collapse of soil structure, loss of porosity and blocking of installed tile drains, with consequent waterlogging and major crop losses (Hodgson & Thompson, 1985). Steeper slopes are generally left under grass in organic systems, so that up and downslope cultivation is not necessary and erosion is avoided. Where arable crops are grown on shallower slopes, cultivations along the contour will be facilitated if the organic farmer is using lighter machinery than that used on intensive conventional farms. The requirement for crop rotations, which rules out opportunities to specialise and some of the economies of scale with which such specialization is rewarded, tends to result in the use of smaller, lighter machinery by organic farmers. As a consequence of greater crop diversity and smaller machinery there is less incentive to remove field boundaries and enlarge fields. Diversity in agricultural landscapes has been shown to be an important factor in the control of pests in organic agricultural systems (Altieri, 1985), providing a further incentive for the retention of hedges and hedgerow trees.
In their report on organic farming the U.S.D.A. (1980a) noted that many of the management practices followed by organic farmers are those highly recommended by the U.S.D.A. itself, and land-grant universities, for improving the productivity and filth of the soil. During the study, little evidence of erosion was seen on any organic farms, not only because of the general nature of organic farming practices, but also because the organic farmers were "strongly committed" to soil and water conservation. Organic farmers using rotations of which 25-40% were given over to grass leys, suffered average annual soil losses one third to one eighth of those occurring under conventionally tilled continuous arable crops (U.S.D.A., 1980a). However, it should be noted that as conservation tillage practices (e.g. no-till) are applied in conventional systems, the erosion control advantage of' organic methods may decrease. Lockeretz et al. (1981) estimated that for a given set of physical conditions (e.g. soil type, slope gradient and length), water erosion was about one third less for the organic rotations. This analysis assumed that the same tillage practices were used on organic and conventional farms, whereas the organic farmers were using less erosive tillage methods (i.e. actual differences in soil loss were still greater (Lockeretz et al., 1981)). Equivalent observations do not exist for organic and conventional farms in the U.K. but, given the accelerating soil erosion on conventional farms and the soil management practices on U.K. organic farms, a similar situation may well exist here, now or in the near future.
Soil degradation consequent upon agricultural practices has so far been discussed more or less exclusively in terms of the resultant losses of` soil. The effect of these soil losses on agricultural land and the environment in general has not been considered in any detail and neither have soil degradation effects other than soil loss by erosion.
The most fundamental agricultural consequence of soil erosion is reduction in subsequent crop yields--this takes place over both the short and the long term, but it is the long-term losses that are most significant. Yield losses result from removal and burial of crop plants, losses of soil organic matter and nutrients and decreases in the depth of the soil available for rooting (Evans & Nortcliff, 1978). In the short term, Evans (1981b), surveying 234 fields in the Midlands and East Anglia, found that though erosion and deposition of soil generally affected less than 4°o of the field area, the proportion could rise to 34.5%. In some instances, particularly of wind erosion, so much of the field was affected that it was necessary to re-drill the crop--a costly operation. Frost & Speirs (1984) estimated that the loss of 79 tonnes/ha from a field in Roxburghshire, Scotland, resulted in the loss of 2.3% of that year's potential yield of` winter barley. A field on the same farm, sown to protein peas, which lost approximately 48 tonnes/ha of soil was calculated to have been subject to a yield reduction of 2.5%. Other short-term effects such as changes in soil filth, reductions in water-holding capacity and infiltration of water and air into the soil are also important factors (Mannering, 1981).
It is very difficult to assess the long-term effects on crop yields of` repeated removals of thin layers of topsoil, not only because of this range of effects, but also because assumptions need to be made about frequency of erosion, rate of surface lowering and the relationship of crop yield to soil depth (Evans,1981). However, some figures for barley and wheat yield losses due to erosion are given by Evans & Nortcliff (1978), Evans (1981b) and Frost & Speirs (1984). These workers estimated reductions in yield over 100 years, consequent upon different annual rates of erosion. Estimated average spring barley yield losses at two different sites subject to annual erosion rates of 3 and 12 tonnes/ha were 1.0-1.8 and 5.3-7.9% respectively. Average winter wheat yield losses at one site were estimated as 3.6% when the erosion rate was 3 tonnes/ha/year, and 13.1% for soil losses of 12 tonnes/ha/year(both after 100 years). Site and crop specific effects make it difficult to generalise about long-term yield losses in a regional or national context. However, the U.S.D.A. (1980b) has estimated that if soil erosion in the Corn Belt continues at current rates (averaging 18 tonnes/ha/year, Berg (1979)), yields will have been reduced by 15-30% by 2030.
The U.S. Soil Conservation Service (S.C.S.) has assigned "soil loss tolerances", so-called T-values, to all soils in the U.S.A., based on soil depth, prior erosion and various other factors affecting productivity. These T values, ranging from 4.5 to 11.2 tonnes/ha, supposedly denote the maximum level of soil erosion that will permit a high level of crop productivity to be sustained economically and indefinitely. The validity of the procedures for assessing these tolerances has recently come under critical scrutiny (e.g. Mannering, 1981). Judging by the productivity effects of soil losses recorded in the U.K., these tolerances may well be too high (Evans, 1981b). Furthermore, T-values are calculated to allow an "acceptable" decline in soil productivity and some workers disagree with this approach (e.g. McCormack & Young, 1981). No attempts have yet been made to estimate soil loss tolerances for U.K. soils, but it seems likely that if such an attempt was made the tolerances set would be considerably lower than those currently applied in the U.S. Morgan (1980) suggested that a soil loss of 2.0 tonnes/ha/year is the maximum tolerable f or sandy and chalky soils in Southern England, yet even this is likely to be too high for some of the thinner soils. On shallower soils, particularly those shallower than the rooting depth of` the crops planted (e.g. 1.2 metres for cereals (Boardman, 1987)), yield declines due to erosion are soon detectable. Yields are substantially reduced in fields where enough topsoil has been removed to expose the subsoil; Evans(1980c)found yields of` winter cereals at a site in Cambridgeshire were a factor of 1.7 greater on the valley floor with topsoil intact, than on the valley sides where the clay subsoil was exposed. Boardman (1987) has suggested that a rational soil conservation policy for the U.K. would seek to exclude arable cultivation from steeply sloping land (> 11°) where soil thickness was less than 20 cm (e.g. on the South Downs). In contrast, current rates of erosion on thick soils in some parts of the country may be tolerable for 200 years before significant effects on productivity and yield are recorded (Frost & Speirs, 1984). j
An important factor in yield losses due to soil erosion is loss of plant nutrients accompanying that erosion. The sediment in the run-off from the eroding fields carries away substantial amounts of inorganic phosphate and organic nitrogen (U.S.D.A., 1980a). Not only are these nutrients lost to subsequent crops, but they also create pollution (eutrophication) problems in the natural water courses and water bodies which they reach. Ketcheson & Webber (1978) recorded annual losses of 87 kgN/ha and 59 kgP/ha from fields in Ontario, Canada, subject to an annual lowering of` the soil surface by 3.6 mm. The long-term effects of such losses on yield will depend very much on the soils in question and future applications of fertilizer. When large applications of` fertilizer are being used, effects on yields may be negligible in the short term, though the applications needed to maintain yields are likely to rise. Cox (1984) showed in the U.S. that the amount of fertilizer required to achieve a given increase in yield in the period 1950-1978, was four times the amount which had to be applied in the period 1910-1949 to achieve the same yield increase. Cox attributed this reduction in the inherent fertility al` U.S. soils to soil erosion, and estimated that by 1978, long term erosion effects were responsible for a 25% drop in the productivity of` U.S. soils.
Organic farmers bear a variety of social and environmental costs which conventional farmers "externalise". Avoidance of` nutrient and sediment pollution of` off farm water supplies is just one of them. The scale of` these problems is difficult to assess but it has been estimated that, in the U.S., soil erosion deposits some 3 billion tons of sediment in inland water bodies (U.S.D.A. (1968), cited in Pimentel et al. (1976)). Of this some three-quarters originates from agricultural land--and there are other sediment damages which were estimated in 1970 to cost the U.S. $500 million annually (Pimentel et al., 1976). Recent U.S. figures suggest that off-farm costs of soil erosion may be far greater than on-farm costs. The Conservation Foundation estimated the annual cost of production losses due to erosion was only $40 million, compared to off farm costs of $3.1 billion/year. There are no comparable figures for the U.K. where the problem has not yet assumed such proportions. Boardman & Stammers (1984) mention costs of £12,000 incurred by a local authority as a result of a single erosion event on the South Downs. Evans (1981a) reported that five county authorities devoted totals of` 120, 250, 280, 500 and 7200 man hours per year to the clearance of eroded soil from roads; Boardman (1987) states that these figures "undoubtedly under-represent the problem".
The costs of` off-farm chemical pollution (e.g. nutrients, fertilizer and pesticides) associated with erosion are even more difficult to assess, but are potentially very high given the effects of these pollutants on aquatic ecosystems. Sediment is one of the most significant transport mechanisms for both nutrient and pesticide pollutants; most of the phosphate and organic nitrogen entering aquatic ecosystems is bound to sediments (Johnson, 1979). The impact of nitrates and phosphates in run-off from agricultural land tends to be most marked in ponds and lakes where the slow rate of water turnover results in progressive accumulation of these nutrients (Briggs & Courtney, 1985). The consequent increase in organic activity and growth of populations at all trophic levels leads to a rapid increase in oxygen consumption which, if not matched, can lead to a dramatic collapse of the aquatic community. Agriculture is by no means the only source of nutrients causing eutrophication and it may be that human sewage effluent and industry together contribute more to nitrate pollution than agriculture does (Briggs & Courtney, 1985). However, in the U.S. it has been estimated that as much as 50% of` the accumulated phosphate in the Great Lakes has been derived from diffuse sources (i.e. mainly agricultural land (Johnson, 1979)). In the U.K. where agricultural land is also an important source of such pollution, it currently appears to be linked to application of inorganic fertilizer rather than nutrient loss during erosion (e.g. Green (1979)). However, the eutrophication of the Norfolk Broads may have been exacerbated by the N and P lost from the credible soils surrounding the Broads (Evans & Cook, 1987).
Deterioration of the soil and its structure can directly affect yields without triggering erosion, as in the case of compaction described above. Similarly, while loss of soil organic matter is an important factor enhancing soil erodibility, it may have a more direct influence on fertility and yields. Soils with higher organic matter contents also retain more water and have higher available water capacities (Cooke, 1977), so that crop yields are less likely to be limited by water shortage. Experiments at Rothamsted and Woburn have shown that annual applications of FYM produce greater yields of` wheat, sugar beet and potatoes than did equivalent dressings (in nutrient terms) of` inorganic fertilizers (Cooke, 1977). It may be that organic manures are a more "efficient" source of nutrients than inorganic fertilizers. In any case, organic matter is certainly important in increasing the cation exchange capacity of` soils, enhancing the ability of the soil to retain nutrients in the face of leaching processes, but in a form available to crops (Russell, 1977). Soil organic matter also contains a variety of trace elements and can therefore serve as important sources of these elements for crops (Mercer & Richmond, 1972). High soil organic matter levels tend to be correlated with large populations of soil flora and fauna which are an important component of natural nutrient cycling processes (e.g. Russell, 1973). The complex effects of soil organic matter on soil fertility are far from fully understood, and it is not yet clear to what extent, or under what conditions, low organic matter levels can induce yield reductions.
Soil is a renewable resource--but the rate of renewal is slow (e.g. 0.2-1.0 tonnes/ha/year in the U.K.) and annual rates of soil erosion can be orders of magnitude greater. Such a discrepancy in the rates of renewal and loss of a resource can mean that the resource is effectively non-renewable, at least on a time-scale to match human needs. In the temperate regions of the world human impacts are still a long way from creating a no soil-no food situation similar to that seen in many arid and tropical areas where desertification has destroyed agricultural economies and inflicted starvation on the dependent communities. Yet the major cost of soil erosion in temperate areas is still the present and future loss of` productivity, even if` present losses are difficult to measure (and probably only locally important in the U.K.). There are economic and resource costs attached to erosion control measures and inputs used to offset yield losses. In the U.K. there is as yet little attempt being made to control what erosion occurs and there is no overall estimate for these costs. However, the U.S. experience again provides an indication of potential costs if this problem escalates After 40 years of public policy aimed at controlling soil erosion, 23.5% of the 415 million ha of U.S. cropland had annual erosion rates equal to or in excess of` 12 tonnes/ha (U.S.D.A., 1980b), i.e. above estimated "tolerable" levels. Between 1950 and 1980 a total of` $45 billion (1980 dollars) was spent on these control measures; in 1980 alone expenditure was $1.44 billion (allowing for inflation, annual expenditure has remained more or less constant over this period (Easter & Cotner, 1970)). The costs of lost production are less easy to calculate, but it has been estimated by Pimentel et al. (1976) that 47 litres/ha/year of fuel equivalents are being used just to offset erosion losses. However, all these costs may be obscured by short-term economic f actors, as in the case of` the increasing export demand f or U.S. grain in the 1970's. Where there is a current price advantage in producing one crop, and given the uncertainty of future demand, farmers will tend to relax longer term soil conservation goals and take advantage of the present profitability of the crop in question (Campbell & Heady, 1979). Between 1971 and 1975, during the worldwide increase in demand for U.S. "rain, 84% of`U.S. cropland had soil loss in excess of the recommended levels (U.S. Congressional Records, 1977). In the E.E.C. an equivalent economic incentive is provided by C.A.P. subsidies on cereals; intensification of cereal production is undoubtedly a significant cause of soil erosion in the U.K.
There remains the question of` whether assigned "tolerable" levels of erosion, which allow a decline in long-term productivity, are in fact acceptable. McCormack & Young (1981) have pointed out that, because this decline increases the amount of energy required per unit of production, it represents an energy tariff on future generations who might be less able to pay than we are today.
With the exception of a flew paragraphs on "blowing soils", soil erosion did not merit a mention in the Strutt Report (A.A.C., 1970). The broad conclusions of the report were that the nutrient fertility of soils ,in the U.K. was largely unimpaired by modern farming techniques, but that the structure of soils in some areas was being damaged. Since the publication of that report the incipient damage noted then has been translated into very severe local erosional events and less damaging erosion rates over wide areas. The Strutt Report identified the following factors contributing to soil damage: reduction of` soil organic matter levels; the passage of heavy machinery; untimeliness of cultivation (often a consequence of tight cropping sequences in intensive production systems); and poor drainage. As the trend to intensification and specialization has continued since the publication of` this report, all these problems have tended to increase. The most critical factor seems to be the lowering of` soil organic matter levels, which the report linked to the ploughing up of` permanent grassland, the adoption of` all-arable rotations on inherently unstable soils and, to a lesser extent, a reduction in the use of organic manures (A.A.C., 1970). Apart from proposing research into improving incorporation techniques and optimising performance of` organic manures, the report specifically recommended a return to some form of arable ley rotation in four out of` the eight agricultural regions identified in England and Wales, namely the Eastern, the East and West Midland and the Yorkshire/Lancashire Regions.
With respect to organic manures there has been no observable trend towards an increase in their use in conventional systems since 1970. Indeed, the problems of` disposal of` surplus animal wastes from intensive animal husbandry units has increased in recent years (e.g. H.M.S.O., 1979)). Neither has the trend towards dispensing with grass leys been reversed. An examination of proportions of land under arable crops, rotational leys and permanent pasture in some of` the counties of` the regions identified above (Norfolk, Suffolk, Nottinghamshire, Northants, Leicestershire, Shropshire, Warwickshire, Stattordshire and Lancashire) reveals that between 1972 and 1983 the area under arable crops has increased in all counties, while that under permanent pasture and rotational leys has decreased (M.A.F.F., 1975, 1982 & 1984a). Decreases in the area of` short to medium-term grass leys have ranged from 17.1-49.8~No, while increases in arable cropland have ranged from 11.6-43.4. The area under cereals has also risen in some countries, particularly in the Midlands (e.g. Northants, Leicestershire, Staffordshire, Shropshire and Warwickshire). Substantial increases in the incidence of erosion have been noted in some of these counties. The Soil Survey Reports for 1982 and 1983 reveal that the number of` eroded fields in a sample taken in Norfolk was eight times greater in 1983 than 1982, in Nottinghamshire the factor of increase was six and in Staffordshire, three (Soil Survey, 1983 & 1984). Some 60% of the eroded fields identified in the 1983 survey were sown to winter wheat. This does not necessarily imply that all these fields were under continuous wheat (or arable) cropping--an important factor in erosion of winter wheat fields was the minimal crop cover during the wet winter. However, the results still suggest that the warnings contained in the Strutt Report have been borne out. Erosion is becoming a serious problem in those areas which were identified as having soils which could not be expected to sustain the farming systems being imposed upon them (A.A.C., 1970). Furthermore, a number of published papers have detailed instances where severe, local erosional events have been specifically linked to farming methods identified in the Strutt Report as having potentially damaging effects on the soil.
In addition to these general warnings the Strutt Report included some specific recommendations on future practices and research, many of which also appear to have been largely ignored. It was noted that intensive cereal cropping was likely to be sustainable only on calcareous soils and that there was a general requirement for more rigorous evaluations of soils capable of supporting all-arable enterprises without suffering damage. It was suggested that minimum organic matter levels for different soils and situations should be established, below which damage was likely to occur. This measure alone would not be sufficient to solve the erosion problem, as serious erosion can still affect soils with high organic matter levels where a combination of other factors such as increased field size, cultivation of` steep slopes and planting of autumn-sown crops can combine to render the soil erodible (e.g. on the South Downs (Boardman, 1983 & 1984)). The report also stressed that economic considerations alone should not determine the techniques and cropping systems applied, but that the importance of` maintaining soil structure and fertility should be taken into account. Both these recommendations were repeated by Newbould (1982), indicating then that the Strutt Report's findings had been ignored. Yet even today, five years after the re-iteration of these recommendations, nothing has been done. Research and farming trends are still largely oriented towards further intensification, and there is no evidence that anything other than economic considerations are governing these trends. Nor is there any indication that intensification will decline. Between 1981 and 1983 the area under short term toys and permanent pasture continued to decrease in all the counties mentioned above while the area under cereals increased in Suffolk, Norfolk, Northants and Warwickshire (M.A.F.F., 1982 & 1984a). The shift to more profitable autumn-sown cereals which leave the soil exposed to rainfall impact throughout the wettest part of` the year (autumn and winter), has further enhanced the susceptibility of these soils to erosion (e.g. Soil Survey, 1984; Boardman & Stammers, 1984). Between 1969 and 1983 the area sown to winter cereals has increased by a tractor of` 3.1 (Evans & Cook, 1987).
There are no indications that organic manuring practices have increased. Machinery is, it` anything, increasing in size; there seems to be a positive feedback mechanism in operation here, too. As soil structure deteriorates, the soil becomes less tillable and drainage is impeded, the farmer's response is often to deploy more powerful tractors drawing heavier tillage implements or to install extensive sub-surface drainage systems, often using heavy--up to 20 tonnes--purpose-built, tracked machines (see soil and Water,11, parts 1, 2 and 3). Other trends which have characterized intensification in the past, such as hedgerow removal, are continuing, though at a slower rate.
To what extent the factors driving the trend towards intensification are inherent in conventional systems development, as opposed to a function of the economic incentives provided by the Common Agricultural Policy, is a debatable point. However, the pattern of conventional systems development has been similar in the United States, the process having been taken a couple of` statics further. The immediate prospects f or U.K. agriculture would seem to be a continuation of` this trend, and the outlook for soil structure and fertility is therefore bleak. Judging by the U.S. experience, the consequent loss of` agricultural productivity and other social and environmental costs is likely to be very high. Despite tour decades of` U.S. government support for soil conservation measures, and the expenditure of` billions of` dollars, it has proved impossible to reduce erosion below "tolerable" levels on nearly a quarter of` U.S. cropland. These "tolerable" levels have, in any case, been called into question as they may yet imply an unacceptable loss of soil fertility. A reduction of` these erosion rates may not, however, be a feasible option as it has been calculated that, in some areas at least, effective control is not possible within the economic framework of` conventional agriculture. This is probably also the case in the U.K. at present (Evans, 1985a). A specific example of this situation is recorded by Alt (1979), who showed that the marginal cost of` reductions in erosion on Iowan cropland, below 12 tonnes/ha/year, increase dramatically (and prohibitively).
A reasonable conclusion from those observations is that the erosion control measures which can be applied in conventional agricultural systems may be insufficient to combat the degradative effects of` these farming methods. Such a situation seems to have developed in the U.K., short-term economic considerations of` conventional farmers precluding the maintenance of` soil structure and fertility. This is by no means entirely the fault of` the farmers themselves--they have only done what they were exhorted to do by successive governments. Indeed, they had little choice given the nature of` the grants and subsidies offered, which encouraged intensification and specialization to maximise production. Is it possible to offer a constructive alternative to current agricultural policy'? The major recommendations of` the Strutt Report can be summarised as: reintroduction of crop rotations incorporating grass leysin some areas currently given over to all-arable production; establishment of minimum levels of soil organic matter; research into organic manuring techniques aimed at optimising their performance; and selection of crops and methods with due consideration for the soil. These techniques and objectives are those of organic farmers while the research proposed would be invaluable to such farmers. The U.S.D.A.'s interest in organic farming techniques owed a lot to its concern about declining soil fertility and intractable erosion problems; the Department concluded that organic farming methods were remarkably similar to well-established soil conservation measures (U.S.D.A., 1980a). If nothing else, the growing threat of soil erosion is the U.K. may prove to be a crucial consideration in the establishment of a government-sponsored research programme into organic farming methods.
With the present problems of overproduction of many crops, largely as a result of E.E.C. agricultural support policies, one suggestion for their reduction is to take large areas of land out of agricultural production--the so-called "set-aside programme" (see, for example, Burnham, 1985; Buckwell, 1986; Potter, 1986). Although such a scheme may be of help in the case of some of the more seriously structurally-damaged arable soils, by putting them down to permanent grass (Evans, 1985b) or some even more extensive usage, a widespread introduction of this scheme would result in continuing--or even increasing--stress upon those soils remaining in intensive production. Application of similar policies in the U.S. has proved ineffective in checking soil erosion, as they have encouraged farmers to plough up all their cropland and as much of the non-crop area as possible, prior to the registration of their cropping base (Berner, 1984). This has led to the ploughing up of even highly erodible land and often to no effective reduction in overall rates of soil loss. This is a problem likely to afflict any form of "set-aside" policy, even if it is carefully constructed. A much more realistic policy, from the point of view of the "health" of Britain's soils, would be a widespread move towards a more organic approach to agriculture; this would reduce levels of production, whilst maintaining farm income, and would institute soil-protective rather than soil-destructive farming techniques.
This review can, perhaps, best be summed up by a recent quotation from Morgan (1986). He states:
"Current management practices on arable lands are unlikely to sustain land quality. An erosion rate of twenty tonnes per hectare each year will result in a loss of 140 millimetres Atop soil in one hundred years. Crude estimates of cereal yields in relation to soil depth suggest that with a starting top soil depth of twenty-five centimetres, the result will be a decline in yield from three or two tonnes per hectare. Organic content is likely to diminish from 1.5 to 0.75 per cent over the same time period with consequent deleterious effects on structural stability, increases in surface compaction and sealing, decreases in infiltration rates, increases in runoff, reductions in the amount of water available for plant growth and further increases in erosion rates."
It is plain that British soils cannot be allowed to sustain levels of` damage of this magnitude, even over what seems to be a relatively long time scale. Weather patterns allied to recent changes in management systems suggest that bad erosion years may become a regular occurrence. Preliminary assessments of erosion during the winter of 1985-86 (Watson, 1986; Boardman, 1986; Evans, 1986; Harrison Reed, 1986c) indicated that it was as widespread as in 1983 or earlier serious erosion years; it is now clear that it was at least as bad as the worst year on record (Arden-Clarke & Hodges, 1987). Also, the cycle of` soil degradation described here by Morgan may result in accelerating soil damage, thus shortening the time-scale over which these processes take place. Thus urgent action needs to be taken to protect the nation's soils, in both the short and the long term, against the damage that recent agricultural trends and economic pressures have imposed upon them. Recommendations as to the action needed are given in the final section below.
This paper reviews in detail the published literature on soil erosion in Britain, covering the recent past--the last 20 years--up to the present. It deals almost entirely with erosion caused by water.
Water-induced soil erosion is widespread throughout the world but, until recently, it was considered to be only a minor problem in Britain, occurring in a few areas where very susceptible soils have been intensively farmed, particularly under arable crops. Britain's soils, combined with a moderate climate and properly adapted farming systems, have been thought to provide conditions where soil erosion would be almost unknown; and whilst traditional, balanced farming systems have been maintained erosion has largely remained at minimal levels.
However, over the past three or four decades British farming has moved towards more intensive and specialized production systems, slowly losing, as it did so, traditional techniques such as mixed farming, crop rotations, etc.; in many cases this has resulted in a slow but significant deterioration in soil structure. In 1970 the Strutt Report drew attention to the problems that were developing with regard to many soils but these warnings were largely ignored. In fact, entry into the E.E.C. in 1972 tended to enhance the trend towards intensification due to the economic incentives incorporated in the Common Agricultural Policy.
Although some authorities still consider soil erosion to be only a relatively limited problem, the majority of' evidence strongly suggests that it is a widespread and increasing problem throughout England, Wales and Scotland, and that it occurs more often than is generally recognized. Erosion appears to have been increasing substantially over the last 15 or 20 years, probably in parallel with the trends towards intensification, such that about 44°o of our arable soils are now clearly at risk. Because, until quite recently, no official support for research was available, an understanding of this problem has been acquired in a somewhat haphazard way, and this suggests that the full extent of' the present and potential problem of soil erosion has yet to be adequately evaluated and understood More extensive research will almost certainly demonstrate it to be more widespread than is presently recognised.
The rates of soil erosion reported across a wide range of' investigations are often greatly in excess of' natural soil renewal rates (0.1-0 5 tonnes/ha/year) and those rates which, on many soils, can be considered as a tolerable erosion loss (2 tonnes/ha/year). Some reports record spectacular soil losses of' between 100 and 200 tonnes/ha/year. Just how long even moderate soil losses can be tolerated, it' they occur on a fairly regular basis, depends very much on the depth of` soil at any one site. On relatively thin soils, such as on the South Downs, regular erosion losses may result in a total destruction of` their productive agricultural capacity within a few decades. On deeper soils regular erosive losses may be maintained for 100 to 200 years without any obvious loss of` productivity. Nevertheless, even in the latter case these losses are not acceptable since they cannot be safely continued indefinitely.
Factors contributing to soil erosion can be divided into two groups, physical and agricultural tractors Physical factors (rainfall characteristics; soil type; landform and slope) are outside the control of the farmer and there is no clear evidence of` changes in rainfall patterns which would have directly increased soil erosion. On the other hand, there are a range of` agricultural management tractors (continuous arable production; converting grassland to arable; increasing cereal acreage; use of heavier machinery; tramlining; working up and down slope; removing field boundaries; untimeliness of` cultivation) all of` which have tended to become more prevalent as a result of` agricultural intensification, and nearly all of` which can have a direct effect on soil structure--and hence on soil erosion. All these tractors are discussed in detail. Probably the most important tractor in relation to erosion is the level of` organic matter in the soil. In most recent studies of` soil erosion, reduced organic matter Ievels have been recognized to be a primary factor in the development of susceptibility to erosion, and many of` the management tractors described can have a direct negative effect on organic matter levels. Thus a major factor in the increase in soil erosion seems to be the recent changes in agricultural practices, and it is suggested that there is a direct relationship between them, the former being the result of` the latter.
Soil erosion is also discussed in a wider sense in relation to economic, environmental and resource implications; and, in particular, it is considered in relation to different systems of farming. Organic farming is inherently a "soil-enhancing" system, as compared to intensive conventional farming which, as has been shown, can directly give rise to soil damage and erosion. It is therefore suggested that the most constructive solution to Britain's soil erosion problems would be to move our farming towards a more organically-oriented system. Such a solution would not only improve soil structure and decrease soil erosion, it could also ameliorate many other environmental problems in the countryside and reduce the difficulties associated with agricultural surpluses.
The following recommendations for action and research are suggested:
I. There is an urgent need for a detailed assessment throughout Britain of the true extent of` soil erosion, both actual and potential; and of` the likely short term trends associated with different soil types.
2. The economic impact of` soil damage and loss resulting from intensive farming systems must be fully assessed and quantified. The cost of soil erosion both to the individual farming enterprise and to the wider community, over the medium to long term, is usually conveniently forgotten when considering the economic viability of` a farming system. Since the soil is the primary resource for farmers, the value of its integrity (or otherwise) must be included in the economic equation. Government and E.E.C. subsidies, however large, will not ensure a vigorous and sustainable agricultural industry in the U.K. if` they are formulated, as they now are, with minimal regard to the impact of the agricultural methods thus fostered, on all the resources on which harming ultimately depends.
3. There is a need for a much more detailed understanding of` the way each of` the "factors contributing to soil erosion" acts upon soil structure and integrity; Mitt also how they may interact with each other, resulting ill stall erosion. In particular, the interrelationships between organic matter levels, soil structure and erosion need much more detailed study.
4. Since organic/biological farming can be shown to be a soil-conserving system, there is a crucial need f or the establishment of a wide-ranging research programme into organic farming techniques and how they may be applied to help solve the present destructive trend towards increasing soil erosion.
The authors wish to thank all those who in a variety of` ways have helped lo make this review possible; and in particular to thank Drs John Boardman, Bob Evans anti Alan Harrison Reed who so willingly helped with their time and expertise
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(Received 18th September 1986; accepted 5th February 1987)
Copyright © 1987 Biological Agriculture and Horticulture. Reprinted with permission. All rights reserved.
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