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THE PRINCIPLES OF ORGANIC FARMING, by Nic Lampkin
There is seldom sufficient manure available for an application to every field every year, so selectivity becomes important. In general, manure and slurry is applied to crops which remove greatest quantities of nutrients, such as conserved forage, and to those which are most nutrient demanding, such as root crops and maize. Excessive applications of manure or slurry to the leys, however, will reduce the nitrogen fixation effectiveness of clover. Although crops like wheat will respond to manure applications, they are usually able to make good use of the residual fertility from the previous crop in the rotation. Application of manures to wheat may not, therefore, make most effective use of the available nutrients in terms of the farm system as a whole, even though the short-term financial gains may seem attractive. In certain cases, such as barley, top-dressing with slurry at tillering may be appropriate.
Various studies have looked at the issue of rates of manure application and their effect on yield in organic systems. Besson et al. ( 1988) in a longterm comparison of conventional, organic and Biodynamic systems with three levels of manure use equivalent to average stocking rates of zero (control), 0.6 and 1.2 livestock units/ha (including both forage and cash crops) found variable responses to manure applications to winter wheat (Table 5.5). Wegrzyn (1983) examined the impact of supplemental nitrogen applications on maize yields on a longstanding organic farm with lucerne in the rotation and concluded that 'in general, supplemental nitrogen applications (mineral or organic) resulted in luxury consumption rather than increased yield. Increasing plant populations increased yields as much or more than supplemental nitrogen.'
Manuring plans can provide a systematic approach to determining the appropriate allocation of manures in the rotation. It is necessary first to estimate the total quantities of manure and slurry available, based on total stocking, length of time that stock is housed and data SUCH as that contained in Chapter 4. An example of a manuring plan is given in Table 5.6.
As mentioned in Chapter 3, nutrient budgets can give a crude indication of the nutrient and losses of the farm. They can be used when planning a conversion to help assess the suitability of a rotation, but inevitably, the figures will not be very accurate and the results will need to be treated with caution. An example is given in Table 5.7 using the same rotation as was used for the manuring plan in Table 5.6 and the crop nutrient removal data in Table 3.6.
In this example, there is a positive nitrogen balance, and the phosphate and potash deficits are within reasonable limits, given the limited accuracy of the approach. Many factors will influence the amount of nitrogen fixed and the quantity of nitrogen and potash lost from the system. It is assumed here that nitrogen losses resulting from volatilisation and denitrification are equal to gains from rainfall and free-living nitrogen fixing bacteria. Potash losses through leaching will depend on the clay content of the soil and will be most severe in light sandy soils. Potash may be replenished from soil reserves at rates greater than the deficit indicated here. In most cases, therefore, soil analysis will provide a more reliable indication of whether action is required to correct any nutrient imbalance.
REFERENCES AND FURTHER READING
Robinson, D. H. (ed.) (1972) Fream's Elements of Agriculture. 15th Edition. John Murray
Spedding, C. (1983) Fream's Elements of Agriculture. 16th Edition. John Murray. (Note:
The 15th and earlier editions are much more useful for indicating possible husbandry . . . Options m orgamc systems. Many pre-1950 books on cropping and crop rotations Various MAFF/ADAS booklets on individual crops Various NIAB lists of recommended varieties
Rotations and mixed cropping (polycultures) Altieri, M. A. & Letourneau, D. K. (1982) Vegetation management and biological control in agroecosystems. Crop Protection I (4): 405 - 430
Copyright © 1990 Nicolas Lampkin
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