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EAP Publication - 31
It's alive! A soil ecologist takes us underground to meet the curious creatures that make their home in earth
Stuart B. Hill
When I was a youngster and filled with idealism--my colleagues will tell you that I haven't lost it--I wanted to farm the sea and solve the world food problem. I had grown up in England's countryside and knew all the fish in the local rivers and wildlife in the local woods and fields. I never thought much about living things in the soil, except for the earthworms, which were always guaranteed to catch some prize fish on one of my frequent trips to the local gravel pit lakes with my grandfather. No, soil was just something you stood on and pulled carrots out of.
So when I set off to Swansea University (the nearest one to the sea in Britain) to study marine zoology, life in the soil was far from my mind. Although I didn't realize it at the time, the ground was being laid for a career in soil ecology. The senior ecologist at the university, Dr. Amyan Macfadyen, had made a name for himself studying soil animals, a subject about which he exuded unlimited enthusiasm. During my last term, Dr. Macfadyen mentioned that the University of the Vest Indies was looking for students to study the ecology of a bat-inhabited cave in Trinidad. Although I was still set on farming the sea, I jumped at this opportunity. I reasoned that humans had made so many errors in our mismanagement of the land that it would be good for me to first study the ecology of a terrestrial environment and then, armed with this wisdom, move on to farm the sea. And where better to study terrestrial ecology than in a tropical cave, a relatively contained and constant ecosystem in which its members and processes could be fairly easily measured.
The cave contained about a quarter of a million bats and consequently the floor was loaded with bat waste, or guano. Now bat guano is like a very fertile soil, and because it was the home of most of the life in the cave it became the focal point of my studies. Well, to cut a long story short, I fell in love with the amazing world within this "soil" and, as they say in documentaries, the rest is history.
At that time, the late sixties, the only substantial English-language textbook on soil animals as written by Dr. Keith Kevan, who had come from Nottingham University to McGill University in the fifties. So I wrote to him from Reading University, where I was writing my PhD), to see whether he was eager to have an enthusiastic soil animal ecologist come and sit at his professional feet. In the spring of 1969, I set out to Canada with my family, looking forward to uncovering the secrets of life in the soil. In the early seventies, there was little research funding for such esoteric pursuits as studying the apparently uneconomically-important organisms that live in the soil. Consequently, I had to broaden my interests, in this case to include insects and ecological approaches to farming. However, my heart has always remained in the soil, so I am going to take this opportunity to introduce those of you who are unfamiliar with this amazing world to some of its inhabitants and their-goings-on.
In most parts of the world there is more life beneath the surface of the soil than above it. Nearly all groups of animals that are not exclusively marine have representatives in the soil. These range from mammals, such as moles and gophers, to the thousands of species of insects, mites and their relatives (collectively called Arthropods), to the smaller animals such as nematodes (roundworms and their relatives) and the one-celled protozoa. My particular favourites are the mites, which in most soils are the most abundant group of arthropods. Most mites are beneficial, feeding on microorganisms or on other small animals. In a few shovelfuls of forest soil there may be as many as 100 different types of mites and all together more than 1,000 different types of organisms. The number of individuals of each type can also be impressive. In a square metre there can be more than 10 million nematodes and protozoa, one million mites and springtails and thousands of other invertebrates, including several hundred earthworms. Even in a teaspoon of soil there can be as many as 10 million bacteria and two kilometres of fungal filaments.
The soil is indeed a living, breathing firmament in which the primary action taking place is the breakdown of organic matter and the release of plant nutrients. The soil is the decomposer part of the cycle in nature that flows from plant and animal production to consumption and use to waste decomposition and recycling, and then back to plant production again. If there were no organisms in soil we would soon be lip to our ears in accumulated dead animals and other forms of organic matter, all of which are efficiently taken care of by our friends in the soil.
Increasingly, however, these organisms are having a harder and harder time getting their work done. Nearly everything that humans do to the soil kills them-- directly or indirectly.'They are poisoned by pesticides and some fertilizers and injured or exposed to the damaging rays of the sun or to predators by cultivation and bare-soil agriculture practices. They're starved of a balanced diet by our habit of not returning all of our wastes to the soil or, because of our tendency to grow the same crop in the same soil year after year, providing the soil population with a monotonous diet. In addition, dessication, flooding, fire, compaction from heavy machinery and contamination with a vast range of pollutants add to the hazards of life in soil.
Yet it is upon these very organisms, which provide this free decomposition service, that the productivity of our farms and forests and ultimately we ourselves rely. Unknowingly--lam being generous here--most humans seem committed to cutting off the hand that feeds them. This is particularly reflected in Canadian statistics on soil erosion: over half of the organic matter in prairie soils has been lost in the past 40 years, and loss from all soils is over twice the rate of new soil formation.
The key to working with (versus against) our allies in the soil is to try to see the world from their point of view. It helps to fantasize what it would be like to be a mite or an earthworm--where you would live, how you would get around, what you would eat.
From the perspective of a soil organism, about half of the average soil consists of solid material, usually mostly mineral particles, and half consists of spaces between the particles. Half of these spaces are filled with water that occurs as a film around the particles. This situation has resulted in the evolution of three primary strategies for living in soil. Protozoa, nematodes and some other small organisms swim or creep around in the water film, feeding primarily on bacteria and on one another. Mites, springtails and other small arthropods wander around in the airspaces (up to their knees in water) browsing on fungi, nematodes and one another. Earthworms, some insects and other larger arthropods and slugs and snails burrow through the soil, independent of the size of the air and water spaces, feeding primarily on the dead organic matter and on the microorganisms that colonize it.
The key soil management question is: what can we do to provide these organisms with optimal food and space conditions so they can get on with their jobs? Clearly, the key is to permit the return of a suitable mix of uncontaminated organic wastes to the soil, and to avoid stressing the system physically, chemically and biologically. In a stressed and starved soil the earthworm can fall to nearly zero, whereas in a well-managed soil their activity may result in the production of more than 250 tonnes of castes (their very fertile waste) in the soil's upper layers. In addition, their burrows help to aerate the soil and provide channels for root growth and water drainage.
As well as these more obvious beneficial roles, most soil organisms have many more subtle positive effects. For example, the breakdown of leaves requires the action of a succession of different species of fungi, solve to digest the sugars, others the cellulose, hemicellulose, lignin and other materials. The problem is that because fungi have limited dispersal powers they soon run out of suitable substrate.
They also pollute the environment around them with antibiotics, making it difficult for other fungi to grow.
The animals, especially my friends the mites and springtails, provide pan of the solution to this problem. Most of the fungal-feeding mites prefer certain species of fungi. So along will come some mites who browse on a patch of their favourite fungi, eating the filaments the spores (like seeds) on the ends of them. However, because they generally digest only the filaments, the spores pass through their guts undamaged and are deposited in little packages of their waste throughout the soil. Because this waste is fertile, like potting soil, these spores will geminate and form new fungal colonies in new locations. In this way the mites help the process of organic matter decomposition. while at the same time essentially practicing a primitive forth of farming. Actually. it is not so primitive because their waste is deposited in a piece of cast-off gut lining that can contain substances that prevent other fungi from growing on their package of waste. Hence, mites practice a fairly sophisticated method of "weed" control. Furthermore, the hairs 011 their bodies may disperse the spores of the fungal species on which they prefer to feed. It is humbling to think that this son of farming has been going on in soil for more than 400 million years and that our version of it is relatively recent. These kinds of relationships operate by means of very delicate balances. Thus, if the mite population is decimated by, for example, the application of a highly toxic pesticide, microbial activity may initially increase in the absence of cropping and then slow down as antibiotics accumulate.
Myconrhizal fungi, which have beneficial relationships with most plant roots, also partly rely on balanced relationships with certain mites and other small arthropods for their dispersal. On the other hand, if the mite or springtail population increases, perhaps because a pesticide has killed their predators, they can overgraze the myconrhizal fungi and reduce their beneficial effects. It is this regulatory function which the animals carry out that is so important for the maintenance of soil fertility and productivity. This has not been recognized, partly because the main effects of these organisms are catalytic and indirect, and also because there has been a tendency to believe that the regular use of chemical fertilizers can keep the soil productive. In fact, as is the case with many analogous processes in the human body, the artificial provision of a naturally produced substance will inhibit its production and create dependence on repeated applications.
It is important to understand that there are many functions performed by the vast array of creatures in the soil, many more than we are fully aware of, and many more than have been touched on in this article. Whenever we eliminate one of these organisms we inherit its job, a job at which it is an expert and we are, at best, novices; at worst, we may be totally ignorant of its role. The message is clear: we must support our allies in the soil, and so help them to supped us and the other organisms with which we share this planet. By conserving this vital resource, we also make it possible for more young idealists to one day fall in love with the amazing inhabitants of the soil and enjoy their goings-on as much as I have.
Stuart Hill is associate professor of entomology in the Faculty of Agriculture at Macdonald College, McGill University and director of Ecological Agriculture Projects. Me display quotations in this article are adapted from Dr Hill's presentation to the Senate Committee on Soil Degradation in Canada, 1984.
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