Soil Ecology: The Basics of Fertility
The Living Soil
“Our life depends on six inches of topsoil and the fact that it rains.” ~Anonymous
Soil “in good heart,” as farmers used to say, is the key to health, both for ourselves and for the animals and plants on whom we depend. Managing the soil for maximum fertility is not something ever to be forgotten or taken for granted—it is always “job one.”
It is best to start with a recognition of our utter ignorance of what constitutes best stewardship of the soil. Without approaching the task with a sense of mystery about the vastness and complexities of the processes involved, we are likely to adopt formulaic and dogmatic soil care solutions. The question is finally not “What is best soil care?” but “What is best soil care for this particular piece of ground; with these particular angles of slope and wet and dry places; in this particular climate; with these unique histories of use (or abuse), insect pressure, crops, etc.?” If we make our ignorance our friend, the piece of ground we have taken into our care will be our own best teacher; and over the seasons, it will teach us the practices that lead to the most beneficial changes. A lifetime should suffice.
Suppose we take as our guide this intriguing question: Why is it that, in natural soil ecologies the world over, soil fertility tends to accumulate spontaneously over time—while human agriculture far too often has led to drastic declines in soil quality, never more so than in the era of industrial agriculture? Whether we look to prairie, or bog, or forest, we find that topsoil deepens and becomes more fertile over time. Why are we humans with our vaunted intelligence and best efforts more likely to destroy than to build soil quality, when natural systems operating on their own produce the opposite result? Even before attempting an answer, one implication is obvious: The key to best soil management is imitation of natural systems, rather than trying to impose novel strategies of our own.
Perhaps the best answer to the riddle is that topsoil is alive, and any approach to agriculture (again, most especially, modern industrial agriculture) which ignores it as a living system and treats it as an inert substance, is almost certain to be destructive.
Topsoil consists of course of tiny particles weathered or worn by geologic processes out of the parent materials (rock, of various types). Both the chemical composition of the parent material and the average particle size help determine fundamental characteristics of soil—whether it is acid, alkaline, or neutral; whether it is sand (large particle size) or clay (extremely tiny particle size); etc. But a layer of small rock particles is not “soil,” and it is not capable of growing a crop.
In addition to rock particles, real topsoil consists of a complex community of living creatures both seen and invisible, each class of organisms with its own strategies for feeding itself (utilizing energy sources), adapting to environmental conditions, and competing against (and cooperating with) its neighbors. Any practice destructive of some or all those classes of organisms—which reduces their diversity and the available pathways of their interactions—is likely to reduce soil fertility.
Living organisms in the soil include bacteria, fungi, protozoa (single-cell animals), nematodes (minuscule non-segmented worms), arthropods (from microscopic to several inches long—insects, spiders, mites, centipedes, etc.), earthworms, and larger organisms such as moles, voles, even gophers, which have their role to play in recycling nutrients and maintaining good soil structure.
Please understand that the mass of living organisms in healthy topsoil is far from trivial: It has been estimated, for example, that total biomass of organisms in a prairie soil exceeds fifteen tons per acre, with the weight of the bacteria alone—invisible to the eye—totaling thirteen tons. A single teaspoon of that soil may contain 600-800 million individual bacteria from a possible 10,000 species; several miles of fungal hyphae; 10,000 individual protozoa; and 20-30 beneficial nematodes from a possible 100 species.
The Soil Food Web
Organic matter is constantly being produced in and over the topsoil—fallen leaves, dead plants and animals, roots shed by living plants, and droppings of passing animals. Different classes of organisms “specialize” in different sorts of organic matter, leaping hungrily on them in accordance with that great principle of nature that every creature’s “waste” is priceless resource for another; and passing their own metabolic residues on to other members of the soil community for their use in turn. Thus the energy represented by the original organic material is passed from one “trophic level” (level of the soil web food chain) to another, rather than being lost from the system. Over time, this “plugging” of all potential nutrient leaks from the system increases fertility. (Remember that the energy of sunlight is constantly being added at the other end of the equation, so, if all nutrients are re-captured by soil organisms, the result has to be added fertility.)
For example, when fresh green materials are added—say in the form of crop residues, or green cover crops cut and used as a mulch—it is the bacteria which take the lead in breaking them down. Nematodes, protozoa, and tiny arthropods feed on the bacteria, and are fed on in turn by larger arthropods and nematodes. Earthworms feed on the bacteria as well, converting them to castings (excreta) rich in minerals and other nutrients in forms easy for plant roots to take up, and conducive to good soil structure. In the meantime, soil fungi, which cannot utilize the more “active” green matter, colonize and feed on the denser plant tissues such as stems and leaf veins, which resist breakdown by bacteria, as well as other more carbon-dense organic materials such as leaves (perhaps in a leaf mulch).
Note that dead organic materials are not the only source of food for soil-dwelling species. Roots of living plants form cooperative, mutually-beneficial associations with various soil organisms. Some plants (many in the mustard family, beets, spinach, etc.) form such associations with bacteria; others (tomatoes, potatoes, corn, and indeed the majority of vegetable crops), with mycorrhizal fungi. In both cases, the plant creates complex sugars and other nutrients in its leaves, then releases them through its roots as exudates to feed its chosen “buddies” in the soil. In exchange, the mycorrhizae or bacteria provide nitrogen, enzymes, minerals, and other nutrients to the roots in forms easy for them to absorb. In some cases, bacteria and fungal hyphae (tiny filaments) do not remain on the outside of plant roots, but actually penetrate between and into cell space and assist with the plant’s metabolic processes.
The above are but extremely simple sketches of what are in fact complex relationships and pathways of energy (food) exchange. The end result of the intricate breakdown process is humus, the final residue of the parent organic materials, no longer recognizable as such, but visible only as a darkening of the soil. The microscopic humus particles are no longer a source of food for soil organisms, but they help with water retention, bond with nutrients in the soil and pass them on to plant roots, bind carbon into soil, etc.
The added fertility is hardly the end of the soil-life story. In various ways, many soil organisms help “glue” soil particles together into larger aggregate particles, thus increasing the size of pore spaces between particles, bringing more air into the soil (most soil organisms need oxygen to thrive) and increasing water flow down into the soil (reducing chance of runoff in heavy rains).
Some soil organisms can be pathogenic (disease-causing), of course, but with maximum possible diversity of species in the mix, pathogens are usually controlled by other organisms in the system—which feed on the pathogens, out-compete them and thus form protective barriers on plant roots, generate metabolites that inhibit them, etc. Diversity of soil organisms is thus a key to plant health.
Most of us have grown up thinking that soil fertility revolves around the question of what we need to buy and add to soil to bring it into balance and heightened productivity. We look to soil tests to guide us in making the proper purchased applications. Certainly when we begin working with a piece of ground, especially if it has been badly abused, there may be additions we need to make. Be cautious about soil tests, however. There is no unified approach to soil testing. Different laboratories use different procedures, such as different solutions to extract soil nutrients; report results differently, and adopt different approaches to interpreting the results. For example, I remember how confused I was by test results I used to get from the Extension Service: They always noted that both phosphorus and potassium were “very high”—and then routinely went on to recommend application of chemical fertilizers containing 10 percent of each. Later, I worked with a soil consultant (a student of William Albrecht), who also noted the high levels of phosphorus and potassium, but who recommended “no fertilizer needed”—and indeed pointed out that it would be easy for my soil to rise to dangerously excessive levels of phosphorus, if I weren’t careful with certain organic matter applications such as manures. Since most soil analyses focus so much on crop needs for nitrogen, imagine my surprise when he also recommended no added nitrogen. When I asked about that, he replied dismissively, “Oh, with organic matter at the level you have, you don’t need any added nitrogen, except maybe a little for really heavy feeders like corn.”
Soil Nurturing Strategies
By all means, find and work with a competent soil consultant if you feel your soil has serious deficiencies or special needs. But I urge that your main focus be not on what you need to add to your soil, as purchased inputs, but on strategies to maximize the diversity, health, and population densities of your friends in the soil.
It is unfortunate but true that the key to doing so is largely to choose strategies directly opposite to almost all current agricultural practices, which are injurious to soil life in three ways:
- Monoculture The growing of a single species on vast tracts reduces diversity of soil life.
- Use of harsh chemicals There is no synthetic chemical—whether used to fertilize crops, kill insects or weeds, or suppress disease—which has been demonstrated not to be destructive to soil populations.
- Excessive tillage Frequent tillage of soil is disruptive to soil life and robs it of its carbon (organic matter) reserves.
The alternative to such destructive practices is to imitate natural soil ecologies in order to:
- Maximize diversity and population densities of soil organisms While homesteaders are unlikely to practice monoculture in the conventional agricultural sense—to grow nothing but carrots, for example, on all their available ground—we should constantly find ways to “mix it up” in how we manage our soil. Crops of different families should rotate over the available ground in succeeding seasons. A diversity of sources of organic matter should be used—composts, mulches, cover crops, etc.
- Feed the soil using sources of fertility grown on the homestead or found close by Deep-rooted cover and fertility crops can “mine” minerals from the deep subsoil and make them available to more shallow-rooted plants. Nitrogen-fixing legumes (clovers, alfalfa, beans and peas) can boost nitrogen for heavier-feeding crops. Recycle autumn leaves and crop residues by composting or using them as mulches. Manures and mulching materials may be available from nearby farming operations. If a soil test does indicate the need to add minerals, use slow-release rock powders (greensand, colloidal rock phosphate, etc.) rather than highly soluble chemical fertilizers, which quickly leach to groundwater.
- Protect and improve soil structure Plant in wide beds and never compact the soil by walking in the growing spaces. Keep the soil constantly covered—by closely-planted crop plants, cover crops, and mulches. Addition of lime to clay soils can help “flocculate” the almost microscopic soil particles into aggregate clumps, resulting in a looser, more open soil structure with better air and water penetration. When tight soil must be loosened, do it with a broadfork rather than a power tiller (or even a spading fork) to avoid inverting soil layers. Chickens can also be used to till in cover crops without serious disruption of soil structure.
The “Soil Care Basics” articles, “Increasing Organic Matter and Mineral Availability” and “Protecting Soil Structure with Alternatives to Tillage”, present specific soil care practices based on the above ideas.