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Soil Ecology: The Basics of Fertility

Table of Contents

Living SoilSoil Food WebSoil Nurture

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, 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 mychorrhizal 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 mychorrizae 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.