Build Better Garden Soil

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Living organisms in the soil include bacteria, fungi, protozoa (single-cell animals), nematodes (minuscule non-segmented worms), arthropods (insects, spiders, mites and centipedes — from microscopic to several inches long), earthworms, and larger organisms such as moles, voles and even gophers, which have their role to play in recycling nutrients and maintaining good soil structure.

The Role of Organic Matter

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 soil organisms “specialize” in breaking down different sorts of organic matter in accordance with the natural principle that every creature’s “waste” is a priceless resource for another, and they pass their own wastes on to other members of the soil community.

The energy represented by the original organic material is passed from one level of the soil food web to another, rather than being lost from the system. The energy of sunlight is constantly being added at the other end of the equation through photosynthesis, so if all nutrients are re-captured by soil organisms, the result has to be added fertility.

For example, when fresh green material is added — say in the form of crop residues, or green cover crops cut and used as mulch — it is bacteria that 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 as leaves and other organic matter, converting it all to castings 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 colonize and feed on the denser plant tissues such as stems and leaf veins, as well as other more carbon-dense organic materials such as leaf mulches.

Dead organic material is 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, including beets and spinach, form such associations with bacteria; others, including tomatoes, potatoes and corn, with mychorrhizal fungi. In both cases, the plant creates complex sugars and other nutrients in its leaves, then releases them through its roots to feed its “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.

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