Good garden soil

• Caring is the key to growing better garden soil and more of our own food

Soil is the key to health, both for ourselves and for the animals and plants we depend on. But soil “in good heart,” as farmers used to say, is not something we can take for granted. For gardeners and farmers, caring for the soil must always be our first priority, and the process of building soil fertility is vast and complex.

The best question to ask is not “What is the best soil care?” but “What is the best soil care for this particular piece of ground?” Over the seasons, the soil itself becomes our teacher and shows us which practices lead to beneficial changes.

Let’s begin with this intriguing question: Why is it that in natural soil ecologies, soil fertility tends to accumulate spontaneously over time, while human agriculture often leads to drastic declines in soil quality? Whether we look at prairies, bogs or forests, we find that topsoil tends to deepen and become more fertile over time. Why are humans more likely to destroy than to build soil quality, when natural systems operating on their own produce the opposite result?

One implication is obvious: The key to soil management is imitating natural systems. But perhaps the best answer to this riddle is that topsoil is alive, and any approach to agriculture that treats it as an inert substance is almost certain to be destructive.

What is Topsoil?

Topsoil is formed from tiny particles weathered or worn from their 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; and whether it is sand (large particle size) or clay (extremely small particle size). But a layer of small rock particles is not “soil,” and it is not capable of growing a crop.

Healthy topsoil also consists of a complex community of living creatures, and each class of organisms has its own strategies for feeding itself, adapting to environmental conditions and coexisting with its neighbors. Any practice that destroys some or all of those classes of organisms is likely to reduce soil fertility.

The number of living organisms in healthy topsoil is enormous: It has been estimated, for example, that the total biomass of organisms in a prairie soil exceeds 15 tons per acre, with the weight of the bacteria alone — invisible to the eye — totaling 13 tons! A single teaspoon of that soil may contain 600 to 800 million individual bacteria from a possible 10,000 species; several miles of fungal hyphae; 10,000 individual protozoa; and 20 to 30 beneficial nematodes from a possible 100 species.

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.

These are simple sketches of complex relationships and food exchange. The end result of the process is humus, the final residue of the parent organic materials, now 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, and bind carbon into the soil. Many soil organisms also help “glue” soil particles together into larger aggregate particles, increasing the size of pore spaces between particles, bringing more air to the soil (most soil organisms need oxygen to thrive) and increasing water flow down into the soil (reducing hance of runoff and erosion in heavy rains).

Some soil organisms are pathogens, but with a diverse mix of species, they are usually controlled by other organisms in the system, which feed on the disease-causing organisms, out-compete them, or otherwise inhibit them. Simply put: Diversity of soil organisms is key to plant health.