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Soil Ecology: The Basics of Fertility

The following is the original version of my article “Build Better Garden Soil”, published in the April/May 2007 issue of Mother Earth News.

Table of Contents

Living SoilSoil Food WebSoil Nurture

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 (miniscule 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.