Build Better Garden Soil

Image Gallery Instead of tilling, Harvey Ussery recommends using a broadfork to loosen the soil for planting. MICHAEL FOCAZIO

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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.

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