Build Better Garden Soil

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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 chance 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.

Strategies for Managing Soil

Most of us have grown up thinking that soil fertility revolves around the question of what we need to buy and add to soil. We sometimes use soil tests to guide us in making the proper purchases. When we begin working with a piece of ground, especially if it has been abused, there may be additions we need to make. Be cautious about soil tests, however, because different laboratories use different procedures, report results differently and adopt different approaches to interpreting the results.

For example, I remember how confused I was by test results I used to get from my local extension service: They always noted that both phosphorus and potassium were “very high” — and then went on to recommend application of chemical fertilizers containing 10 percent of each. Later, I worked with a soil consultant (a student of pioneering soil scientist William Albrecht), who also noted the high levels of phosphorus and potassium, but recommended no fertilizer application.

Indeed, he pointed out that it would be easy for my soil to rise to dangerously excessive levels of phosphorus if I wasn’t careful with certain organic matter applications, such as manures. Since most soil analyses focus so much on crop needs for nitrogen, imagine my surprise when he also recommended no added nitrogen. When I asked about that, he replied dismissively, “Oh, with organic matter at the level you have, you don’t need any added nitrogen, except maybe a little for really heavy feeders like corn.”

By all means, find and work with a competent soil consultant if you feel your soil has serious deficiencies or special needs. But your main focus should not be what purchased inputs you need to add to your soil, but on strategies to maximize the diversity, health and population densities of soil organisms.

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