Compost and the Living Soil

by Steven Wisbaum. Revised November 2021

While most gardeners are now are familiar with the concept of using compost, the specific role that compost plays in building soil and plant health isn’t always as well understood.  For some background, it’s useful to consider how soil fertility is created in more natural ecosystems such as forests and grasslands.

Early agriculture scientists often viewed the soil as a lifeless anchor for plant roots. However, we now know that the soil is actually a highly complex eco-system teeming with a dazzling array of organisms including: bacteria, fungi, protozoa, algae, actinomycetes, nematodes, earthworms and insects.

These organisms have evolved to play an essential role in transforming plant and animal tissue into the nutrients that plants need to grow.  A physical by-product of this subterranean digestive process is a substance known as humus, which among other benefits, enhances the ability of the soil to hold water and oxygen, which are also essential for a healthy soil-plant eco-system.

However, in a garden or farm where a large portion of the vegetation (ie. the crops) are harvested, this natural supply of organic matter is removed from the system. To make matters worse, the soils in these unnatural environments can become compacted from the activity of people and/or machinery, which further compromises the oxygen and water holding capacity of the soil.  Additional negative impacts of growing crops can include the use of synthetic fertilizers which are toxic to many soil organisms, and  cultivation which reduces the volume of organic matter in the soil due to increased oxidation, erosion, and exposure to UV-radiation. Ultimately, the collective impacts of these gardening/farming activities can lead to a depleted and lifeless soil, which in-turn impairs plant growth, leaving them more susceptible to damage from pests, diseases, and draught.

Since it takes thousands of years to create the deep, fertile soils found in nature, the use of compost along with crop rotation and growing “cover crops”, has become the cornerstone of a biological approach to agriculture. Aside from providing a source of pre-digested organic matter, compost also contains stable plant nutrients, as opposed to the soluble and chemically reactive forms contained in synthetic fertilizers and raw manure. Compost that has been properly made and is sufficiently “mature”, also serves as an important source of beneficial soil microbes, which will not only help stimulate nutrient cycling, but also can suppress a wide variety of soil-borne plant diseases. And if uniformly exposed to temperatures above 130 degrees F during the decomposition process, compost will also be free of the weed seeds and pathogens associated with raw, or improperly composted materials.