The Passionate Gardener: Soil — What's Going On in There, Anyway?

Ron Kushner, for the Shuttle

Soil is the basis for all living things, supplying food, fiber, habitat, shelter, recreational space, clean air and water.  So what actually is “soil”?

Physically, soil is a mixture of various solids, air and water in varying proportions. The solid components are mineral and organic, both living and non-living. 

Most of the solids are mineral — stone fragments, sand, silt and clay. These materials are defined by the size of their particles. The relative proportions of sand, silt and clay determine texture of the soil. Texture is one of the fundamental characteristics for determining how soil is functioning.

For example, the amount and type of clay can greatly influence the ability of soils to hold and exchange nutrients and to store organic matter. The surfaces of most clays are negatively charged, so positively charged nutrient ions “stick” to them. This ability of soil particles to hold on to nutrient ions and exchange them with water is called the soil’s “cation exchange capacity” (CEC). It is a measure of the organic matter in the soil and the CEC number is in most soil test results. The scale is from sand (1) to clay (30). The number should be in the high teens or low 20s.

Organic matter, made up mainly of carbon, is any material that originates from living organisms. As organic matter decomposes, nutrients are released and become available to plants. Organic matter also contributes to the soil structure’s capacity to store water, providing drought resistance.

Spaces between solid soil particles are called “pores.” These spaces are filled with air, water and living things. Water and air are essential for all life in the soil. Air is constantly moving through the soil, providing oxygen for cell function in plant roots and living organisms. Water allows for nutrient transport and enables plant uptake of them. It also allows organisms such as nematodes and bacteria to move through the soil. A well-structured soil with a range of pore sizes allows plant roots and soil-dwelling organisms to have access to the proper balance of air and water.

Soil is teeming with life. Soil-dwelling organisms range in size from easily seen (earthworms and arthropods, i.e. bugs) to microscopic (bacteria). The initial source of food for this life is organic material like leaves, roots and “exudates” (sticky substances produced by living organisms). 

Earthworms drag organic material into the soil from the surface, exposing it to the activity of other organisms. They are generally a sign of healthy soil. As they burrow through the soil consuming solids, they digest the nutritious material and release what’s left as “casts.” Worm castings are coated with microbial cultures, which contribute to both building stable soils and suppressing plant disease. Worms help break down organic matter, mix materials into the soil, alleviate compaction and develop soil pores.

Arthropods (spiders, mites and many insects) also interact with organic matter, breaking it down into smaller pieces, mixing materials and exposing organic matter to microbes.

Bacteria and fungi produce digestive enzymes that they release into the soil. This action releases nutrients that plants can absorb. Protozoa are single-celled organisms, larger than bacterial cells, which they actually consume. They also consume other cells with access to sunlight and therefore energy through photosynthesis, as plants do.

Nematodes are microscopic, multicellular animals, a type of roundworm. They consume bacteria, fungi, protozoa and other nematodes. Some are parasitic and feed on plants and other animals. Nematode diversity can help determine soil health.

Mycorrhizal fungi, from the Greek words for fungus and root, are  fungi joined with plant roots. The plant host provides sugars to the fungus, which grows through the soil, absorbing more nutrients (especially phosphorous, which is poorly soluble) than the plant roots alone could. They also help the plant resist disease and tolerate drought and salty conditions and contribute to the stabilization of soil aggregates.

“Soil health” is basically the capacity of the soil to function as a vital, living ecosystem, sustaining plants and animals. A healthy soil has the following characteristics:

  • Good “tilth”: Crumbly, well structured, dark with organic matter and no large or hard clods.
  • Good depth: No compacted layer preventing roots from growing and finding water and nutrients.
  • Good drainage.
  • Sufficient nutrients.
  • Few insect pests and plant pathogens.
  • Large population of beneficial organisms.
  • Low weed pressure.
  • Free of chemicals and toxin contamination.

Cornell University’s soil testing laboratory has developed a comprehensive assessment of soil health. Developed by Bob Schindelbeck, it is the first commercially available soil test for gardeners and growers for field-specific information on their soil’s biological and physical properties. For more information, visit

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