Soil is the basis for everything that life on earth needs to live including food, fiber, habitat, shelter, recreational space, clean air and water. What actually is “soil”?
Physically, soil is made up of a mixture of materials including various solids, air and water in varying proportions. The solid components are mineral and organic, both living and non-living.
Most solids are mineral, consisting of stone fragments, sand, silt and clay. These particles are defined by their sizes. The relative proportions of sand, silt and clay determine texture of the soil. Texture is one of the fundamental characteristics for determining how a 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 surface of most clays are negatively charged so that positively charged nutrient ions can “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 measures the organic matter in the soil and the CEC number is located on most soil test results. The scale is from sand (1) to clay (30). The number should be in the high teens or low twenties.
Organic matter is made up of mainly carbon, any material that originated 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. Water allows for nutrient transport and enables plant uptake of them. It also allows microbes such as nematodes and bacteria to move through the soil. Air is constantly moving through the soil, providing oxygen for cell function in plant roots and living organisms. 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 & arthropods) 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). All this life needs energy and they get it from plants. Without plants and their organic input, the soil food web couldn’t thrive.
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 nutritious material and release it as “casts”. These worm castings are coated with microbial cultures which contribute to both building stable aggregates and suppressing plant disease. The 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 soil materials and exposing organic matter to microbes.
Bacteria and fungi both produce digestive enzymes that they release into the soil. This action releases nutrients for plant uptake.
Protozoa are single-celled organisms, larger than bacterial cells and actually consume them as organic matter degrades further. They also consume other cells with access to sunlight where they get 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. There are reports that certain nematodes contribute to suppression of plant disease by consuming plant pathogens. Nematode diversity can help determine soil health.
Mycorrhizal fungi is named from Greek words for fungus and root as these fungi are joined with plant roots. The plant host provides sugars to the fungus, used for growth and metabolism in exchange for nutrients. The fungus grows through the soil, absorbing more nutrients (especially phosphorous which is poorly soluble) than the plant roots alone could. They help the plant resist disease and tolerate drought and salty conditions along with providing nutrients. They also 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 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 their website soilhealth.cals.cornell.edu/.
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