Among all the things we take for granted, the soil beneath our feet might be near the top of the list. While it may be the stuff that gets one in trouble when it shows up on the carpet, it’s also the very thin layer of stuff upon which virtually all terrestrial life depends. And while we’re totally dependent on it, it also depends on us to manage and maintain its life-giving nature.
Soil generally is made up of four things: mineral particles, air, water, and organic matter.
The mineral part is fragments of weathered rocks, and “secondary” minerals, such as clays which are synthesized within the soil (more about clays later). The mineral portion may average around 45%. The pore space between these particles is occupied by air and water in approximately equal parts (20 to 30% each) in a productive soil. The fourth component is biological, and although it may comprise only about 5%, it’s the workhorse, and its management is the key to maintaining a healthy soil.
Soil organic matter includes both living plants and animals, and their decaying tissues. The decay process is carried out by burrowing animals, insects, earthworms, and other micro-animals as well as an army of bacteria, fungi, algae, and many other kinds of micro-plants. A half-teaspoon of soil may contain over a billion bacteria alone; and the total weight of living matter in the soil may be over 20,000 lb per acre. The activities of all these organisms may be the most intricate biological cycle on earth. Some of these biological processes break down complex organic compounds into elements that plants need, such as calcium, magnesium, and especially nitrogen; without this breakdown process plants would starve.
The breakdown process also produces humus, a dark-colored, organic “glue” that provides a loose, crumbly structure that’s easy to cultivate and work, and that retains optimum amounts of air and water. This is particularly important with high clay soils (many of our soils in Sheridan County). Humus is also chemically-active, that is, it has a negative charge that attracts and holds plant nutrient elements such as potassium, calcium, and magnesium. Soil clays also perform this retention function, and are critical players in plant nutrition, although their capacity is significantly less than humus. Soil humus also withdraws and sequesters huge amounts of carbon dioxide from the atmosphere, and binds toxic compounds such as heavy metals, and pesticides. Soil microbes also safely process organic wastes, such as in septic system leach fields. One can begin to see how our life and future on the planet is so closely tied to the life of our soil.
The organic material in soils is a dynamic system and has to be continually maintained; this happens naturally in grassland and forest soils through their dense, long-lived, perennial root mass (about 3/4 of the total living plant material on a grassland soil is below ground), and the natural, annual additions of decaying plant materials. Annual crops however, such as most food and horticultural crops that require cultivation open up the soil to accelerated organic matter oxidation and depletion, potentially yielding eroded, hard, sterile, soils. Gardeners in particular, because we use the soil intensively, learn that our success hinges on providing optimum conditions for the livings things in our soil. The solutions lie in maximizing the use of perennial plants, limiting tillage, and regular additions of organic matter such as organic mulches, compost, crop residues, and cover crops.
These topics are covered in detail in several excellent University of Wyoming publications & videos; a good place to start is uwyo.edu/barnbackyard. Look also for future articles on specifics of soil management. Constant attention to the life of our soils is the key to sustaining the healthy, critically valuable stuff beneath our feet (and sometimes on the carpet).
Jerry Forster is a master gardener initiate in Sheridan through the University of Wyoming program.