Gardeners are always working to improve their soil through the addition of compost, use of a fall planted cover crop, a green manure crop or turning under organic mulch. The improvement of the soil in one’s garden is accomplished by soil microorganisms that are involved in the mineralization of organic forms of carbon, nitrogen, sulfur, phosphorous and other compounds, and turning them into inorganic forms available to higher plants. This process is essential for the maintenance of life on our planet.
A teaspoonful of soil may contain billions of living organisms. In fact, an acre furrow slice of fertile soil can contain 1,000 pounds of bacteria and an equal amount of fungi, protozoa and algae. Plant growth, soil fertility and soil development are directly and indirectly dependent on soil microorganisms. Some microorganisms improve soil fertility by converting atmospheric nitrogen into nitrogen compounds used by plants to synthesize proteins. Other microorganisms mineralize organic residues such as proteins into inorganic substances like ammonium compounds and nitrates, making them available to plants. Generally, a high level of microorganisms indicates a healthy soil.
Few environments provide as great a variety of microorganisms as fertile soil. The number of bacteria, fungi, algae, protozoa and viruses may reach billions of organisms per ounce of soil. The size and diversity of the soil microbial population is dependent upon a soil’s physical and chemical characteristics, climate, whether the soil is cultivated and the type of growing plants. Other soil conditions such as the amount and type of nutrients, available moisture, degree of aeration, temperature, pH and agricultural practices also influence the population of microorganisms.
Bacteria are the smallest and most numerous of the free living organisms in the soil. It takes 25,000 bacteria to measure an inch. Despite bacteria’s small size, their total weight in the top foot of an acre of fertile soil may be as much as 1,000 pounds or 0.03 percent of the weight of the soil. Poorly aerated soils and sandy soils generally have few bacteria. Bacteria are very important in the capture of gaseous nitrogen, such as the Rhizobia or nodulating bacterial found on the roots of legumes.
Another of the microscopic organisms is called actinomycetes, which in many respects resemble bacteria. They can be found in most soils but are only about one-tenth to one-fifth as numerous as bacteria. The actinomycetes are important in the decomposition and the humification of organic residues. They cause much of the aroma found in a handful of soil.
A sustainable garden system capitalizes on the knowledge that intricate biological processes exist within the soil. Because soil microorganisms play an indispensable role, cultural practices that damage the microorganism population must be minimized. The end result will be a healthy, diverse population of soil microorganisms contributing to soil fertility by increasing organic matter decomposition, mineralization of organic compounds, nitrogen fixation, soil stability and tilth and reducing damage from soil pathogens through competition. When you take that walk in the woods, check the soil humus out which is nature’s compost.
Bill Lamont is a professor and extension vegetable specialist in the department of plant science at Penn State and can be reached by email at firstname.lastname@example.org.