Microorganisms Found in soil
Soil organism any organism inhabiting the during part or all of its life. Soil organisms, which in size from microscopic cells that digest organic material to small mammals that live primarily on other soil organisms, play an important role in maintaining fertility, structure, drainage, and aeration of soil. They also break down plant and tissues, releasing stored nutrients and converting them into forms usable by plants. Some soil organisms are pests. Among the soil organisms that are pests of crops are nematodes, slugs and snails, symphylids, larvae, fly larvae, caterpillars, and root aphids. Some soil organisms cause rots, some release substances that inhibit plant growth, and others are hosts for organisms that cause animal diseases.
Soil organisms are commonly divided into five arbitrary groups according to size, the smallest of which are the —including , actinomycetes, and algae. Next are the , which are less than 100 microns in length and generally feed upon other microorganisms. The microfauna include single-celled protozoans, some smaller flatworms, nematodes, rotifers, and tardigrades (eight-legged invertebrates). The are somewhat larger and are heterogeneous, including creatures that feed on microorganisms, decaying matter, and living plants. The category includes nematodes, mites, springtails (wingless insects so called for the springing organ which enables them to leap), the insectlike proturans, which feed on , and the pauropods.
The fourth group, the , are also quite diverse. The most common example is the potworm, a white, that feeds on fungi, bacteria, and decaying plant material. The group also includes slugs, snails, and millipedes, which feed on plants, and centipedes, beetles and their larvae, and the larvae of flies, which feed on other organisms or on decaying matter.
constitute the largest soil organisms and include the largest , perhaps the most important creatures that live in the topsoil. Earthworms pass both soil and organic matter through their guts, in the process aerating the soil, breaking up the litter of organic material on its surface, and moving material vertically from the surface to the subsoil. This is extremely important to soil fertility, and it develops the structure of the soil as a matrix for plants and other organisms. It has been estimated that earthworms completely turn over the equivalent of all the soil on the planet to a depth one inch (2.5 cm) every 10 years. Some vertebrates are also in the megafauna category; these include all sorts of burrowing animals, such as snakes, lizards, gophers, badgers, rabbits, hares, mice, and moles.
One of the most important roles of soil organisms is breaking up the complex substances in decaying plants and animals so that they can be used again by living plants. This involves soil organisms as catalysts in a number of natural cycles, among the most prominent being the carbon, , and sulfur cycles.
The begins in plants, which combine from the atmosphere with water to make plant tissues such as leaves, stems, and fruits. Animals eat the plants and convert the tissues into animal tissues. The cycle is completed when the animals die and their decaying tissues are eaten by soil organisms, a process that releases carbon dioxide.
Proteins are the basic stuff of organic tissues, and nitrogen is an essential element of all proteins. The availability of nitrogen in forms that plants can use is a basic determinant of the fertility of soils; the role of soil organisms in facilitating the is therefore of great importance. When a plant or animal dies, soil organisms break up the complex proteins, polypeptides, and nucleic acids in their bodies and produce ammonium, ions, nitrates, and nitrites that plants then use to build their body tissues.
Both bacteria and can fix nitrogen directly from the atmosphere, but this is less vital to than the symbiotic relationship between the bacteria genus and leguminous plants and certain trees and shrubs. In return for secretions from their host that encourage their growth and multiplication, Rhizobia fix nitrogen in nodules of the host plant’s roots, providing nitrogen in a form usable by the plant.
