Someone has said, and not too inaccurately, that soil management is nothing more nor less than the care and feeding of bacteria. Kill them off and what is left is no longer a true soil but an inert mass of rock debris contaminated with remains of dead plants and animals- wastes that without organic breakdown must remain permanently fixed in their sterile grave. Although they have an almost split-second life span, microorganisms such as bacteria, fungi (including mycor-rhizae), actinomyces, rotifers, and protozoa are all vital to the reduction of organic and mineral wastes into plant nutrients, thus recycling the elements of existence from one generation to the next.
Fortunately for mankind as well as for all living things, these organisms are doggedly invasive and wonderfully capable of finding their way to places where they are needed. Few spots on earth are unfit for them to do their vital work. (By microorganisms is meant all living organisms, whether of plant or animal origin, that are too small to be seen with the naked eye.)
When we speak of the microorganisms of the soil, we are speaking of a living, throbbing community of well-nigh infinite numbers. Is it any wonder, then, that air, moisture, food and growing conditions are so vital to the production of Gardener's Loam? These trillions upon trillions of cells need to be fed, watered, warmed and protected so they can carry on their many functions in safety. Perhaps the most important single service performed by soil organisms (particularly bacteria) is to supply nitrogen in a form that can be used by higher plants and eventually by man. They do so in two ways: (1) by direct fixing of nitrogen from the air, and (2) by releasing nitrogen locked up in organic matter.
DIRECT NITROGEN FIXATION
There is no general agreement among microbiologists as to the extent of direct fixation of nitrogen by soil bacteria. Most authorities agree that Azotobacters (there are several species) carry on a process known as non-symbiotic fixation (to distinguish it from nitrogen fixed by bacteria on the roots of clovers and other legumes). Non-symbiotic fixation is aerobic; that is, Azotobacters work only in the presence of oxygen. Another direct-fixation type, Clostridium pastorianum, is what is known as a facultative anaerobe; that is, it can work either in the presence or absence of oxygen. This last form is thought to take its nitrogen from ammonia in soil gases, thus preventing the ammonia from escaping into the air.
These direct-fixation bacteria work best when they have access to plenty of calcium, carbon dioxide and glucose. At a pH of 5.5 or below, nitrogen fixation practically comes to a standstill. Potassium chloride (muriate of potash) is particularly harmful and stops all action.