Multiplication of the Bacteria.—Marshall Ward and Duclaux have made some special observations in regard to the rapidity with which certain bacteria multiply. Duclaux summarizes the final conclusion thus: taking as a basis the time of 35 minutes for the subdivision into two, which has been frequently observed by Ward, there would be four millions of bacteria produced in twelve hours. The first filaments had plenty of room in a drop culture of one cubic millimeter; but at the end their total volume amounted to the tenth part of the total volume of the drop. At the above rate, making 48 generations in 24 hours, 281,500 billions of organisms would be produced. (Grandeau, Ann. Sci. Agron. Vol. 1, 1905, p. 456).
Aerobic and Anaerobic Bacteria.—As may readily be inferred, the cultural and other surface conditions exert a potent influence both upon the kinds and abundance of the bacteria and molds; since the life-functions of some are dependent upon the presence of free oxygen (“aerobic”), while others flourish best, or only, in the absence of air (“anaerobic”), or are able to avail themselves of the presence of combined oxygen, by reduction of oxids present. Their number is found, in general, to be greatest in cultivated lands, and bacteria are there by far predominant over the moulds. On the other hand, the moulds gain precedence in woodlands and meadows, at least so far as air can gain access; while in the deeper layers of the same, as well as in peaty lands, bacterial life is always scanty. This holds particularly in respect to the nitrifying organisms, and others whose life-functions are dependent upon abundant access of oxygen (aerobic).
Food Material Required.—All bacteria, like the fungi, are dependent for their development upon the presence of adequate amounts of some organic food-material, best apparently in water-soluble form. In the soil it seems to be chiefly compounds of the carbohydrate group, especially various gums derived from the decaying plant substance, or from stable manure; in artificial cultures, glucose is mostly found to be a highly available food. When the decaying substance reaches the state of humus, the latter seems to be available as food only to comparatively few bacteria. The very abundant development of bacterial life seems to be among the most important effects produced by stable manure upon the surface soil, in establishing good tilth (“Bodengare” in German).
Functions of the Bacteria.—While there is still much uncertainty as to the exact functions performed by most of these bacteria in respect to soil-formation and plant growth, there are several kinds whose activity has been proved to be of the utmost importance in one or both directions; it having been shown that when the soil is sterilized either by heat or antiseptic agents, certain essential processes are completely suppressed until the soil is re-infected and the conditions of bacterial life restored.
Probably the chief in importance are those connected with the processes of nitrification and denitrification, bearing as they do upon the supply to plants of the most costly of the three substances furnished by fertilizers. These organisms have been first extensively studied by Winogradsky, while the conditions of their activity have been largely developed by R. Warington.
Fig. 15.—Nitrosomonas.
(Winogradsky).
Fig. 16.—Nitrobacterium.
(Winogradsky).
Nitrifying Bacteria.—The conversion of ammonia into nitrates is accomplished under proper conditions by two organisms, or groups of organisms; the first stage being the formation of nitrites by the round, often flagellate cells of nitrosomonas (or nitrosococcus). The second, the oxidation of the nitrites into nitrates by very minute rod-shaped bacilli, named nitrobacteria. The conditions under which these bacteria can act are quite definite in that, aside from a supply of the nitrifiable substance, a fairly high temperature (24° C. or 75° F.) and a moderate degree of moisture, there must be a free access of oxygen (air); and there must be present a base (or its carbonate) with which the acids formed by oxidation can immediately unite. In an acid medium (“sour” soils) nitrification promptly ceases; as it also does whenever the amount of base present has been fully neutralized. The bases most favorable to nitrification are lime and magnesia in the form of carbonates, an excess of which does no harm; while in the case of the carbonates of potash and soda, the amount must be strictly limited.