Bacterial Activity in the Soil

BACTERIAL ACTIVITY IN THE SOIL The organic material produced by green plants by means of photosynthesis provides a source of energy which maintains a dense population of micro-organisms in the soil. The bacteria, fungi and actinomycetes are of fundamental importance in the economy of the soil owing to their action in destroying dead plant remains and in producing those end-products of decomposition, such as nitrates upon which the new crop feeds. Two other necessary functions are performed by soil bacteria, the fixation of atmospheric nitrogen, which balances the washing out of nitro gen compounds from the soil by rain, and the solution of minerals such as potassium compounds and phosphates which are thus rendered available to the crop. Since the soil bacteria have usually been studied in relation to specific biochemical processes which take place, it is convenient to divide them into physiological groups according to the most important changes that they bring about. It must be remembered, however, that most soil organ isms are not specific in their activity, but produce different chemi cal changes according to the nature of the available food supply.

The Decomposition of Plant Residues.

When plant re mains are added to the soil the simpler sugars and starches are amongst the first components to be decomposed since they form a readily available source of energy to a great variety of micro organisms. Pentosans, though not available to so many species are attacked by a number of bacteria and fungi. During the rotting of manure and in compost heaps pentosans play an im portant part in providing a supply of energy enabling bacteria and fungi to assimilate soluble ammonia and nitrate, thereby reducing the loss of valuable nitrogen. The bulk of dead plant remains consists of cellulose and lignocellulose. These substances, especially the latter, are much more resistant to decomposition and disappear more slowly than the simpler carbohydrates. The first cellulose decomposing organisms to be studied were anaerobic spore-forming rods which break down the cellulose into organic acids, CO2, H2 and methane. These organisms are of importance in such waterlogged soils as occur in paddy fields. The majority

of agricultural soils, however, are well supplied with air, and the cellulose is attacked by aerobic organisms,a number of which have been isolated and studied. The most interesting of these is Spiro cizoeta cytophaga, an organism of remarkable appearance and doubtful systematic position. It possesses a life-cycle, the most typical stage being that of a flexible tapering filament staining with difficulty. This passes by intermediate stages into a spherical deeply-staining body resembling a spore, though it is not resistent to heat nor is there evidence that it is a resting condition. In addition to this organism a considerable number of aerobic bac teria claimed to be cellulose decomposers, have been described. The most thoroughly investigated of these, Microspira agar liquif aciens attacks filter paper rapidly and also possesses the unusual property of liquefying agar. Some actinomycetes and fungi are capable of attacking cellulose and it is likely that the latter are mainly operative in destroying the cellulose in acid soils where the environment is unsuited to the bacteria. The rela tive importance of bacteria, actinomycetes and fungi in this, as in other biochemical processes, is difficult to judge at present owing to the lack of an adequate technique for estimating the population of the latter two groups in soil. The cellulose in plant tissues is commonly combined or associated with lignin. The decomposition of lignin by bacteria, actinomycetes and fungi has been claimed to take place, but an exact study of its decomposi tion is hindered by the difficulty of obtaining this substance in a state of purity.

In addition to carbohydrates there are a number of other non nitrogenous organic compounds released in the soil during the decomposition of plant remains. Amongst these are the aromatic compounds, such as phenol and indol, which are derived from proteins. A considerable number of bacteria that can decom pose phenol, cresol and naphthalene occur in soil, while three organisms have been isolated which possess the power of oxidis ing indol into indigotin.