A more important source of oxygen under these conditions is nitrate, which can supply oxygen to a larger number of bacteria. The stage to which the reduction can be carried varies according to the organism. A very large number of bacteria are capable of reducing nitrates to nitrites. Many can reduce nitrate to ammonia, and some can produce an evolution of nitrogen gas from nitrate. The effects of nitrate reduction, therefore, appear under water-logged conditions in soils. For example, in swamp soils in which rice is grown, it has been found by Nagaoka,[53] in Japan, that treatment with nitrate of soda depresses the yield, probably owing to the formation of poisonous nitrites by reduction.
Under normal conditions of well aerated soil, however, it is unlikely that the reduction of nitrate is of great importance. In such soils the activities through which bacteria acquire their energy are, as we have seen, of vital importance to the plant, resulting in the disintegration of plant tissues, with the ultimate formation of humus, and in the production of nitrate.
In their activities connected with the building up of their protoplasm, bacteria may, on the other hand, compete with the plant. These activities and their consequences will be reviewed in the following chapter.
CHAPTER III.
SOIL BACTERIA.
C. Activities Connected with the Building-up of Bacterial Protoplasm.
(1) Composition of Bacteria.
The activities of the soil bacteria that we have yet to consider are those connected with the building-up from simpler materials of the protoplasm of the bacterial cell. It is important to bear in mind that this process is one requiring an expenditure of energy on the part of the organism. The sources of energy we have already considered.
The bodies of bacteria contain the same elements common to other living matter. Analyses of various bacteria have been made by a number of workers. About 85 per cent. of their weight is made up of water. This analysis of Pfeiffer’s Bacillus by Cramer[15] shows the typical percentages of carbon, nitrogen, hydrogen, and ash in the dry matter:—
Composition of Pfeiffer’s Bacillus (Cramer).
| C | 50 | per cent. |
| N | 12·3 | „ |
| H | 6·6 | „ |
| Ash | 9·1 | „ |