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 „ About 65-70 per cent. of the dry matter of bacteria consists of protein. (2) _Sources of Carbon._ The biggest constituent of the dry matter of bacteria is therefore carbon. In the soil, bacteria find an abundance of organic matter from which they may derive their carbon supply. A special case, however, is furnished by the nitrifying organisms, certain sulphur oxidising bacteria, and others that derive their carbon from the CO₂ of the soil atmosphere. The sources from which these special groups obtain the necessary energy to accomplish this, we have already considered. (3) _Assimilation of Nitrogen Compounds._ Of chief importance in its consequences are the means adopted by bacteria to obtain their nitrogen supply. There is some reason to believe that soil bacteria do not take up protein and peptones as such, but must first break down these bodies into simpler compounds. When a sufficient amount of easily decomposable organic nitrogen is present in the soil, the ammonifying bacteria use such compounds as sources of energy, and in this case have a nitrogen supply exceeding their requirements.