(2) Short non-sporing organisms, related to Pseudomonas fluorescens, that are rapid gelatine liquefiers. These form another 10 per cent. of the numbers.
(3) Short rod forms that liquefy gelatine slowly or not at all, and develop colonies very slowly. These form 40-75 per cent. of the numbers, and may therefore be of considerable importance in the soil.
(4) A few micrococci also occur.
These groups comprise the larger portion of the bacterial flora of the soil, but, in addition to these organisms, that develop on the media commonly used for plating, there are special and important groups that appear only on special media, either owing to their being unable to grow on ordinary media or because they get swamped by other forms. Examples of such groups are the ammonia and nitrite oxidising bacteria, the nitrogen fixing groups, the cellulose decomposing organisms, and the sulphur bacteria.
In order that we may apply the results of the study of a definite organism to other localities, a knowledge of the geographical distribution of the soil bacteria is clearly needed. We have, unfortunately, very little knowledge of the distribution of soil organisms. The common spore-forming groups appear to be universally distributed. Thus Barthel, in a study of the bacterial flora of soils from Greenland and the island of Disko, obtained soil organisms belonging to the groups of Bacillus subtilis, B. amylobacter, B. fluorescens, B. caudatus, and B. Zopfii, which are common groups in European soil, indicating that the general constitution of the bacterial flora of the soil in arctic regions is not widely different from that of Western Europe. Bredemann, who made an extensive study of the Bacillus amylobacter group, obtained soil samples from widely scattered localities, and found these organisms in soil from Germany, Holstein, Norway, Italy, Morocco, Teneriffe, Russia, Japan, China, the East Indies, Samoa, Illinois, Arizona, German East Africa, and the Cameroons. Some soil organisms, on the other hand, are apparently absent from certain districts. This may be due to the conditions, such as climatic environment, being unfavourable to them. A study has recently been made at Rothamsted of the distribution over Great Britain of a group of bacteria that are capable of decomposing phenol and cresol. One of these organisms, apparently related to the acid-fast B. phlœi, has an interesting distribution. It has been found in 50 per cent. of the soils samples examined from the drier region, where the annual rainfall is less than 30 inches, but in only 20 per cent. of the samples in the wetter parts of Britain. Another example of limited distribution is found in the case of Bacillus radicicola, the organism that produces tubercles on the roots of leguminous plants. The distribution of the varieties of this organism follows that of the host plants with which they are associated, so that when a new leguminous crop is introduced into a country, nodules may not appear on the roots unless the soil be specially inoculated with the right variety of organism. In cases where a group of soil organisms is widely distributed over the globe, it may yet be absent from many soils owing to the soil conditions not suiting it. Thus, phenol decomposing bacteria, though abundant in the neighbourhood of Rothamsted, are yet absent from field plots that have been unmanured for a considerable period. The occurrence of the nitrifying organisms and the nitrogen fixing Azotobacter is also very dependent on the soil conditions.
Owing to the method by which our knowledge of soil bacteria has been acquired, by studying first the chemical changes in the soil and then the bacteria that produce them, it is natural for us to divide them into physiological groups according to the chemical changes that they bring about. This grouping is the more reasonable since so little is known as to the true relationships of the different groups of bacteria that a classification based on morphology is well-nigh impossible. In considering the activities of bacteria in the soil, it is convenient to group the changes which they bring about into the two divisions into which they naturally fall in the economy of the organisms.
In the first place, there are the changes that result in a release of energy, which the bacteria utilise for their vital processes.
In the second place, there are the processes by which the bacteria build up the material of their bodies. These building up processes involve an intake of energy for their accomplishment.
It will be convenient to deal first with the release of energy for their own use by bacteria, and its consequences.