a certain stage in this life-cycle, small actively motile rounded cells are produced which are able to migrate rapidly through the soil. It is probably in this stage that it reaches the roots of the host legume. The bacteria enter the plant through the unicellular root-hairs. They attach themselves to the root hair near the tip and appear to soften the cell wall at the point of the entrance. Inside the hair they multiply and form a thread-like strand of mucilaginous material in which the bacteria are im bedded. This thread grows down the hair and passes inwards through the cortex of the root, actually penetrating the cells. As it progresses, the cells in its neighbourhood start to divide and the infecting thread of bacteria ramifies through the mass of dividing cells. The infected cells swell in size and the bacteria pass out of the infecting thread and come to lie in the cytoplasm of the host cells. At about this time vascular strands grow out into the cortex so as to surround the mass of infected cells. The presence of these strands is essential to nitrogen fixation within the nodule and it is probable that they are the avenues along which carbohydrates are brought to the bacteria and the products of nitrogen fixation are removed. The bacteria that form nodules on legumes are divisible into physiological varieties each of which can form nodules on only a small group, sometimes a single genus of legumes. These varieties can also be distinguished by sero logical tests. For many years attempts have been made to im prove the growth of legumes by supplying the appropriate variety of the nodule organism either by spreading soil from a field where the said legume has been grown or by treating the seed with a suspension of the bacteria. When a legume has been introduced into a new district the soil of which does not naturally contain the specific variety of nodule organism, considerable benefit may result from such treatment. This is especially the case with lucerne (Medicago sativa; alfalfa) which has recently been intro duced into new areas all over the world where it often will not make satisfactory growth unless supplied with the bacteria.
Bacterial Activity Under Anaerobic Soil Conditions.— The atmosphere existing in the pore spaces in the soil is usually similar to that of the overlying air though somewhat richer in
In normal cultivated soil the conditions are therefore suit able for the growth of aerobic bacteria. It is probable that, even in well aerated soil, conditions of deficient oxygen supply exist locally, for example in the centres of the masses of colloidal mate rial. In waterlogged soils, moreover, the oxygen dissolved in the water is soon used up so that anaerobic conditions prevail. As mentioned above, such processes as cellulose decomposition and nitrogen fixation can be brought about by anaerobic organisms. Ammonia production from proteins also takes place rapidly under such conditions. Nitrate formation is, however, inhibited by a deficiency of free oxygen. Many anaerobes obtain their oxygen by reducing nitrates or nitrites and some by reducing sul phates, such reduction processes being therefore characteristic of waterlogged soils.
The organic acids and
pro duced by bacteria in the soil have an important action in attack ing insoluble phosphates and potassium salts and rendering these available to the crop. In the laboratory and probably in the field,
rock phosphates can also be attacked by the acids produced by the nitrification of ammonium carbonate. There is also an inter esting group of soil bacteria that can oxidise sulphur and sul phides producing sulphuric acid. The solubility of rock phos phates can be increased by composting with sulphur owing to the activity of such bacteria.
In keeping with the complexity of the biochemical changes that they bring about in soil is the enormous variety of morphological groups of micro-organisms that can be isolated from it. Indeed the num ber and variety of these has so far defeated efforts at systematic study and classification. The soil is continually liable to con tamination from above and there is evidence that many of the bacteria that exist in the soil are not active inhabitants but are present in a resting condition. When a given source of energy is added to soil it usually causes an increase, not in the whole bacterial flora but in one or two specific types. It is thus prob able that, in the future, soil bacteriologists will concentrate their attention on those types that multiply in the soil itself when given substances are added thereto. The study of the soil micro organisms is further complicated by the fact that their numbers in a field soil are constantly fluctuating. These fluctuations take place not only from day to day but from hour to hour and their cause is not yet clearly understood, although they are related to similar fluctuations in the protozoan fauna. There is clearly a changing equilibrium between bacteria and active amoebae and probably between other groups of the soil population. In con sidering the effect of changes in the physicochemical environ ment on the soil bacteria, the existence of this state of equilibrium has to be borne in mind. It is probably for this reason that changes in soil temperature and moisture can seldom be corre lated directly with bacterial numbers. Extremes of temperature usually result in an increase in bacterial numbers probably be cause they are less harmful to the bacteria than to the protozoa which normally keep their numbers down. The reaction of the soil has been found to have a marked effect on soil bacteria, both the numbers and the quality of the flora being affected by it.
Certain groups such as the nitrifying bacteria and Azotobacter are very intolerant of acid conditions. The effect of alkali salts on soil bacteria has been studied on account of the importance of these salts in dry districts. There is evidence of antagonistic action between the various ions, two toxic salts being some times less harmful in combination than alone. Since potassium and phosphorus are essential constituents of protoplasm, it is found that potash salts and phosphates usually increase the num bers of bacteria in the soil. Lack of available phosphates are indeed a factor limiting bacterial growth in many soils, and this fact has been utilised as a test for phosphate requirement.
(H. G. T.)