The Soil Population

Other studies have been made of the microbial population of the soil near the roots and the rhizosphere, with somewhat different objectives. The flora of the rhizosphere is much larger numerically, and more active physiologically, than that of the adjacent soil in the same horizon. The presence of the rhizosphere flora must surely affect the development of the plant; conversely, the nature of the plant influences the character of the microflora. Especially may this be true in the case of perennial plants. The incorporation of organic residues may produce substantial changes in the micropopulation of the general soil mass, but may have little effect on the microflora associated with crop roots.

Studies of the nutrient requirements of organisms isolated from the root region have shown that more types dependent on growth factors or amino-acid nitrogen are to be found therein than in soil away from roots. The view that microbial activity in the rhizosphere is maintained chiefly on sloughed off root cells is inadequate. Direct evidence has been obtained of the excretion from roots of soluble substances that are utilized in the rhizosphere and are responsible for the activity of the flora therein. This may be a mechanism that affects the attack on plants by root pathogens. Resistant and susceptible varieties of a crop may differ in the nature of the root excretions. M. I. Timonin in Canada has studied particularly wilt-susceptible and wilt-resistant varieties of flax in this connection, and has adduced evidence of the excretion of minute quantities of hydrocyanic acid by roots of the latter.

Virgin genetic soil types have characteristic and distinctive microbial Populations that reflect the influence of the factors involved in their development. When these soils are brought under cultivation, modifications may be impressed upon the flora. The changes that result are sometimes great if the environment is much altered by the new land use, as, for example, when poorly drained peat soils are drained, or forest podzols cleared and plowed to a depth that mixes the shallow upper horizons. The responses that follow different management practices subsequently applied may be much smaller and more difficult to detect. As a measure of these, little reliance is placed upon bacterial and fungal counts because it is appreciated that only a small fraction of the population is represented on agar plates, and, moreover, that the slow-growing forms, which probably constitute the basal autochthonous (native) flora, will not be included.

However, quantitative procedures have been much improved by the studies of N. James and M. L. Sutherland, of Winnipeg, Manitoba, who have examined statistically the reproducibility of the results and have made recommendations for modifications that increase the accuracy of plate counts. In field studies in Manitoba they demonstrated an apparent relationship between numbers of bacteria and soil moisture content. Evidence pointing to a similar conclusion had previously been obtained in counts of organisms in grassland soil, and, inasmuch as the moisture films between soil particles are the actual loci of the active bacteria, it is not surprising that such a relationship should be found.

Each horizon in the soil profile develops a characteristic microflora. Organisms from one horizon introduced into another do not necessarily establish themselves in the new environment. Each population may be likened to a team of compatible organisms, the activities of which fit together so precisely that the available energy is utilized most efficiently. A certain measure of population stability comes from the presence of inhibitory or antibiotic substances produced by some of the established microbial inhabitants. Introduced organisms would be likely to maintain themselves only if they accomplish as well or better some step in the sequence of transformations by which energy material is utilized and are unaffected by any inhibitory substances that may be present. This has implications when serious erosion has occurred since the newly exposed surface soil does not have, and may not readily acquire, a micropopulation similar to that of the uneroded surface soil.