A growing interest in microbial population is displayed by plant growers as well as by soil scientists. The importance of microorganisms in the life of plants, as shown by present data, in very great but still little investigated. The interaction of soil microorganisms with higher plants is very complex and multiform. The effect of the former on the latter may be positive or negative. Depending upon the plant cover on the same soil under equal external climatic conditions, the composition of the microflora changes sharply.
Plants are a very strong ecological factor.
As a result of wrong agricultural practices and crop rotation, the soil becomes infested with harmful microbial forms. The influence of soil microorganisms on growth and development of higher plants is of great diversity. The role of microbes is not by any means limited to the mineralization of substances in nature. At present, microorganisms should not be considered as only a link in the circulation of substances, as agents delivering sources of mineral nutrition to plants. The positive role of symbiotic root-nodule bacteria, mycorhizal fungi and others is well known though little investigated.