BIOLOGICAL STATE OF THE SOIL 5 



the chemical elements comprise the framework of the soil and are used 

 to only a limited extent by plant and microbial life. Others form im- 

 portant nutrients (for example, C, N, S, P, H, and O) or serve as cata- 

 lysts for the continuation of life (Zn, Fe, Mn, Cu, Mo, B, and even K 

 are often considered as belonging in this category). The function of 

 most of these elements in the life of microorganisms is not fully under- 

 stood. In view of the fact that some of the elements in the latter group 

 have been found to form important constituents of certain enzyme sys- 

 tems, the difference between the two functions is not significant. 



BIOLOGICAL STATE OF THE SOIL 



The abundance of higher plant and animal life in and upon the sur- 

 face of the soil influences considerably the nature and extent of the 

 microbiological population. Certain plants harbor in their roots specific 

 microorganisms that act as symbiontsj this is true of the root nodule 

 bacteria of leguminous plants and the mycorrhiza-forming fungi found 

 in orchids, evergreens, and many other plants. Higher plants also 

 offer a favorable environment for the growth of certain other types 

 of bacteria and fungi, this specific environment being designated as the 

 rhizosphere. The bacterial population of the rhizosphere is not very 

 different qualitatively from that found some distance away from the 

 plants, except that certain types of bacteria are more prominently repre- 

 sented. 



The growth of plants results in the production of waste materials 

 and residues left in and upon the soil in the form of roots, leaves, 

 needles, and other products, all of which offer favorable nutrients for 

 microbial development. The root systems of plants also bring about bet- 

 ter aeration of the soil, thus making conditions more favorable for the 

 development of aerobic organisms. The presence of higher plants often 

 leads to the development of certain types of bacteria, fungi, and nema- 

 todes that are pathogenic to the plants, such as the causative agents of 

 root rots, damping-off diseases, root-galls, and various others. Some of 

 the pathogens may become well established in the soil and may persist 

 there long after the specific host plants have been removed. They may 

 even be able to attack other hosts. Plant life thus exerts a variety of in- 



