218 MODIFICATION OF THE SOIL POPULATION 



a definite change in reaction is produced. Since this buffering 

 capacity resides principally in the colloidal material, the finer- 

 textured soils require more neutralizing agent to cause a certain 

 change. Consequently, one would not expect all soils to react 

 equally to the same applications of lime, even though the reactions 

 of both were alike previous to the treatment. The effects of lime 

 are further complicated by the fact that soils differ so greatly in 

 reaction. Acid soils will generally respond to lime, while soils 

 which are alkaline in reaction may show no appreciable change in 

 biological activities or plant growth following the same treatment. 

 Lime causes its effects principally through its influences on the soil 

 reaction, but it may produce other changes in the soil micro- 

 population, both directly and indirectly. Lime is particularly 

 effective in improving the physical structure of the soil, by causing 

 flocculation of the colloidal particles with the development of 

 granular structure. Plant growth may be favored by the creation 

 of a more suitable soil reaction and physical condition. As a 

 result of this enhanced growth, the plant residues furnish the soil 

 organisms with larger amounts of food material. 



Thus, the influences of lime on soils cannot be analyzed in 

 simple terms. The effects are numerous and diverse, and may be 

 exerted upon plants and microorganisms both directly and indi- 

 rectly over a period of years. The improved plant growth follow- 

 ing liming is a suggestion of the extent to which the concealed 

 microbes are affected. The accelerated activity of the soil popula- 

 tion contributes to the improvement in plant development, and, 

 over an extended period of time, one of the factors which causes 

 the pronounced increases in the abundance of soil organisms is the 

 modified plant growth itself. Referring again to Table 11, one 

 can readily observe that liming practices may modify the distribu- 

 tion of soil organisms to a marked degree. The fertilizer and lime 

 treatments created marked differences in fertility of these soils, 

 and the microbial population changed with the growth of plants. 

 When such soils are compared with respect to the nature and 

 abundance of their soil population, it is found that, at reactions 

 below pH. 6.0, the more acid the soil the less favorable it is to the 

 development of bacteria and actinomyces and the more favorable 

 it is to the growth of fungi, provided that other conditions are the 

 same. Table 53 indicates that organic materials added to soils 

 of different fertility also lead to different responses in the develop- 



