64 THE SOIL POPULATION AND ITS DISTRIBUTION 



the total number of bacteria, as well as the development of certain 

 specific types, will decrease. When an acid soil of humid regions 

 is treated with lime, the numbers of bacteria rapidly increase. 



As a result of this, the idea prevails that bacteria are more 

 favored by an alkaline reaction while fungi are more abundant 

 in acid soils. The results given in Table 11 show the effects 

 upon bacterial development of differences in reaction resulting 

 from differences in treatment of soil. These results bring out 

 further the fact that the reaction is not the sole limiting factor 

 in the development of microorganisms. Plot 5A is slightly more 

 acid in reaction than 9A, but still contains a much larger number 

 of bacteria because it contains more organic matter, as shown by 

 the higher nitrogen content. Plot 9A is more acid than plot 7B, 

 but still it contains a greater number of bacteria, because of the 

 higher organic matter content. 



When a black alkali soil having a pH value of 9.6 to 10.0 is 

 treated with sulfur, sulfuric acid, aluminum sulfate, or other acid- 

 reacting substances, thus making the soil less alkaline, an increase 

 in the number of bacteria takes place, because of the fact that 

 conditions become more favorable for their development. 



Influence of Season of Year. — The greatest numbers of 

 bacteria are usually found in the soil in the spring time and in the 

 fall (Fig. 37). During the winter months the soil becomes so 

 altered chemically and physically by the low temperatures that 

 there is a pronounced acceleration of microbial development when 

 the soil habitat becomes warm again. After this period of 

 unusual activity, the microorganisms decrease in numbers some- 

 what and fluctuate from this lower level, in response to changes in 

 their environment brought about by rainfall, plant development, 

 and cultural treatments. In the fall of the year, the organisms 

 reach a second peak in development which may be explained in 

 part by the incorporation of considerable amounts of organic 

 material accompanying maturity of plants, in part by the higher 

 moisture content resulting from decreased transpiration and evap- 

 oration. During the winter months, when the soil is frozen, the 

 organisms are quite inactive, although a large portion of the 

 microbial population persists in spite of the adverse conditions 

 (Fig. 44). 



Appreciable changes in temperature, in the moisture films 

 about the soil particles, or in the amount of available food materials, 



