70 



THE SOIL POPULATION AND ITS DISTRIBUTION 



influences upon the activities brought about by microorganisms in 

 a normal soil environment. 



The conditions which favor the development of bacteria also 

 appear to favor the growth of protozoa. The explanation becomes 

 apparent when the fact is considered that bacteria are an impor- 

 tant part of the protozoan diet. In fertile soils rich in organic 

 matter and relatively high in moisture, the protozoa are found in 

 greatest abundance. The small flagellates and amoebae are con- 

 siderably more numerous than the ciliates. They develop best 

 at slightly alkaline reactions, but appear to be as tolerant to acid 

 conditions as most of the soil bacteria. Under unfavorable con- 

 ditions, the soil protozoa go into a dormant state in the form of 

 cysts, which again become active when the environment is favor- 

 able. The active trophic stage of protozoa is probably more com- 

 mon in moist soils. Although they may occur in the soil in much 

 smaller numbers than bacteria, as shown in Table 13, their cells 

 occupy considerably more space, as brought out in Table 14. 

 As with most of the other members of the soil population, the 

 protozoa occur in greatest numbers near the surface. 



TABLE 13 

 Numbers of Protozoa in American Soils (from Sandon) 



Development of Algae in Soil. — Algae occur in greatest 

 numbers in cultivated land and to depths of even 40 to 50 cm. 

 below the surface. As chlorophyll-bearing organisms, they are 

 capable of utilizing the rays of the sun photosynthetically when 

 growing on the surface, but, at lower depths of soil, they function as 

 heterotrophic organisms. Since their distribution below the sur- 



