DEVELOPMENT OF PROTOZOA IN SOIL 69 



important factors affecting their growth and development in the 

 soil is the abundance of organic food. However, even in soils low 

 in organic substances there may be relatively extensive fungus 

 development; such a condition develops most frequently in 

 quite acid soils. This does not necessarily signify that fungi 

 develop best in soils under extremely acid conditions, for it has 

 been observed that they develop quite well over a wide range of 

 reaction. Since the bacteria and actinomyces may be at a low 

 order of activity in the acid environment, the fungi, which can 

 develop under these conditions as well as in neutral soils, have less 

 competition for the available food supply and consequently appear 

 in greater abundance. 



The results of determinations of the numbers of fungi in variously 

 treated plots, as shown previously in Table 11, are illustrative of 

 the above conclusions. Limed soils contain fewer fungi than un- 

 limed soil, while soils treated with acid fertilizers contain the largest 

 numbers of fungi. In soils more nearly neutral in reaction, there 

 is such a large number of microorganisms which are capable of 

 developing under these conditions, that the factor of competition 

 for food by the cells may keep the fungi from making an extensive 

 development. 



The fungi are favored by a relatively high oxygen content in 

 the air since they are distinctly aerobic organisms. Consequently, 

 we would expect to find fungi developing best under the aerobic 

 conditions close to the soil surface. The fact that there is greater 

 abundance of organic matter in the surface soil is also responsible 

 for the most extensive fungus development being restricted to the 

 superficial layers. 



From the response of the actinomyces and fungi to the soil 

 reaction it becomes apparent that frequently the total abundance 

 of these organisms, and also the portion of the total soil population 

 composed of either fungi or actinomyces, are functions of the soil 

 reaction. However, the responses of these two groups to reaction 

 are opposite, one appearing in greatest abundance in acid soils, 

 the other in more nearly neutral or basic soils. 



Development of Protozoa in Soil. — To a large degree, the 

 biological transformations in soil are brought about by the bac- 

 teria, actinomyces, and fungi. The lower animal life may be of 

 particular importance under certain soil conditions and in connec- 

 tion with some processes, but it would appear that it exerts minor 



