NUMBERS OF MICROORGANISMS 



49 



ported in detail due to the difficulty of their development on the com- 

 mon media. In soils with a moisture content not in excess of the 

 physical optimum, protozoa exist mainly in a nontrophic state. 116 The 

 protozoa become active in the soil whenever there is excessive moisture 

 present for a period of several hours. Such conditions are common, 

 especially in rainy seasons, in poorly drained soils, in spring, etc. The 

 protozoa then excyst and, after a period of active growth, they repro- 

 duce. When conditions become again unfavorable, they either encyst 

 or die. Protozoa are readily found in field ditches, furrows with stand- 

 ing water, etc. Since the protozoa chiefly use bacteria as their food 



20 

 18 I 

 16 |.25 

 14 \. ■<& 

 12 |20| 



10 I I 

 8 Sl5* 



el 



"9 



8 15 22 29 

 October 



5 12 19 26 

 Nnvfml)or 



3 10 17 24 31 7 

 December 



14 21 28 



Jancary 



Fig. 6. Numbers of active flagellates, amoebae and bacteria in the soil, in 

 relation to its moisture content (from Cutler and Crump). 



(perhaps also organic matter and soil extracts, especially in the case of 

 small flagellates), they are more abundant in rich fertile soils than in poor 

 acid soils. Their greatest numbers are concentrated in the top four or 

 six inches of soil, where the bacteria are also at a maximum, while below 

 twelve inches the soil is practically free from protozoa. 117 Isolated 

 species may be found, usually in a cyst condition, even at very low 

 depths. 118 By the use of the method above described, Cutler and 



116 Fellers, C. R., and Allison, F. E. The protozoan fauna of the soils of New 

 Jersey. SoilSci.9: 1-25. 1920. 



117 Crump, L. M. Numbers of protozoa in certain Rothamsted soils. Jour. 

 Agr. Sci. 10: 182-198. 1920. 



118 Sandon, 1927 (p. 329). 



