Protozoa 91 



4. Sporozoa. These are parasitic forms, the motility of which is 

 greatly reduced. 



Ciliates are present in the soil largely in an encysted condition 

 and cannot, therefore, function as a factor limiting bacterial activity 

 in the soil, a property often ascribed to protozoa. Smaller amoebae 

 and flagellates were at one time beliexed to play the most important 

 part in the phenomenon of "sick" soils. The limiting factor as 

 regards their activity in the soil is the quantity of water. An ex- 

 tensive protozoan fauna nonnally occurs in the soil in a trophic 

 state; this fauna is most readily demonstrated in moist soil well sup- 

 plied with organic matter, like heavily manured soils, sewage soils, 

 and especialh' greenhouse "sick" 

 soils. The forms predominating 

 in the soil are not necessarily the 

 same as those that develop on 

 artificial media, such as ha>' in- 

 fusions inoculated with soil. ^ ,^ „ ., , t. 777 r 



biG. 45. Sou amoeba, \ ahlkampfia 



Protozoa that are present in the „,,,• (f,„„, j^j^^.tin and Lewin). 



soil in the form of cysts, especially 



after a continuous dry period, will be rapidly transformed into a 

 trophic state by the first rain that brings the moisture content of the 

 soil to optimum. Some protozoa are found in an actixe state even 

 in soils containing a low percentage of water. The flagellate Cer- 

 comoims crassicauda is capable of excysting and reproducing in 

 air-dried soils brought to one-sixth of their water-holding capacity. 

 Various other common protozoa behave normally in soil previously 

 dried and restored to one-half to one-third of its water-holding 

 capacity. 



Some investigators have reported that ciliates and flagellates are 

 more abundant in the soil than are amoebae; others have found 

 amoebae and thecamoebae to be most prevalent. The discrepancy 

 may be due to the difference in methods used, especially in view 

 of the sensitiveness of the amoebae and thecamoebae to the compo- 

 sition of the medium. The largest numbers of protozoa are present 

 in the soil in spring, after the thawing of snow, or in summer, after 

 heavy rainfall; only cysts are found in dry soils. 



The protozoan fauna is largely confined to the top 6 inches of soil. 

 In arid regions, especially in poor sandy soils, protozoa are found in 

 greatest abundance somewhat below the surface. Irrigation of arid 

 sofls stimulates considerably the development of numerous proto- 



