92 Occurrence of Microorganisms in Soil 



zoa. The richer the soil is in organic matter, the richer it is in 

 protozoa, especially in amoebae and thecamoebae. 



The majority of soil protozoa are cosmopolitan, since they are 

 found throughout the world, although not all the species are found 

 in every soil. 



Cutler and associates found six species of protozoa occurring con- 

 stantly in the soil in sufficient numbers to admit the application of 

 statistical methods to the results. These are: (1) Dimastigamoeba 

 gruberi, (2) a small limax amoeba, (3) Heteromita sp. resembling 

 Bodo repens, (4) a small soil flagellate, 3-6 by 2-3 ju; (5) Cercom- 

 onas sp., and (6) Oicomonas termo. 



Sandon found the following average number of species of protozoa 

 in 107 soils examined: 7.2 flagellates, 3.4 ciliates, 2.45 amoebae, and 

 2.0 testaceous rhizopods. Some species grew in all media employed; 

 others developed only in special media. In all, Sandon recorded 250 

 species of protozoa, some of which were observed in every soil, often 

 in very large numbers. The flagellates Heteromita globostis, 

 Oicomonas termo, and Cercomonas sp.; the ciliates Colpoda ciicul- 

 lus and C. steinii; and the limax amoebae Ndegleri gruberi and Hart- 

 manella hijalina were most common and most abundant. Most pro- 

 tozoa found in the soil are also present in various other habitats, 

 such as standing and flowing fresh waters, sea water, and plankton; 

 a few are found only in the soil. The extreme climate of arctic land 

 is not in itself an obstacle to the abundant development of protozoa, 

 provided the soil is well manured and in good condition. 



In general, the soil contains an extensive population of protozoa, 

 consisting largely of amoebae and flagellates, and to a lesser extent of 

 ciliates. These organisms are specifically adapted to a terrestrial 

 form of life. The protozoa, in comparison with other groups of 

 microorganisms, form only a small part of the microbial population 

 of the soil. Theii- ability to reduce the numbers and control the ac- 

 tivities of other groups of microorganisms in soil is very limited. 

 Some protozoa feed only upon certain types of bacteria, others con- 

 sume protozoa, and still others take an active part in the decomposi- 

 tion of plant and animal residues; even by consuming certain specific 

 bacteria, they may favor the process for which these bacteria are 

 responsible. Partial sterilization of soil does not destroy all the 

 protozoa. 



