72 THE SOIL POPULATION AND ITS DISTRIBUTION 



Lower Invertebrates in Soil. — The lower invertebrates 

 occur in greatest abundance in light soils, other conditions being 

 alike. As with the other soil organisms, environmental conditions 

 greatly affect their prevalence; probably these effects are even 

 more marked than with those forms which do not have the ability 

 to migrate appreciable distances. The abundance of organic 

 matter is a particularly important factor. In unmanured field 

 soil cultivated to wheat, Morris found 4,885,400 of the larger 

 invertebrate animals per acre. In the manured soil there were 

 14,795,000 individuals. However, these figures represent only a 

 very small fraction of the total numbers of invertebrate animals 

 present in the soil. The nematodes alone have been found in hun- 

 dreds of millions of cells per acre of land (Table 16). 



TABLE 16 

 Abundance of Nematodes in Soils (from Cobb and Steiner) 



The effects which these small animals exert are probably more 

 physical than chemical. They aid in the decomposition of organic 

 substances to some extent, but it is doubtful that they play roles 

 which could be compared with the bacteria and other related 

 microorganisms. 



The Activity of the Soil Population as a Whole. — Many 

 methods have been used to determine the response of the total soil 

 population to changes in its environment. Of these methods the 

 determination of the amounts of carbon dioxide formed in a certain 

 unit of time has proved to be most valuable. The carbon dioxide 

 of soils originates almost entirely through biological agencies. 

 Practically all of the soil microbes produce carbon dioxide as one 

 of the principal waste products of their development, and the 

 amounts are closely proportional to the degree of their activity. 



