360 PRINCIPLES OF SOIL MICROBIOLOGY 



some selecting light, sandy soils and others living only in soils saturated 

 with water. The optimum soil habitat is determined by the ratio or 

 balance between the amount of available oxygen and the amount of 

 carbon dioxide which can be endured without injury. 54 Loam soils 

 have a more abundant insect fauna than clay and sandy soils. 51 



Table 19 shows the influence of manuring of soil upon the distribu- 

 tion of the invertebrate fauna; these are not maximum numbers, but 

 are in many cases, especially in the case of nematodes, far too low, 

 as can be seen from the data recorded previously (p. 347). The dis- 

 tribution of the organisms with depth of soil is greatly influenced by 

 manuring and plowing of the soil, the greatest number of insects and 

 most other invertebrates occurring within the upper 3 inches of soil. 



The great majority of free-living soil nematodes are "world wide" 

 or are found everywhere when conditions are favorable for their develop- 

 ment. To what extent this is true of the other members of the inver- 

 tebrate fauna of the soil still remains to be investigated. 



Economic importance of the invertebrate fauna of the soil. The treble 

 role of the invertebrate fauna in the soil may be included under the heads 

 of (1) mechanical effect upon the soil, as discussed above; (2) a relation 

 to the transformation of organic substances in the soil; (3) a relation 

 to the growth of higher plants. The influence of the invertebrate fauna 

 upon the bacteria, fungi, actinomyces and algae of the soil is still a 

 matter of speculation. There is no doubt that the fauna feeds to some 

 extent upon the soil microflora and thus influences its activities. The 

 earlier idea of Pasteur that the distribution of anthrax bacteria in the 

 soil is brought about by worms inhabiting the soil may hold true also 

 for the true soil microflora. 



McColloch 55 suggested that there is a reciprocal relation between 

 the soil and its insect population : these utilize the soil for shelter, pro- 

 tection, as an avenue for travel, and find there their food, moisture 

 and atmosphere. The soil is benefited, on the other hand, by being 

 well mixed by the mechanical separation of particles, improvement 

 of aeration and drainage, and addition of organic matter. The me- 

 chanical effect may be injurious as a result of the fact that the soil may 

 become porous leading to an increase in evaporation and plant injury. 

 Morris found that the nitrogen content of the invertebrate fauna 



64 Adams, 1915 (p. 358). See also Hesse, R. Tiergeographie auf okologischer 

 Grundlage. Jena. 1924. 



65 McColloch, J. W. The role of insects in soil deterioration. Jour. Amer. 

 Soc. Agr., 18: 143-159. 1926. 



