WORMS AND INSECTS IN THE SOIL 



39 



living bacteria and using them as food. A nutrient solution 

 inoculated with some fresh soil first shows an extensive develop- 

 ment of bacteria; this is soon followed by the appearance of 

 numerous protozoa, which use the bacteria as food. 



This succession of microscopic forms 

 need not indicate that all the protozoa 

 depend for their nutrition upon the 

 bacterial cells and that they appreciably 

 limit bacterial development. It simply 

 points further to the complex inter- 

 relationships of the soil organisms. It 

 gave rise to a theory which tended to 

 explain soil fertility on the basis of the 

 interrelationships of the protozoa and 

 the bacteria. The protozoa feeding 

 upon bacteria were supposed to limit 

 soil fertility, since the bacteria were 

 looked upon as the sole organisms bring- 

 ing about the important processes of 

 decomposition of soil organic matter. 

 However, this theory has not found 

 confirmation on further study and investigation. 



Worms and Insects in the Soil. — The higher animal popu- 

 lation is represented in the soil by numerous worms and insect 

 larvae which inhabit the soil permanently and take an active part 

 in soil processes. Some live in the soil only for certain periods of 

 their life. Some of these groups are very abundant in most 

 soils; others occur only in certain soils under a given set of con- 

 ditions. Among the various worms, it is sufficient to mention 

 nematodes and earthworms. The nematodes (Figs. 34 and 35) 

 alone are found in billions per acre of soil. The insects may find 

 in the soil a temporary habitat, by passing there a certain 

 part of their life cycle; some of them spend in the soil the 

 major part of their life, only coming to the surface occasionally. 

 All these members of the animal population are very active in 

 bringing about an intimate mixture of the soil and the organic 

 matter, in macerating the organic matter and the fungus mycelium 

 attacking it, and thus assisting in the disintegration of the dead 

 materials. Some of them cause damage to living plants by 

 attacking their roots or certain parts growing above ground. 



Fig. 29. — Soil flagellates 

 (from Fellers and Allison). 



