CHAPTER XX 



ORGANISMS IN THE SOIL 



A VAST number of organisms, animal and vegetable, live 

 in the soil. By far the greater part of these belong to 

 plant life, and these comprise the forms of greatest influ- 

 ence in producing the changes in structure and composition 

 that contribute to soil productiveness. Most of the 

 organisms are so minute as to be seen only by the aid of 

 the microscope, while a much smaller proportion range 

 from these to the size of the larger rodents. They may 

 thus be classed as microorganisms and macroorganisms. 

 The latter class will be considered first. 



MACROORGANISMS OF THE SOIL 



Of the macroorganisms in the soil the animal forms 

 belong chiefly to (1) rodents, (2) worms, and (3) insects; 

 and the plant forms to (1) the large fungi and (2) plant 

 roots. 



342. Rodents. — The burrowing habits of rodents — 

 of which the ground squirrel, the mole, the gopher, and the 

 prairie dog are familiar examples — result in the pulveri- 

 zation and transfer of very considerable quantities of soil 

 While the activities of these animals are often not favor- 

 able to agriculture, the effect on the character of the soil 

 is rather beneficial and is analogous to that of good tillage. 

 Their burrows also serve to aerate and drain the soil, and 



421 



