D. 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 

 effect in producing those changes in structure and 

 composition which 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 macro-organ- 

 isms and micro-organisms. 



I. MACRO-ORGANISMS OF THE SOIL 



Of the macro-organisms in the soil the animal 

 forms belong chiefly to (1) rodents, (2) worms, (3) 

 insects; and the plant forms to (1) the large fungi and 

 (2) plant roots. 



257. Rodents. The burrowing habits of rodents, 

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

 dog are familiar examples, result in the pulverization 

 and transfer of very considerable quantities of soil. 

 While their activities are often not favorable to agri- 

 culture, the effect upon the character of the soil is 

 quite beneficial, and analogous to that of good tillage. 

 Their burrows also serve to aerate and drain the soil, 

 and in permanent pastures and meadows are of much 

 value in this way. 



(388) 



