CHAPTER XX 



SOIL ORGANISMS, 1 CARBON, SULFUR, AND 

 MINERAL CYCLES 



A vast number of organisms, both vegetable and animal, 

 live in the upper layers of the soil and determine to a very 

 large degree its dynamic character. 2 By far the greater por- 

 tion of these organisms belong to plant life, producing those 

 changes, both organic and inorganic, which control, in large 

 degree, the productivity of the soil. While most of the or- 

 ganisms are so minute as to be seen, if visible at all, only by 

 the aid of a microscope, a small proportion attain the size of 

 the larger rodents. For convenience of discussion the life of 

 the soil may be classified into macro-organisms and micro- 

 organisms. 



209. Macro-organisms — animal forms. — Of the macro- 

 organisms in the soil, the animal types are chiefly (1) rodents, 

 (2) worms, and (3) insects; and the plant forms (1) the 

 large fungi and algae, and (2) roots. 



The burrowing habits of rodents — of which the ground 

 squirrel, the mole, the gopher, and the prairie dog are familiar 

 examples — result in the pulverization of considerable quanti- 



1 General references : 



Lipman, J. G., Bacteria in Eelation to Country Life; New York, 

 1908. 

 Conn, H. W., Agricultural Bacteriology; Philadelphia, 1918. 

 Marshall, C. E., Microbiology ; Philadelphia, 1917. 



2 It has been estimated that every acre of soil contains at least 2000 

 pounds of living material exclusive of roots. If these organisms were 

 confined to a surface foot of soil, weighing, when moist, 4,000,000 pounds 

 to the acre foot, they would make up .05 per cent, by weight of the nor- 

 mal field soil. 



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