244 IMPORTANCE OF MICROBES IN SOIL FERTILITY 



of organic matter per acre of soil. A large part of this organic 

 matter is returned to the soil in the form of plant stubble, leaves, 

 branches, green manures, stable manures, and various waste 

 products of plant utiHzation by man, while another part is used as 

 the source of human and animal food. 



Animals, being unable to manufacture their own organic sub- 

 stances from simple inorganic materials, depend upon the plants 

 for the organic matter. The synthesized products resulting from 

 the development of higher plants furnish the sole source of energy 

 which keeps the other living organisms active in the world. The 

 animal, using the energy derived from the disintegration of some 

 of these compounds, changes the composition of the materials 

 manufactured by the plant, and forms, from these, substances 

 which are needed for the synthesis of its own tissues and for its 

 numerous functions. A large part of this animal organic matter 

 returns to the soil sooner or later in the form of animal excreta 

 and other waste materials. Finally, as the animals themselves 

 die, their own bodies, from those of man to the lowest insects or 

 microscopic worms, are returned to the soil. Consequently, 

 whatever is taken from the soil and from the atmosphere is sooner 

 or later returned, but generally in an entirely different form. The 

 plant has originally taken its nutrients from the soil and from the 

 air in the form of simple inorganic salts; these are returned in the 

 form of numerous complex compounds of plant and animal origin. 



Since the amount of carbon dioxide in the atmosphere is quite 

 limited, being not more than three-hundredths of one per cent of 

 the atmospheric gases, and since nitrogen is present in the soil 

 in mere traces in the inorganic forms of nitrates and ammonium 

 salts, it would take but a short time before growth of plants and 

 animals would stop, were it not for the activities of the micro- 

 organisms in the soil. The microbes of the soil must constantly 

 replenish the supply of available nitrogen and carbon for the 

 plants and keep these two most important elements in constant 

 circulation. As soon as this stops, which rarely, if ever happens 

 under natural conditions, the soil is unable to support further life 

 of plants and animals. The elements hydrogen and oxygen are 

 present in such great abundance in available forms in the water 

 and as gases that they rarely become limiting factors in the cycle 

 of plant and animal life. 

 , When fresh organic substances, whether of plant or of animal 



