172 SOIL ORGANISMS IN RELATION TO SOIL FERTILITY 



and animals finally disappear after they die? If this were not so 

 the world long ago would have become choked with dead material. 

 Most of the rubbish of the earth is buried in or is thrown upon the 

 soil, and through decay, it is reduced to the fundamental elements, 

 becoming again the dust of the earth, water and gases. This decay 

 is the work of many kinds of microorganisms, particularly the 

 bacteria and fungi. It is a wise provision of nature that all organic 

 matter can again break down into the elements of which it is com- 

 posed, since through these changes sustenance is provided to pro- 

 long the life on the earth and to make .possible new life. 



Fig. 114. — The important soil bacteria. A, common bacteria causing decompo- 

 sition; B, nitrifying bacteria; C, nitrogen-fixing bacteria; a, free nitrogen-fixing; h, nodule 

 bacteria. 



No Crops Without Decay. — Soils continue to weather^ after 

 they are formed from rocks; and because of weathering, crops are 

 able to secure from the mineral rock particles the mineral elements 

 so necessary for their growth. But crops also require nitrogen — 

 and the nitrogen in the soil is held there, not in the mineral par- 

 ticles, but mainly in complex, insoluble compounds in the form 

 of organic matter. Crops cannot absorb this organic matter any 

 more than they can consume the mineral particles of the soil.; 

 Before plants can secure any nitrogen from this organic matter, 

 and before any crops can be grown successfully at all, the organic 

 matter must first undergo decomposition. Herein myriads of 

 microorganisms do a most important work. , 



Crops Secure Mineral Elements From Two Sources. — At the 



1 Weathering is a broad term meaning the breaking up and decay of 

 material things wrought by natural forces. Decay of soil is largely the result 

 of chemical forces acting independently or through the aid of microorganisms. 



