SUMMARY 19 



carbon dioxide, various organic acids as well as certain inorganic 

 acids (notably nitric and sulfuric) which have a marked solvent 

 action upon rocks and rock constituents and thus assist materially 

 in the weathering processes which lead to soil formation. By 

 assimilating certain chemical elements or compounds, by chang- 

 ing the nature of other chemical compounds, by the processes of 

 decomposition of organic matter, as well as by the synthesis of 

 new organic complexes, microbes play a very prominent part in 

 the constant transformation of the soil. 



The elements which plants require for their nutrition come 

 directly or indirectly from the soil. The microbes play prominent 

 roles in transforming these elements into forms available for 

 plants. Out of these elements the plants synthesize their tissues: 

 the roots, the stems, the branches, the leaves, the grain, and the 

 fruits. Only a part of these plant products are utilized by men 

 and animals for their food. Another, frequently the larger part, 

 is returned to the soil. Even the very bodies of men and animals 

 that live by consuming plant substances will sooner or later also 

 return to the soil, and with them a large part of the elements 

 removed from the soil through the agency of the plants. 

 These are acted upon by the soil microbes, liberating the important 

 elements, carbon, nitrogen, phosphorus, sulfur, and potassium, 

 into circulation, where they are again available for plant growth. 



The soil is an excellent medium for the development of microbes 

 since it contains all the elements essential for their activities. 

 The plant and animal residues offer suitable sources of energy to 

 keep the various microbes in a state of constant activity and repro- 

 duction. The relationships between soils and plants, on the one 

 hand, and the microbes, on the other, are mutual. The soil and 

 plant supply the medium for the growth of the microbes, and the 

 energy and other nutrients for their activities and reproduction; 

 the microbes carry out the processes which keep the elements in 

 constant circulation, thus enabling the plants to develop with a 

 limited supply of nutrients. 



