204 MODIFICATION OF THE SOIL POPULATION 



simple and of short duration, they are frequently preferable to 

 vegetation experiments with higher plants to test the fertility of 

 soils. 



Influence of Soil Treatment upon the Nature ANt) 

 Abundance of Microbes. — Each modification of the physical 

 and chemical condition of the soil will be accompanied by a change 

 in the biological soil condition, as expressed by the nature and 

 abundance of microbes in the soil. Some treatments result in 

 profound alterations of the biological activities which persist for 

 considerable periods of time; other treatments exert only slight 

 effects of short duration. Among the important treatments of the 

 soil which might be considered here are the following : 



(1) The addition to the soil of organic matter in the form of 

 stable manures, green manures, artificial manures, plant 

 stubble, or other plant and animal residues, as well as the 

 organic fertilizers of commerce. 



(2) Physical modification of the soil by plowing, cultivation, 

 and similar mechanical processes. 



(3) The addition of different inorganic fertilizers or lime 

 in its various forms — carbonate, oxide, and hydroxide. 



(4) Cropping the soil according to various systems. 



(5) Partial sterilization of soil by heat or antiseptics. 



(6) Air-drying of soil. 



Influence of Organic Matter upon the Soil Microbes. — 

 The addition of organic matter to the soil brings about a number of 

 changes in the soil population, depending upon the nature of the 

 organic matter and upon the conditions in the soil. Green 

 manures bring about a rapid development of various bacteria and 

 fungi which use the water-soluble substances, proteins, cellulose, 

 and the various hemicelluloses. These organisms are soon fol- 

 lowed by protozoa that feed upon the bacteria and by various 

 larvae and worms that feed upon both the residual organic matter 

 and upon the fungi and bacteria. Finally, when the whole organic 

 mass has become largely transformed, other bacteria and actino- 

 myces begin to develop, attacking the more resistant residual 

 substances. The entire mass of decomposing organic matter 

 loses its identity as plant material and is transformed into the 

 finely divided mass which is referred to as soil organic matter or 

 humus. 



