SOIL INOCULATION 71 



from a given area, and with it humus is taken. Because of the 

 erosion, or wearing away by water, the surface layer of black soil 

 is not so deep in a humid region as in regions of less rainfall. In 

 arid regions the decayed organic matter which has accumulated 

 for centuries makes the black layer of surface soil much deeper. 

 The decayed matter will penetrate more deeply into a loose, open 

 soil because of better ventilation. Root growth is deeper and 

 the decay of such roots will form humus at a great depth. These 

 conditions help explain the fact that in the arid regions the black 

 layer of soils may be several feet deep, while it may be only a few 

 inches deep in a more humid region, particularly if the conditions 

 are favorable for erosion. 



Live Organisms in Soil. A good soil contains much life. It 

 may almost be said to be alive with bacteria, molds and other 

 organisms. They are so abundant in kind that specialists fre- 

 quently discover new ones. Their work is only half understood 

 by soil bacteriologists, and farmers are only just beginning to 

 believe in their benefits. They attack the tissues of the dead 

 plants and hasten their decay; the plant-food is thus rendered 

 available to crops. Certain bacteria bring on chemical changes 

 which result in making nitrogen available for plants. Other 

 kinds produce other series of changes. There exists among soil 

 bacteria a specialized division of labor. 



If the sample of soil be baked in a hot oven until well heated 

 through, the bacteria and other organisms will probably all be 

 killed. Such a soil would then be much less productive, and if 

 no bacteria were allowed to enter such a lot of soil, its productivity 

 in the growing of crops would soon be reduced to a minimum. If a 

 little soil containing a number of kinds of bacteria were added 

 the whole mass would again become thoroughly alive with them. 

 Under favorable conditions the process of reproduction of bacteria 

 by division is very rapid. The process of division takes place 

 within a few minutes. Simple calculation will demonstrate the 

 great number which may be produced in a single day from a small 

 beginning. It is estimated that a small handful of good soil may 

 contain as many as a billion or more micro-organisms. 



Soil Inoculation. The process of adding certain desirable 

 bacteria to a soil for any particular purpose is called " soil inocu- 

 lation." It is well known that certain bacteria enable alfalfa to 

 acquire nitrogen from the air. A different kind enters into partner- 

 ship with the roots of red clover. Still other kinds help other 



