The study of the microorganic population of the soil has revealed many interesting facts but none more significant nor of more far-reaching importance than the discovery that molds occur in soils and perform various functions which directly or indirectly exert considerable influence on soil fertility.

The occurrence and activities of bacteria in the soil has been the subject of extensive inquiry and much knowledge along this line has been accumulated. These organisms have been found to occur in large numbers in practically all soils and to play a prominent part in the reactions which must necessarily take place in soils in order that plant food shall be made available and crops properly nourished. In short, it has been definitely proven that bacterial activities bear a direct relation to soil fertility and to crop production and that .permanent agriculture is very largely influenced by the presence . and action of these microorganisms.