MICROORGANISMS AND SOIL FERTILITY 71 



tility, and upon the rate at which the chemical, physical, 

 and biological factors are preparing it for the green plant. 

 Of these the biological factors are the most important. The 

 various kinds of microorganisms feed upon the organic 

 matter, changing it into simpler compounds, until the vari- 

 ous dements are in a form usable by the green plant. At 

 the beginning of each growing season there is a minimum of 

 available plant food; hence the unavailable reserve must be 

 changed into an available supply. This means that the 

 farmer must grow a crop of microorganisms to prepare the 

 way for the visible crop. The supply of food for his field 

 crop, and therefore the yield of the same, depends in no 

 small degree on the success with which he grows his micro- 

 scopic crop. 



In the newer portions of the world, the farmer pays little 

 at tout ion to any other source of plant food than the soil. 

 In the older agricultural regions, the store of plant food 

 has been partially exhausted. The plant must grow on the 

 material the farmer adds to the soil in the form of organic 

 matter. Under these conditions, the chief interest is in 

 the efficiency of the soil organisms, and in their ability to 

 transform the added potential plant food to the available 

 food supply. 



From this point of view the soil becomes a factory where 

 multitudes of workers are working over the raw material, 

 converting it into the finished food for the green plant. 

 The soil must therefore be favorable for the growth of the 

 microorganisms that are concerned in the cycles of carbon, 

 nitrogen, sulphur, and phosphorus, the four main elements 

 the plant derives from the soil. The lack of any one of 

 these elements in compounds available to the plant will 

 limit the growth of the crop. Nitrogen is most often the 

 element that acts as a limiting factor to the development of 

 the plant ; for, in the final form in which it appears in the 



