CHAPTER XXII. 

 FIXATION OF NITROGEN. 



At the time when life first appeared on the earth the 

 soil, of course, contained no organic matter for such ma- 

 terial is the product of living things, plants and animals. 

 The soil was presumably composed of clay, sand, and 

 gravel, the result of the processes of disintegration of 

 the original rocks of which the earth was composed. 

 During the thousands and thousands of years that have 

 passed, organic matter has been accumulating in the soil 

 because all of the material that the soil produced, vege- 

 table or animal, was returned to it. This material has 

 been decomposed by the bacteria and other forms of life 

 with the production of the various substances that have 

 been previously mentioned. A part of the organic mat- 

 ter has not been completely decomposed, but remains in 

 the soil in the material called humus. On account of its 

 effect on the physical properties of the soil, and because 

 it is a source of nitrogen, humus is one of the most im- 

 portant constituents of the soil. It is not certain that 

 the soil at the time life began to exist on the earth con- 

 tained any nitrogen. The question at once arises as to 

 how the present store of nitrogen found in the soil has 

 been gathered from the air. 



It is evident that soils under natural conditions tend 

 to accumulate nitrogen until some of them contain such 

 large amounts as are found in the soils of our western 

 prairies. Under artificial conditions, as under cultiva- 



