44 



Agricultural Chemistry. 



soil must contain available nitrogen, and this must have been de- 

 rived originally from the air. Small quantities of combined 

 nitrogen, as stated in a previous chapter, are carried into the 

 ground by the rain water and though small in amount, are prob- 

 ably sufficient to enable plant growth to begin. Bacteriologists 

 believe that certain species of bacteria, which can live on mineral 

 food alone and derive all their nitrogen supply from the air were 

 the first agencies and are still important factors in accumulating 

 the nitrogen supply of the soil. Certain simple forms of plant 





Diagram illustrating the formation of a soil on a limestone hill (after 



Vivian). 



life, as lichens and mosses, it is believed, can also derive their nit- 

 rogen from the atmosphere. When, after death, a plant becomes a 

 part of the soil, all the plant food it contained is returned. Food, 

 once used by plants, is readily made available to succeeding crops 

 through processes of decay and nitrification. The soil is thus 

 made richer and more fertile. In this way growth gradually 

 becomes more abundant. The plants upon decay give rise to 

 "humus," the chief nitrogen containing body of the soil and 

 from which the higher plants, through ammonification and nit- 

 rification, derive their necessary supply of nitrogen. 



Legumes enrich soil with nitrogen. This particular class of 

 plants to which the clovers, alfalfas, vetches, lupines, peas, and 



