SOIL FERTILITY 



251 



Obviously the materials applied to the land in the form 

 of commercial fertilizers should be easily soluble. As 

 a rule the ingredients of these fertilizers are of this sort, 

 and one can easily tell from the guaranteed analysis 

 given by the manufacturers to what extent this is true. 



Among the various chemical elements which are needed 

 for the growth of plants, nitrogen is one of the most 

 important. It is commonly furnished in the form of 

 commercial fertilizers as well as in connection with the 

 various organic fertilizers that are applied to the soil. 



Every one knows that a 

 large part of the air is com- 

 posed of nitrogen, and one 

 might think that an ele- 

 ment which is so abundant 

 would be easily available 

 for plant growth. The 

 nitrogen of the air, how- 

 ever, is not in a form which 

 plants can utilize, so that 

 the free-air nitrogen cannot 

 be depended upon for 

 direct help in the growth Modules Containing Nitrogen 

 . On the roots of a leguminous plant. 



01 liowering plants. 



Fortunately, there are certain low forms of germ life 

 which are able to take the free nitrogen of the air and fix 

 it in a condition in which it can be used by the higher 

 plants. This constitutes a sort of partnership between 

 the bacteria and the various members of the great family 

 of legumes — the clovers, vetches, beans, peas, alfalfa, 

 and related crops. These bacteria produce nodules 

 upon the roots of such plants, and in the nodules the 



