MINERAL NUTRIENTS AND NITROGEN 



205 



nitrogen which has been, taken from the air by fixation. The 

 great abundance of these organisms in connection with legumi- 

 nous crops makes these crops very important as soil improvers. 

 Leguminous plants growing without these organisms must ob- 

 tain their nitrogen in the same manner as other plants and are 

 of no more value as fertilizers than other plants. 



The use of legumes for this purpose was recognized long be- 



FIG. 104. Nodules on the roots of a legume. 



fore mankind knew anything about bacteria. The fungi which at- 

 tack the organic materials in the soil and reduce them to soluble 

 compounds are very generally referred to as molds. The rela- 

 tive value of these various: organisms is not well understood. 



Other Soil Organisms. There are many other bacterial and 

 fungous organisms in the soil, some of which are beneficial to 

 agriculture, and others which are injurious. Among the latter 



