78 



WATER AND MINERAL NUTRIENTS 



air is taken into the soil. Dig up a clover, vetch, pea, bean, 

 cowpea, alfalfa or other legume plant. Carefully wash the 

 soil away from the roots. Nodule swellings will probably 

 be found on the roots. (Figs. 

 126, 127.) In these nodules are 

 certain bacteria that secure 

 nitrogen from the air, and from 

 which they build up more 

 complex nitrogenous com- 

 pounds. The legume host-plant 

 then appropriates some of the 

 nitrogen fixed by bacteria and 

 the remainder, of course, re- 

 mains in the bacteria. 



164. Only the leguminous 

 plants bear these nodules. 

 The legumes are plants of the 

 great family Leguminosse, 

 comprising all pea-like, bean- 

 like, clover-like, acacia-like and 

 other pod-bearing plants. It 

 has been demonstrated that 

 over 100 pounds of nitrogen 

 per acre can be fixed by these 

 nodule-forming bacteria dur- 

 ing a growing season. These 

 bacteria are not present in all 

 fields. They must be intro- 

 duced to fields on which 

 legumes have not grown. 

 Moreover, the bacteria that 



127. Nodule, on root of Canada field pea. j^^ the df a j f & wiU not j^^ 



the cowpea. A different "strain" or variety is necessary for 

 almost every legume. So important are the bacteria that 

 the farmer who desires to enrich his soil and secure good 



