172 FORAGE CROPS 



tioiis by inoculation alone; the other conditions 

 of growth must also be made favorable. 



The amount of nitrogen gathered 



It does not follow that even when these organ- 

 isms are present and all other conditions are fav- 

 orable, all of the nitrogen in the legume crop has 

 been gathered from the air. It has been shown 

 that the plants preferably take soil -nitrogen rather 

 than air- nitrogen. On good soils containing much 

 available nitrogen, or directly well supplied with 

 this element, the proportionate amount of nitrogen 

 appropriated from the air will be much less than 

 when the crop is grow^n on soils poor in nitrogen, 

 even if inoculation has been made. The amount 

 of nitrogen gathered by a crop, therefore, cannot 

 be exactly determined, although, as just indicated, 

 it is thought that the usefulness of the legumes as 

 a means of acquiring atmospheric nitrogen and 

 adding to the stores in the soil, is greater when 

 they are grown on soils rather poor in this 

 element. 



It has also been clearly demonstrated that the 

 pi'oportion of nitrogen gathered from the aii", par- 

 ticularly on poor soils, even when the proper or- 

 ganisms are present, depends on the supply of the 

 other necessary plant- food ingredients. Soils poor 

 in nitrogen and uncongenial in physical character 



