172 FORAGE CROPS 
tions 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 grown 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 
proportion of nitrogen gathered from the air, 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 
