LEGUMES AND SOIL FERTILITY 



177 



The results for this comparison have been obtained from Illi- 

 nois and Delaware experiments and are tabulated below: 



Legume 



Dry Matter per 



Nitrogen per 



Per cent of 



Total 



Acre {lbs.) 



Acre {lbs,) 



Nitrogen in 



Tops 



Sweet Clover 











Tops 



9,029.0 



174.0 



76.0 





Roots and residues 



3,784.0 



54.0 





Crimson Clover 











Tops 



4,512.0 



103.0 



70.0 





Roots 



2,022.0 



41.0 





Alfalfa 











Tops 



2,267.0 



54.8 



60 





Roots 



1,980.0 



40.4 







In the clover three-fourths of the total nitrogen is found in the 

 plant above ground and only one-fourth in the roots, whereas 

 alfalfa shows a greater proportion in the roots — ^40 per cent. 

 This represents the proportion for the first-year growth for 

 alfalfa, and it is not likely that in the older plant this proportion 

 of the total nitrogen would be maintained in the roots. Hence, it 

 is quite certain that if only two-thirds of the total nitrogen of the 

 plant is obtained from the air the quantity returned to the soil 

 with the roots and plant residues does not exceed that removed 

 from the soil by the growing plant. This would give no increase 

 in soil nitrogen from the growing of a legume where the entire 

 crop is removed even though the roots are allowed to remain and 

 decay. Yet we find farmers who remove the crop and later the 

 roots as a nuisance from the soil and even then expect an increase 

 in their soil nitrogen. 



Legumes Feed on Nitrates. — It is, therefore, rather certain 

 that the legumes, where the crop is harvested, does not increase 

 the soil nitrogen of the fertile soil of Illinois and other soil fairly 

 rich in nitrogen. But what will happen on the arid and semi-arid 

 soil where nitrogen in many cases is the limiting element of crop 

 production and is present in much smaller quantities than it is in 

 the soils on which the experiments considered were conducted. 



