ORGANIC MATTER AND NITROGEN 217 



untreated land, and 64 per cent on land treated with phosphorus 

 and potassium, the average annual yields of nitrogen per acre 

 being 29.1 pounds for oats, 25.9 for wheat, and 50.1 for hay, on 

 untreated land, and 36.15 pounds for oats, 35.5 for wheat, and 75.5 

 pounds for hay, on treated land. While the calculation of 65 per 

 cent is probably near the truth for the treated land, where the 

 nitrogen is likely to be the limiting element in crop production, 

 the marked reduction in yield of nitrogen between the oats and the 

 wheat crops on the untreated land is probably not a true index of 

 the change in available soil nitrogen, because on these plots phos- 

 phorus is certainly the limiting element for wheat, as will be seen 

 from later discussion. 



In any case, we are safe in concluding that soil which will fur- 

 nish from 26 to 36 pounds of available nitrogen for a crop of oats 

 or wheat will also furnish approximately as much for the hay crops, 

 whether timothy or clover. 



Clover and other legumes take available nitrogen from the soil 

 in preference to the fixation of free nitrogen from the air, the latter 

 being drawn upon only to supplement the soil's supply and thus 

 balance the plant-food ration. In other words, the legumes have no 

 nitrogen limit in yielding power when properly infected, but with 

 abundance of available soil nitrogen constantly provided to fully 

 balance other essential elements or factors, there is little or no 

 development of root tubercles, and little or no fixation of free nitro- 

 gen occurs. 



From the experimental data here presented or referred to, and 

 from many other calculations approximating exactness, the con- 

 clusion may be drawn that on normally productive soils at least 

 one third of the nitrogen contained in legume plants is taken from 

 the soil, not more than two thirds being secured from the air. 

 This proportion would apply to the nitrogen content of the roots 

 as well as to the tops; so that, if one third of the nitrogen of the 

 entire plant is in the roots and stubble, and two thirds in the crop 

 harvested, the soil would neither gain nor lose in nitrogen because 

 of the legume crop having been grown, the soil having furnished 

 as much nitrogen to the plant as remains in the roots and stubble. 



When grown on richer soils, such legume crops leave the soil 

 poorer in nitrogen; but on poorer soils, furnishing less than the 



