The absorption of nitrogen from the air and from added fertilizer was measured in legumes when varying quantities of fertilizer were added. The plants were grown in gravel cultures and supplied with mineral elements in nutrient solution through an automatic irrigating device. The ammonium nitrogen applied to the gravel culture was enriched with N15 to provide a measure of the absorption from the fertilizer and from the air;To relate the nitrogen supply obtained in the nutrient gravel culture to soil conditions, grasses were grown in both a number of typical soils and in the gravel cultures;In the legumes studied, with the exception of birdsfoot trefoil, the lowest rate of nitrogen applied to solution culture resulted in an increase in yield and in the amount fixed. The higher rates of nitrogen addition, with the exception of the case with soybeans, resulted in progressive increases in yield and nitrogen absorption, but decreases in the amount of nitrogen fixed;Under the conditions of the experiment, the fixation mechanisms did not furnish sufficient nitrogen for maximum growth of the legumes. Added nitrogen tended to increase yields and to replace the fixation processes;The influences of available nitrogen on the fixation processes were similar in all of the legumes studied when considered from the stand point of nitrogen applied in relation to the total nitrogen needs of the legumes;Recoveries of added ammonium nitrogen ranged from 76 to 94% in the legume crops, to from 79 to 97% in the grasses;Nitrate and ammonium ions from ammonium nitrate fertilizer were absorbed in about equal proportions by alfalfa, soybeans, and Sudan grass, indicating that similar results in nitrogen uptake by legumes, and inhibition of fixation should be expected had a nitrate fertilizer been employed in place of ammonium;The conclusion is reached that in soils containing from 0.2 to 0.3% of nitrogen, alfalfa and sweet clover normally fix from 60 to 75% of the nitrogen absorbed. For Ladino clover and birdsfoot trefoil, under the above soil conditions, fixation would range between 25 and 60% of the nitrogen absorbed; the lower fixations occurring in the first growing season, and particularly with birdsfoot trefoil.