Isotopic nitrogen (Nl5) was used in greenhouse pot expts. with oats to measure recovery of fertilizer N as affected by rate of appln. and treatment of the soil with crop residues. Plant yield and N uptake were lower in soils to which corn fodder had been added than in soils to which alfalfa or no plant residue had been applied. Fertilizer N in the cropped soils had essentially disappeared from mineral form, and plant uptake of the fertilizer constituent had almost ceased by the time the oats had reached the boot stage. Plant uptake of N from the fertilizer increased in direct proportion to the rate of appln. Recoveries in the total crop range from 27 to 54% of the N applied. The lower % recoveries were generally associated with the lower rates of N appln. and with the addition of plant residues, especially corn, to the soil. Changes in the mineral N contents of the cropped soils were compared to those of duplicate soils kept fallow during the growing period. In absence of crop withdrawal, residual fertilizer N decreased slowly and at the same time was diluted with N mineralized from organic sources. Under fallow conditions the supply of mineral N was larger at the end of the expt. than initially, except where corn residues had been added. Under cropping, mineral N contents decreased rapidly during the vigorous growth stage, irrespective of soil, treatment, or rate of N addition. The content of soil mineral N increased gradually as the crop matured.