The uptake by Sudangrass of N applied to two ammonium‐fixing subsoils at rates ranging from 0 to 400 pounds per acre was similar to the uptake from a nonfixing surface soil. N uptake from NaNO3 and banded (NH4)2SO4 was higher than that from (NH4)2SO4 mixed throughout the fixing soils, but the differences could not be attributed entirely to ammonium fixation. Where (NH4)2SO4 was mixed with the soil, A values calculated from the N15 data appeared constant for all N rates. A N‐balance sheet prepared from analyses of soil and plant materials accounted for 93.6% of the N15‐tagged fertilizers as an average of all soils and treatments.