SUMMARYA combination of mathematical analysis and computer simulation, using parameters readily measured in a nitrogen‐15 field experiment, is employed to determine rates of mineralization, immobilization and nitrification under a growing crop. The procedure also yields the proportion of crop nitrogen uptake occurring as ammonium and nitrate.When applied to ‐results from grass lysimeters receiving 250 or 900 kg N ha–1a–1as ammonium nitrate, the analysis suggested that at 250 kgN ha–1a–164–66% of crop nitrogen uptake was as ammonium; at 900 kg N ha–1a–1the figure was 43–49%. Nitrification at 250kgNha–1was only 13–19kgN ha–1over 160d while at 900 kg N ha–1between 191 and 232 kg N ha–1were nitrified.The results suggested that the apparent inhibition of nitrification in grassland soils may simply reflect poor substrate competition by nitrifying bacteria. Finally, there was a suggestion that mineralization/immobilization was lowe