Transformation of ammonium and nitrate fertilisers in two soils of low and high nitrification activity

Transformations of (NH 4 ) 2 SO 4 and Ca(NO 3 ) 2 applied to a soiI of low (Wharekohe silt loam) and a soil of high (Waimate North clay loam) nitrification activity were studied using both unlabelled and 15 N‐labelled fertilisers. The problems associated with both techniques are briefly discussed. Total recovery of applied N in soil and plant estimated from 15 N recovery, was similar for both forms offertiliser 84 days after application (about 65%) to Waimate North clay loam, but was superior for (NH 4 ) 2 SO 4 (70%) relative to Ca(NO 3 ) 2 (33%) on Wharekohe silt loam. Isotopic dilution occurred in the A horizon, decreasing the 15 Nenrichment in the mineral N pool and leading to an underestimation of the effect of fertiliser N on N uptake by pasture.