An improved chemical extraction method for predicting the supply of available soil nitrogen

An improved method for predicting the supply of available soil N has been developed, based on the concept of determining simultaneously the mineral N (NO 3 ‐N+NH 4 ‐N) present in the soil at the time of sampling, and the mineralisable N that would become available during the growing season through mineralisation of organic matter. The method entails the determination, by automated colorimetry, of NO 3 ‐N and NH 4 ‐N extracted from the soil by boiling with 1M KCl. The method was evaluated, in comparison with others, using 21 soils. The values for predicted soil N supply from the various methods were correlated with the amounts of N, summated over six harvests, in the herbage of perennial ryegrass grown on the soils in pots kept under uniform conditions. The method based on extraction by boiling with 1M KCl accounted for 80% of the variation between soils in herbage N yield. A second method, based on the determination of NO 3 ‐N+NH 4 ‐N extracted by 0.05M Ba(OH) 2 , was equally as effective but was considered less suitable for routine use.