Dr Mike Beare¹, Dr Denis Curtin¹, Mr Craig Tregurtha¹, Mr Weiwen Qiu¹, Mr Richard Gillespie¹

¹Plant and Food Research, Lincoln, New Zealand

Improved fertiliser management is critical to lifting the economic and environmental sustainability of agricultural production systems. Forecasting fertiliser N requirements depends on predicting the supply of plant-available N from soil and the demand for that N by crops/pastures during their growth. The nitrogen released by mineralisation of soil organic matter can contribute a large (but variable) amount of plant-available N. Accurately predicting the supply of N from mineralisation remains a key limitation to properly forecasting the amount and timing of fertiliser N additions. Predicting the supply of plant-available N under field conditions requires knowledge of the soil’s N mineralization potential (i.e. N released under “optimal” conditions of temperature and soil moisture) as well as capability to predict how much of that N will actually be mineralised under varying environmental conditions (e.g. soil temperature and moisture). Soil type and management history affect the quality and quantity of soil organic matter that determines the amount of potentially mineralisable N (PMN). PMN is best measured using a longer-term aerobic incubation, but the procedure is laborious and time-consuming. A reliable, “laboratory-friendly” test for soil N mineralization potential is not available; this remains a major barrier to implementation of best management practices for N on farm. This paper will describes recent advances in measuring PMN based on analysis of hot water extractable organic matter from a wide range of soils and land uses across New Zealand. We also describe preliminary results from field validation trials that shows how PMN may be used to predict the supply of plant available N under field conditions.