TRANSFORMATION OF NITROGEN FERTILISERS AND MOVEMENT OF NUTRIENTS FROM THE SURFACE OF A RHYOLITIC PUMICE FOREST SOIL

The transformations and movement of urea, diammonium phosphate (DAP), ammonium sulphate (AS), ammonium nitrate (AN), calcium ammonium nitrate (CAN), sulphur-coated urea (SCU) and isobutylidene diurea (IBDU) were followed over a 16-week period using intact cores from the surface 10 cm of a rhyolitic pumice forest soil. Fertiliser was applied at equivalent to 200 kg N/ha and the cores were leached with 5 cm of distilled water per week. AS, AN and CAN exhibited similar, rapid leaching rates with over 90 % of the applied N passing through the cores in 10 weeks: 50 % of the N applied as NH4-N passed through in this form while the remainder was nitrified and passed through as N03-N. Urea and DAP showed a slower initial leaching, but this increased as nitrification peaked at 7 weeks. After 16 weeks about 70 % of the N applied in urea and DAP had passed through the cores with about 70 % of this as N03-N. The two slow-release fertilisers, SCU and IBDU showed steady leaching patterns, identical to each other. About 50 % of their N was leached over the 16 week period, nearly all as N03-N. Very little unhydrolysed urea was leached from any of the urea-based sources. The bulk of the residual fertiliser N appeared to be in organic form, but it was readily mineraliseable. All fertilisers greatly increased the leaching rate of Ca, Mg and K, with AS having the greatest and DAP the least effect. The fertilisers had little effect on the movement of P in this soil. The practical implications of the results are discussed