Acidification of volcanic ash soils under oil palm in Papua New Guinea: effects of fertiliser type and placement

Soil acidification is a widespread degradation problem, but little information is available for oil palm production systems. The aim of this work was to determine the effect of fertiliser type and placement on acidification of volcanic ash soils under oil palm in Papua New Guinea. A field trial, which had various combinations of N (114-120 kg N/(ha.year)) and K (329 kg K/(ha.year)) fertilisers applied over 13 years, was examined. Over that period, pHwater of the 0-0.2m layer declined by 0.38 units in the control treatment and 0.52-0.96 units in the fertilised treatments. Application of fertiliser and plant residues are not uniform in oil palm plantations, so the effect of placement was measured in another trial, which had been operating for 6 years. In the 0-0.05 m layer, the decrease in pHwater due to fertiliser addition (ammonium chloride plus kieserite) was least (0.52 units) when fertiliser was applied in the weeded circle and greatest (1.03 units) when it was applied in the frond pile. Soil pH buffer capacity was different between zones and fertiliser treatments, but the difference in acidification effect of fertiliser between zones was attributed primarily to differences in the water balance and N cycling rather than pH buffer capacity