Influence of soil nitrogen fertilisers on soil acidification in Papua New Guinea

Soil acidification is a worldwide concern with many cropping systems where nitrogen fertilisers are used. Inorganic N fertilisers are generally used to improve and maintain high yields especially with oil palm, however little is known on their effects on soil acidification in tree crops especially oil palm cropping systems. This study looked at the effects of different sources of N fertilisers on soil pH and suggests strategies to sustainably grow oil palm in Papua New Guinea. Soil samples were collected in a grid designed from a nitrogen fertiliser trial that has been receiving five different N fertiliser types for more than eight years. The palms were receiving nitrogen at 0, 420, 840 and 1680 g N/palm/year. Soil pH was measured for composite samples for all the plots and individual grid points for two of the plots that received the highest AMC rates. The effect of N fertiliser types and the rates significantly (p<0.001) reduced soil pH but the effects reduced with depth. At the highest N rate, AMC had the most effect in reducing pH to depths greater than 90 cm while urea had the least. However at the normal N rates, pH was reduced by 0.2 – 0.4 units. pH was highly variable between 3.86 and 6.87 at different grid points and was significantly different (p