Surface soil changes in base cation concentrations in fertilised hardwood and softwood plantations in Australia

Soils at seven nitrogen (N) and phosphorus (P) fertiliser experiments in Australia were sampled to determine the changes in exchangeable and soil solution cation concentrations resulting from fertiliser application. Nitrogen fertiliser was applied as ammonium sulphate and/or urea at cumulative rates in the range 0–1600 kg N ha^1 and the P fertiliser as single or triple superphosphate at cumulative rates in the range 0–600 kg P ha^-1. High cumulative rates of fertiliser application (greater than 600 kg N ha^-1 and 250 kg P ha^-1) resulted in acidification of the topsoil at five sites, but had no effect on soil pH at the other two sites. Acidification of deeper soil layers was also observed at the one site where sub-soils were sampled. Exchangeable cations were generally lower in the fertilised treatments than the non-fertilised treatments whereas, there was evidence of a short-term increase in base cation concentrations in solution. Exchangeable and soil solution cation concentrations provided contrasting interpretations of site rankings and the effects of N fertilisation. The fate of cations depleted from surface soils probably included increased uptake by fertilised trees, as increased growth rates have been reported for a number of the experimental sites, and there was some evidence of increased base cation leaching in the fertilised treatments.