Comparison of some soil properties under Pinus radiata and improved pasture

Some properties of surface mineral soils under Pinus radiata were compared with those under adjacent pasture at ten farm-forestry sites in the Manawatu, New Zealand. None of the sites had received lime in the last 10 years. Generally, the soil samples under P. radiata had lower pH and higher extractable aluminium concentrations than their counterparts under pasture. Exchangeable calcium values were lower under P. radiata, by 96-1275 kg/ha. Tree uptake and forest floor development can account for up to 550 kg/ha; at six sites the difference was less than 550 kg/ha, suggesting that calcium generally was conserved by the P. radiata ecosystem. Soil exchangeable sodium and magnesium values were usually greater under P. radiata than under pasture; this probably resulted from the interception of airborne sea-salt by the P. radiata canopy and subsequent transfer to the soil. There were no general trends in the data for available phosphorus and exchangeable potassium. Total nitrogen was often lower in the samples under P. radiata, and the C/N ratio was generally greater under P. radiata. Loss of soil nitrogen may arise from elimination of legumes, accumulation by the trees, and possibly by leaching. Mineralisation of nitrogen, together with production of organic acids and uptake of excess cations over anions, may be possible causes of soil acidification.