Investigating the relationships between soil acidity and phosphorus fractions in high country farmland of New Zealand

Soil fertility remains a limitation to pasture production in the South Island high country of New Zealand, despite a strong history of aerial superphosphate fertiliser applications. This may in part be due to the acidic nature of these soils. This study of extensively farmed high country soils (n = 19) aimed to investigate the current status of soil fertility, and to quantify the effects soil acidity and exchangeable aluminium (Al) have on the accumulation of phosphorus (P) in these soils. To a depth of 7.5 cm, mean soil pHH2O across these soils was 5.2 ± 0.1, and exchangeable Al concentrations increased exponentially as pH decreased (R2 = 0.87), exceeding 3 mg kg–1 at pHH2O ≤ 5.1. Sulfur (S) concentrations were highly deficient for pasture production in these high country soils, with sulfate S (SO4-S) and organic S (Org-S) only exceeding 10 mg kg–1 at 6 and 3 out of the 19 sites, respectively. Total soil P concentrations, measured by Hedley fractionation, ranged from 587 to 1570 mg P kg–1, but only 7.2 ± 0.5% of this P was plant-available P (20.1–152 mg P kg–1). The greatest concentrations of P that these soils contained were in the moderately-labile organic P fraction (176–730 mg P kg–1), which may have the potential to become available due to organic matter mineralisation following liming. However, using Pearson’s correlations and principal component analysis, no strong correlations between the quantities or proportions of P contained in each P fraction were found due to the acidity or concentrations of exchangeable Al in these soils. This suggests that liming these soils is unlikely to result in substantial increases in plant P availability from the mineralisation of P in non-labile soil P fractions.