Soil fertility status of the Taranaki hill country

A survey of 35 farms in the eastern Taranaki hill country was conducted to determine fertiliser practices and the fertility status of hill soils using the New Zealand MAF soil test methods. Two north‐facing and two south‐facing slopes on each farm were soil sampled. Most sites sampled (69%) were predominantly volcanic ash soils (average phosphate retention (PR) = 91%), with the remaining 31% being predominantly sedimentary soils (average PR = 45%). The volcanic ash soils had an average Olsen phosphate (Olsen P) value (0–7.5 cm) of 8.3, with only 14% of samples with Olsen P values > 12. The sedimentary soils had an average Olsen P value of 11.4, with 41% of samples with a value > 12. Volcanic ash soils had higher soil sulphate test values (average 16.4) than sedimentary soils (average 8.6), with 76% and 41% of the samples respectively having soil test levels > 10. There was no effect of aspect on soil test levels for either volcanic ash or sedimentary soils. Soil sulphate levels and pH increased, whereas MAF Quick Test cations and Olsen P decreased with sampling depth. Trends in soil test levels were related to fertiliser history only for phosphate. This study suggests that, on Taranaki hill soils of high PR (predominantly volcanic ash), the major soil fertility limitation for maximum pasture production is low soil phosphate level. On the lesser areas of sedimentary soils (low PR), phosphate levels are not low, but sulphate levels are marginal.