Long-term fertiliser application and fertility of hill soils

Abstract

Application of phosphatic fertiliser to legume/grass pastures, and increased stocking rate, is commonly assumed to increase soil organic matter (estimated from soil carbon) content, to lower soil pH, and to increase soil Olsen P status. Changes in these variables in soils derived from sedimentary parent material were monitored over 16 years, in a longterm grazing experiment on southern North Island hill country. There were 2 fertiliser treatments, principally high (capital applications followed by 375 kg/ha/year) and low (125 kg/ha/year) superphosphate application. There was a net loss of soil C of approximately 200 kg/ha/year in both treatments, although annual C inputs to the soil were greater (by 1000 kg/ha) in the high fertiliser soils. Soil pH increased slightly; and Olsen P status reflected P application, although it was less responsive to P application (13 kg P/ha applied above maintenance for 1 Olsen P unit increase) than has commonly been assumed is the case (4-7 kg P/ha per unit). The most likely reason for net loss of soil C is increased decomposer activity in the soil, and the greater proportion of plant-fixed C ingested by grazing animals as a result of intensification. The lack of a decline in soil pH may have been related to the declining soil organic matter level lowering hydrogen ion-supplying power. The lower-than-expected responsiveness of Olsen P to fertiliser P application was probably because the experimental area had a very poor fertiliser application history, in comparison with previous experiments against which Olsen P was calibrated. Keywords: carbon, fertiliser application, hill country, Olsen P, organic matter, pH, soils