Changes in soil quality following humping/hollowing and flipping of pakihi soils on the West Coast, South Island New Zealand

Abstract

Humping/hollowing and flipping are land development practices widely used on the West Coast to overcome waterlogging constraints to pasture production. However, there is very limited information about how the resulting "new" soils function and how their properties change over time following these extreme modifications. We hypothesised that soil quality will improve in response to organic matter inputs from plants and excreta, which will in turn increase nutrient availability. We tested this hypothesis by quantifying the soil organic matter and nutrient content of soils at different stages of development after modification. We observed improvements in soil quality with increasing time following soil modification under both land development practices. Total soil C and N values were very low following flipping, but over 8 years these values had increased nearly five-fold. Other indicators of organic matter quality such as hot water extractable C (HWC) and anaerobically mineralisable N (AMN) showed similar increases. With large capital applications of superphosphate fertiliser to flipped soils in the first year and regular applications of maintenance fertiliser, Olsen P levels also increased from values <10 ìg/g to values well within the target range (20-30 ìg/g) after about 7 years. Humps and hollows responded differently following modification. The increases in total soil C and N, HWC and AMN levels on humps were similar to those of flipped soils over 8 years of development, whereas soil quality changes in the hollows were much slower. This has important implications for nutrient availability and losses. As soils develop, fertiliser and effluent applications should be adjusted to optimise production while minimising nutrient losses. Keywords: humping and hollowing, flipping, soil quality, soil organic matter, effluent