Effect of soil slip erosion on Wairoa hill pasture production and composition

Abstract

Dry matter production, botanical composition, soil fertility parameters, and foliar nutrient concentrations were measured on hillslope pastures in Wairoa in the North Island of New Zealand, for up to 3 years starting in 1982. Measurement sites were on 10 physiographically different landform units (pre-1942, 1942, 1970, and 1977 slip scars, 1970 and 1977 accumulation zones, footslope ridges, spurs, flats, and lower pumice ridges) representing the erosion-sedimentation . continuum of the area. Average annual pasture production on pre-1942 scars was 7.5 t dry matter/ha. Similar yields were recorded from 1942 and 1970 scars, but the 1977 scars produced only 2.6 t dry matter/ha. A model derived from analysis of these results indicated that pasture recovery was exponential — recovery was most rapid in the first 10 years after slipping, and after 30 years there was no further increase in production. The proportions of the grass and clover pasture components increased and decreased, respectively, with increasing length of time since slipping. Soil pH and calcium levels generally declined with scar age and were lowest for spurs, but the reverse trend was generally true for potassium, phosphate retention, and sulphur. Moisture content and phosphate retention were higher on treads than scarps. All other landform units produced more dry matter annually than any slip scar. The results show the necessity of studying landform units separately in any research on hill country pastoral productivity, and confirm many of the findings from previous work in the Wairarapa, particularly those relating to the rate of pasture recovery.