Soil–landscape relationships of downlands soils formed from loess, eastern South Island, New Zealand

Abstract

The Timaru and Claremont soil series occupy >145 000 ha of loess-mantled downlands of the South Island. The soils are mapped in large map delineations and the accompanying soil reports provide very generalised descriptions and definitions of the units. Almost no information is recorded on the nature and causes of soil variability. In this study soil variability is described along transects at 12 locations. The sites selected encompass a range of aspect (sunny and shady), topography (easy rolling to strongly rolling terrain), and land use (non-ploughed, pastoral, and mixed cropping).

The definitions of the Timaru and Claremont series are refined and the variability in soil morphology is related to landscape features. Morphological data from the transects are collated into generalised soil–landscape models. Similar arrays of soils occur within the Timaru and Claremont soil–landscape models. The models vary mainly in the occurrence and depth to reducing conditions. The soil–landscape models provide a framework within which to derive soil attribute values for use in predictive models and land-use interpretations, and provide a baseline from which future studies may assess the impacts of land-use practices.

In both soil series, the thickness of topsoils, depth to reducing conditions, and depth to fragipans are greatest on footslopes and generally decrease to shoulderslopes. Penetration resistance is lowest on footslopes and increases to shoulderslopes. In cultivated land, footslope sites have markedly over-thickened topsoils. Relocation of topsoil material from upper to lower slopes is attributed mainly to the effects of cultivation, either directly, through mechanical movement of soil material during cultivation operations, or indirectly, through the promotion of soil erosion. Variations in depth of soil materials are expected to have attendant affects on potential productivity.