Studies of soil development and distribution in the eastern hill country, central South Island, New Zealand

Abstract

Hill country in eastern South Island has potential for increased primary production. Reconnaissance surveys have delineated the soil resource into broadly defined mapping units. The objectives of this study were to establish models of soil landscape relationships for two such units.

Soil resource surveys may be divided into three phases; research, field survey, and interpretation. A research phase involves the synthesis of soil landscape models. An analytical procedure applicable to hill country is outlined and followed in this study.

In the eastern hill country of the central South Island, the bedrock regolith is predominantly formed in fractured, weakly to moderately weathered, indurated, sandstones and interbedded siltstones. The two areas studied in detail, Coopers Creek Research Area (Hurunui steepland soils) and Mt Pleasant, Hunua (Haldon steepland soils) have temperate climates with annual rainfalls of 1400mm and 700mm respectively, and semi-improved grassland vegetation. Soils are developed in stratified coarse stony colluvium, with little evidence of soil periodicity. Soil catenary relationships within first order valley basins, and soil development on south and north aspects were evaluated from laboratory analyses of 36 soil profiles and supporting field studies.

The results obtained are summarised below:

The first order valley basin was a suitable apriori model. Properties of the debris mantle regolith were strongly influenced by the lithology, stratigraphy and structure of the bedrock regolith. The debris mantle was shallower (0-50cm) on noses and thicker (1-3m) in hollows. Where outcrops of bedrock were less numerous, a sedimentological differentiation between nose and hollow was recognised with the latter having a higher volume of stones and gravels. The spatial variations in thickness, and in the skeletal and fine earth texture profiles characteristic of the debris mantle, determined the form and distribution of soil profile classes.

The key criteria for these classes were; soil horizon sequence, solum depth, and skeletal and fine earth texture profiles. Of the 35 classes recognised, 27 occurred in the two detailed study areas. The distribution of soil profile classes was different on noses and similar in hollows, at both Coopers Creek and Mt pleasant.

The soil pattern was delineated into 7 compound soil mapping units, at a map scale of 1:4000, at the former and into 3 at the latter area. It is suggested that, both Hurunui and Haldon steepland soils are defineable as associations, within which both catena and complex delineated landscape bodies may be differentiated.

Soils, with similar morphology, occurring on different aspects, or on noses or hollows within an aspect, could differ in soil chemical properties and this is illustrated by their taxonomic classification as either Dystrochrepts or Hapludolls.

Catenary differentiation of soil chemical properties, within or between nose and hollow transects, were recognised and differed between aspects at Coopers Creek, but were not evident on either aspect at Mt Pleasant. The above contrasts between the two areas are caused by differences in leaching conditions, on south and north aspects.

Umbric Dystrochrepts occurred in catenary relationship with Aquic Hapludolls and/or Typic Haplumbrepts at Coopers Creek (Hurunui association), while Lithic Dystrochrepts were subdominant on noses and interfluves. At Mt Pleasant (Haldon association) Typic Hapludolls were dominant in hollows and Lithic Hapludolls on noses and interfluves. Family criteria were loamy skeletal, mixed, mesic for all soil taxa.

The relative development of soils, with respect to south and north aspects could be interpreted by combining results of this study with previous hill country studies in the eastern South Island. It is concluded, that the relative stage of leaching and soil development, on south and north aspects, changed from being comparable, to being more advanced on south aspects, to becoming more advanced on north aspects, as annual rainfall increased from 700 to 1400mm.