Morphological characteristics and interpretation of some polygenetic soils in loess in southern South Island, New Zealand

Abstract

Abstract Soils with morphological features resulting from more than one cycle of soil formation occur widely in the loess terrains of New Zealand. Fragiochrepts (Fragic Pallic Soils) developed in loess on seasonally dry terraces and downlands in the southern South Island have long been considered to be polygenetic in that the fragipan is a relict feature. It follows, therefore, that at least some of the Dystrochrepts (Firm Brown Soils) developed in loess in more humid adjacent regions may also be polygenetic, as the two great groups form a continuum. Field evidence shows that some Dystrochrepts in the humid areas have fragipan-like features in advanced stages of destruction suggesting polygenesis and implying climatic change. Other features that suggest polygenesis include pedosphere stripping whereby the solum of a soil is removed by erosion down to the upper surface of a consolidated layer, such as a fragipan, prior to the deposition of a subsequent loess layer. Several Fragiochrepts in the region have a very sharp upper surface to the fragipan; the morphological features of the horizon itself are in marked contrast to those of the overlying solum; and polygonal fissure patterns in the respective horizons do not match vertically. Offset fissure patterns have also been observed in some Dystrochrepts, though soil development tends to obscure any morphological breaks. Dystrochrepts developed in loess over gravels show generally similar features: the gravels are compacted; are in a position equivalent to that of a fragipan; have a stone line or lag at the surface indicative of deflation; and overlying loess forms a solum comparable in thickness to sola in Fragiochrepts (ca. 50 cm) in the region. These three similar features suggest varying climate as well as polygenesis.