Soil variation on steep greywacke slopes near reefton, western south island

Abstract

In a steeply sloping area of 335 ha underlain by greywacke and minor argillite, and covered by mixed podocarp/beech forest, forty‐one soil profiles were examined in order to investigate soil variability. Observations were made every 100 m along fixed traverse lines set at right angles to main water courses between 320 m and 620 m a.s.l. More than 2 m thicknesses of fossil scree or greywacke colluvium mantle most of the lower two‐thirds of the 32°‐38° slopes; however, rock is closer to the surface beneath spurs, and there are some rock outcrops forming bluffs in midslope positions. Rock underlies most ridge crests and upper slopes, which average 22°. Hard beech with some rata dominates on upper slopes. Red beech is increasingly dominant downslope, with the largest trees (up to 30 m high) on the lowest‐altitude slopes. At soil order level, Brown Soils comprise 57% of the profiles described; other proportions are Recent Soils, 20%, Organic Soils, 14%, and Podzols, 9%. Podzols and certain subgroups of the Brown Soils indicate soil stability for about 60% of the soils and, by implication, sites described. These soils are found mainly in mid‐ and lower slope positions. Tree overturn and hard rock near the surface are considered the main factors influencing the maintenance of Recent Soils and some Brown Soils. The subdivision of slopes into ridges, stable intermediate steep slopes, eroded slopes and accumulation slopes was not practicable in this study area. An alternative model, dividing the area into ridgetops and a complex of colluvial and bedrock steeplands, would better encompass the observed soil variability. Comparisons with soil studies in the two other known areas where climate and other environmental factors are similar, namely the Marlborough Sounds and Mount Harata, show a similar pattern of soil development on steep slopes. © 1994 Journal of the Royal Society of New Zealand.