SOIL VARIATION ON STEEP GREYWACKE SLOPES NEAR REEFTON, WESTERN SOUTH-ISLAND

In a steeply sloping area of 335 ha underlain by greywacke and minor a rgillite, and covered by mixed podocarp/beech forest, forty-one soil p rofiles were examined in order to investigate soil variability. Observ ations were made every 100 m along fixed traverse lines set at right a ngles 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 t he lower two-thirds of the 32-degrees-38-degrees slopes; however, rock is closer to the surface beneath spurs, and there are some rock outcr ops forming bluffs in midslope positions. Rock underlies most ridge cr ests and upper slopes, which average 22-degrees. Hard beech with some rata dominates on upper slopes. Red beech is increasingly dominant dow nslope, with the largest trees (up to 30 m high) on the lowest-altitud e slopes. At soil order level, Brown Soils comprise 57% of the profile s described; other proportions are Recent Soils, 20%, Organic Soils, 1 4%, 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 slop e positions. Tree overturn and hard rock near the surface are consider ed the main factors influencing the maintenance of Recent Soils and so me Brown Soils. The subdivision of slopes into ridges, stable intermed iate steep slopes, eroded slopes and accumulation slopes was not pract icable in this study area. An alternative model, dividing the area int o ridgetops and a complex of colluvial and bedrock steeplands, would b etter encompass the observed soil variability. Comparisons with soil s tudies in the two other known areas where climate and other environmen tal factors are similar, namely the Marlborough Sounds and Mount Harat a, show a similar pattern of soil development on steep slopes.