Argillite soil characteristics influencing sustained pastoral production, east coast, North Island, New Zealand

Abstract

Shoulder, mid-, and foot-slope profiles from two representative soil toposequences developed on siliceous mudstone, on steep land under cut-over forest and scrub, were described morphologically and analysed for selected chemical, physical and mineralogical properties, principally to determine the constraints to long-term sustainable pastoral production. The soils are low in both available phosphorus and sulfur, but the chief constraint appears to be high subsoil acidity giving rise to toxic levels of exchangeable Al. This is presumed to restrict root growth and N-fixing ability of the mixed grass/clover pastures, thereby severely limiting production through frequent and prolonged summer droughts. According to the revised N.Z. Soil Classification System, five of the six profiles are Brown Soils, and one shoulder profile is a Recent Soil. Close associations with Pallic Soils were noted in down-profile changes in Ca/Mg ratios and relatively high exchangeable Na levels. Following Soil Taxonomy, five of the profiles were classified as Dystrochrepts and one as a Fragiochrept. These classifications indicate site stability for soil formation under the indigenous vegetation cover. Adjoining steep sites under pasture are prone to erosion, however. Soil analytical results indicate that extensive erosion is not necessarily a prerequisite for problems with pastoral production in the region.