Soils of the cotter catchment area, A.C.T. : Distribution, chemical and physical properties

Abstract

Studies on soil distribution and classification, together with their chemical and physical properties, were carried out in the Cotter Valley, a major water supply area for Canberra. Vegetative cover is mostly native eucalypt forest at high altitude and plantations of P. radiata at lower elevation; geology is diverse. Based on geologic diversity, five major soil associations were recognized, with significant differences in profile morphology, chemical and physical properties. Yellow earths occur on Silurian volcanic parent material; these soils are shallow and sandy, and calcium is the main exchangeable cation. They are moderately permeable but have low water retention. Gentler slopes on the same parent material are covered by red and yellow podzolic soils, with often dense A2 horizons. Exchangeable calcium is high in A horizons but in subsoils magnesium exceeds calcium. In the A2 and deeper horizons more than 30% of the exchangeable cations is aluminium. While red podzolics are reasonably permeable, yellow podzolics are not, and are likely to suffer surface waterlogging and are erosion prone. Profile water storage of these soils is good. Krasnozems and red earths are dominant soils on metamorphosed sediments. They are of clay loam to light clay texture and are largely base unsaturated, with aluminium the dominant exchangeable cation. These soils are permeable and have reasonable water retention, although krasnozems tend to be overdrained. The yellow podzolic soils on adamellite are coarse textured and have high bulk density. Calcium is the dominant exchangeable cation throughout the soil profile. These soils are reasonably permeable to 0.6 m depth, but water storage is low. The red and yellow earths found on granodiorite are not as dense as those on volcanic parent material, their clay content is higher, and soil profiles are deeper. About 30% of exchangeable cations is aluminium, and exchangeable potassium is rather high. These soils are moderately permeable and have reasonable water storage. All soils of the area are acid, with krasnozems and red earths being the most acid, and all have low CEC and thus low content of the metal cations. Their salt content is low, and exchangeable sodium and hydrogen are very low. The likely effects of forest management practices on these soils and on soilrelated catchment water quality aspects are discussed.