Studies on the pedology of five soils above Tertiary sediments in the lower Coal River Valley, South-East Tasmania

A pedological study was made of five soils occupying a small relatively elevated (30-50 metres ASL) area of the University of Tasmania Farm, Cambridge, Tasmania. The soils formed above unconsolidated Tertiary sediments. Examination and sample collection sites were chosen to represent areas of minimal erosion/deposition (UF1 and UF2)and a sequence on a straight uniform toeslope (UF3, UF4 and UF5) exposed to colluvialactivity. Soil morphology was recorded at each site. A major difference in primary profile form was noted as between a uniform, black cracking clay (UF1) and four duplex soils (UF2,UF3, UF4 and UF5) three of which (UF2, UF4 and UF5) were very strongly duplex. Laboratory examination included physical, chemical and mineralogical analysis, viz, particle size analysis, bulk density measurement, soil reaction (pH 1:5 H\\(_2\\)0), electrical conductivity (EC 1:5 H\\(_2\\)0), cation exchange capacity and exchangeable cations (Ca, Mg, K, Na, Al and H), X-ray diffraction of oriented clay samples, micromorphology, and separation of light and heavy mineral fractions and magnetic minerals in selected sand separates. All of the soils were shown to be slightly saline but strongly sodic and magnesic in the subsoil, increasing slightly in salinity but markedly in sodicity and magnesic character with depth with an apparent reversal of this trend deeper in the profile. The major source of salt was arguably atmospheric cyclic salt of marine origin. The black cracking clay soil was shown to have formed by in situ weathering of a granular, base-rich, basalt glass sediment deposited on a Tertiary clay surface, now seen at 150 cm depth in the profile. Weathering and soil formation produced a mixed smectite, kaolinite material (80% clay) to 55 cm depth below which smectite was found to be dominant. The less weathered material below 74 cm depth contained only 25-30% clay above an abrupt lithologic discontinuity to dense, lenticular, Tertiary clay (up to 80% clay) at 150 cm depth. The strong to very strong duplex form of the texture-contrast sodic soils was attributed to in situ weathering of clay-forming minerals and/or inheritance of clay from parent Tertiary sediments. No evidence in the form of depositional argillans was found to indicate an eluvial/illuvial process in the formation of the B2 horizons. The sandy A horizons were considered to have formed in part by weathering and by wind sorting with a strong indication of aeolian sand accession from the sandy foreshore of Pitt Water, a shallow embayment receiving the flow of the Coal River, with extensive sandy flats exposed at low tide. The UF2 soil, located on the broad summit of a gently domed and sloping spur, was considered to have formed within undisturbed Tertiary sediments apart from the effect of aeolian processes on the sandy A horizon. It contained secondary carbonate and manganese oxides in the lower B2 horizon. The UF3, UF4 and UF5 soils, on a straight, 9% - 6% toeslope, were considered to have formed in colluvium of which Tertiary sediments were the dominant source. The parent material of the (uppermost) UF3 soil was strongly stratified with alternating sandy and clayey layers. A striking feature of this soil was the occurrence of a very large concretionary mass of iron oxides beginning at the base of the sandy A horizon and extending into the upper B2 horizon. Also notable was the occurrence of secondary carbonates just below the large glaebular mass of iron oxides. The sandy layers in the stratified sand-clay sequence below were marked by precipitation of iron oxides, disseminated or in small, soft glaebules. These features were considered to be the result of groundwater movement from a ridge of weathered Jurassic dolerite a few hundred metres to the west and directly connected with the study site via a backslope and footslope. The UF4 and UF5 soils contained only small to medium, soft, iron-enriched glaebular segregations and no secondary carbonates were detected. However, the UF5 soil formed in fine sediment deposited above a basal channel deposit of well-rounded (water worn) coarse stones in filled by clay (a clast-supported deposit). Physical and chemical characteristics and the clay mineralogy of the soils provided an explanation of management problems encountered in their use for agriculture. In the case of the black cracking clay (UF 1) these include wide and deep cracking as the soil dries, a dispersed condition of the material within coarse structural units (peds) at shallow depth resulting in high bulk density and unfavourable pore-size distributions both of which were associated with sodicity. Together with the clay mineral composition this has resulted in a very narrow range of moisture content within which soil working and trafficking can be conducted with satisfactory results. The duplex soils have a moderately thick sandy topsoil which has had the effect of reducing the rate of drying of the upper subsoil, with greatly reduced shrinkage and cracking. However, high bulk densities were measured for the sodic, dispersed, clay subsoils. The observed very slow internal drainage, poor aeration and poor root development may thus be explained. Finally the soils have been classified in the new Australian soil classification (Isbell, 1996) in which it has been possible to place them precisely, providing an accurate summary of their, properties.