School of Ecosystem and Forest Sciences - Theses
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ItemSalting in Victorian catchments: an investigation of soil salinity problems arising under non-irrigated condititons in Victoria( 1956)Salting, that is, the accumulation of excessive amounts of soluble salts in soils, is found in many parts of the world. In Victoria salting is associated with the presence of a permanently saline water table in irrigation districts, or with local accumulations of salt in catchment areas. The results of an investigation, carried out between April 1952, and December 1955, into this second problem, referred to here as catchment salting", are set out in this thesis. The introductory part of the thesis includes evidence regarding the incidence, and economic importance, of catchment salting in the state. The area affected is estimated as nearer 10,000 than 100,000 acres; it is mainly the better class land of valleys, and flats. Salting is extending, but at present its main importance lies in the severe erosion which it induces. In the second part of the thesis, the ecosystems, under which the accumulation of salt in the soils or a catchment can take place, are examined. The factors contributing to such an ecosystem are listed here, with summaries of the conclusions reached: i) Accessions of salt to the soil. In Victoria these salts are largely chlorides of oceanic origin, their presence in soils inland can best be accounted for by the cyclic salt theory. The evidence for this theory is reviewed, and reasons are put forward to account for variations in cyclic salt accessions at different sites. ii) A low precipitation/evaporation ratio. Such a ratio prevails in many parts of Victoria. iii) Vegetative cover with a high water usage. The catchments, in which salting now occurs, were formerly covered with Eucalypt forest, or woodland, making use of rainfall. iv) Soils having an impermeable horizon. at no great depth. Salting in Victoria is confined to catchments with soils of the solodic or solonetzic, type, characterised by a light textured A horizon overlying a heavy clay B horizon. In an undisturbed catchment of this type the hydrological equation can be expressed by: Precipitation = Evaporation + Transpiration. So that, with annual accessions of salt, some accumulation in soils of the catchment is to be expected. In these catchments, which are found chiefly in the 20” – 30” rainfall areas, drastic changes in the hydrological balance have followed settlement. All too frequently catchments have been overcleared, and overgrazed, and the resulting reduction in transpiration has produced a surplus of groundwater, with accelerated water movement downslope. The sub-surface flow over the clay horizon of solodic type soils carries down any soluble salts from the catchment; should this flow be impeded, and saline water brought to the surface, salt is concentrated by evaporation, and salting occurs. A knowledge of the factors responsible for salting will enable an assessment of salting liability in catchments to be made. Three types or salting, seepage, wetpan, and hardpan, found in Victoria, are described, with the types of vegetation which occur on each. The soils of seepage, and wetpan, areas are described as “saline”, and those of hardpan areas as "salinealkali", following the classification used by the U.S. Salinity Laboratories, Riverside, The compacted surface structure of hardpan soils, despite the high soluble salt content, is attributed to the compacting action of raindrops whilst the surface soil is temporarily washed : free of salt, and is in the “alkali” condition. Hardpan formation accounts for the characteristic shallow, and extensively dissected, pattern of erosion on salted land. In the third part of the thesis, the control of salting, and the reclamation of salted land, are dealt with. Correct catchment management, which ensures sufficient vegetative cover, is the surest control measure. Measures such as drainage, the establishment of trees and shrubs, and of improved pastures, are dealt with in a general way. Detailed results are given of trials with fertilizer, and soil improver, treatments; also of trials with some sixty species, and varieties, of grasses, and legumes, on salted land. In the final chapter the evidence, and conclusions, of previous chapters are reviewed, together with recommendations for the treatment of salted land; it therefore provides a more detailed summary, should that be required. Details of soil sampling, analytical methods, plot layout, statistical methods, and some lists, and photographs, of plants, are given in seven appendices.