The occurrence of sodic soils in Queensland is more related to soil genetic factors of the past than to the current rainfall pattern, with lower sodium accessions and smaller occurrence of saline lands than other areas of Australia. A soil sodicity map of Queensland is presented. On an area basis, 55% of soils in Queensland are non-sodic, 25% are strongly sodic and 20% are of variable sodicity. The map was prepared using exchangeable sodium percentage (ESP) values at 0.6 m depth from 2 009 soil profiles, as well as the soil boundaries of the 1:2000000 Atlas of Australian Soils maps (Northcote et al. 1960-68). There is general agreement with the earlier sodicity map of Northcote and Skene (1972). The relationships between exchangeable sodium and field-measured soil hydraulic properties and plant-available water capacity are discussed. Behaviour of sodic soils depends on the exchangeable sodium percentage, clay content, clay mineralogy and salt levels. The binary component particle packing theory has been used to explain soil behaviour and identify those soils most susceptible to sodium. Cracking clay soils with dominantly smectite mineralogy and high clay contents are less susceptible to a given ESP level, as determined by their hydrological behaviour, than soils of moderate clay content and mixed mineralogies. The sodicity and the salt content of an irrigation water are important in maintaining permeability of soils. The naturally occurring equilibrium salinity-sodicity relationships of a wide range of subsoils in Queensland is compared to the published relationships between stable permeability and decreasing permeability based on sodicity and salt content. Aspects of management of sodicity under dryland and irrigation are discussed.
The steady-state leaching requirement (LR) model (USSL 1954) and the transient solute mass balance model of Rose et al. (1979) were applied to soil chloride data from 42 sites in Queensland, Australia, to evaluate utility of the models for assessing the impact of irrigation on soil salinity and leaching. Data were taken from previous studies on salinity in irrigated soils, and covered a wide range of soil types and irrigation managements. The time taken for soil chloride levels to reach steady-state was assessed from the transient model, and was calculated to be years ( > 3) or decades. These calculations showed that 19 sites had been sampled prior to steady-state, so that a strict application of the LR model was invalid. However, at only seven of these sites were there significant differences between sampled soil chloride and the calculated final values. At these seven sites leaching fluxes calculated with the two models differed by 3-162 mm y-1. At two sites leaching fluxes were _<0 and the LR model could not be used to interpret soil chloride dynamics. Although both models assume constant inputs through time, variations in irrigation management practices at most sites had little practical effect on model predictions. However, where there were extreme variations in irrigation application or irrigation water chloride concentration, calculated leaching fluxes and thus predicted chloride levels were markedly affected. These models should not be applied in this situation.The transient model was preferred to the steady-state LR model for assessing the effect of irrigation on soil salinity, because of its ability to provide predictions of future soil chloride levels under non-steady conditions, and where leaching flux values are < 0.
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