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.
Various models for predicting profile available water capacity (PAWC) from laboratory measurements were compared with published field values for the same sites. The intention was to choose the best model/s to predict PAWC, by using a database, for a wide range of soils in the Burdekin Irrigation Area, North Queensland. Effective rooting depth for all models was estimated from the chloride profile. It was found that the predictive abilities of all models used were dependent on soil types. A conventional model (ASWC) based on the difference between water retained at -33 and -1500 kPa matric potentials was higher (P < 0.01) than field measured PAWC. An empirical model (PAWC2) based on cation exchange capacity (CEC) and depth was suitable only for cracking clays and sodic duplex soils. Another empirical model (PAWC1) based on -1500 kPa water retained and depth, predicted field PAWC particularly well on cracking clays, sodic duplex and related soils. There were strong indications that the PAWC1 model is also suitable for the better drained, lighter textured soils but there was a shortage of comparable field data to confirm this. The practical implication of these findings was that an analytical database can be used to predict PAWC on many Burdekin soils, providing immediate assistance to those designing irrigation channels and farm layouts.
Estimates of rooting depth are necessary parameters in predicting available water capacity (AWC) of soils. In a recently assembled database for the Burdekin River Irrigation Area, no single criterion, commonly used to estimate rooting depth, was available for all sites. Therefore a number of methods of estimating rooting depth which give interchangeable results were required. This paper compares eight methods of estimating rooting depth within three AWC models and compares the outcome with field determinations. Soil properties used to estimate rooting depth were laboratory-based (two chloride methods, electrical conductivity and pH), morphological (carbonate and mottling) and two fixed depths (0.9 and 1.0 m). For all soils tested, the laboratory-based methods used within one AWC model (based on regression equations by using -1500 kPa water retained) resulted in predicted AWC values not significantly different (P< 0.05) from field measurements. The suitability of mottling was limited to cracking clays and sodic duplex soils and other rooting depth methods had varying applicability depending on soil type. This work shows that a range of rooting depth methods can be used to predict AWC of Burdekin soils. The results should have application to soils of other areas.
Morphologically similar scrub and forest basalt soils were investigated for differences in soil phosphorus and potassium levels. Surface samples (0-10 cm) from 181 scrub and 111 forest sites were analysed for five soil tests: 0.05 M H2SO4-extractable phosphorus, 0.5 M NaHCO3- extractable phosphorus, 0.05 M HCl-extractable potassium, total phosphorus and total potassium by X-ray fluorescence. Highly significant differences (P <0.001) between scrub and forest soils were found for each soil test when all sites were assessed in terms of regional differences based on vegetation. These differences were then investigated further for each of the three common Great Soil Groups - black earths, euchrozems and lithosols - and then for six individual soil series common to both the scrub and forest areas. An unexplained geographic trend in phosphorus values previously reported (1972) in the same area could be accounted for by this difference between scrub and forest soils. Similar differences in soil phosphorus and potassium for scrub and forest soils on other parent materials are also discussed. It was clearly shown that the scrub means were always greater than the forest means for all five soil tests for all soils combined, each Great Soil Group and each soil series, and that there is a strong association between vegetation distribution and these soil differences.
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