A method has been developed for the speciation of arsenic (AsO−2, AsO3−4, and dimethylarsinic [DMA]) in natural soil solutions from contaminated sites in Australia. The separation of these anions was achieved by capillary zone electrophoresis (CZE) using a fused silica capillary (72‐cm by 50‐μm i.d.) with a basic chromate buffer and on‐column indirect UV detection at 254 nm. Method parameters, such as electrolyte pH, run voltage, and capillary temperature were studied in order to establish suitable analytical conditions. The ideal separation for As(III) and DMA was achieved with a buffer pH of 8.0, a run voltage of 25 kV, and a capillary temperature of 30°C. Under these conditions, As(V) and orthophosphate ions comigrated. However, the use of a chromate buffer at pH 10, a run voltage of 20 kV, and capillary temperature of 20°C led to complete separation of As(V) and phosphate peaks. Results of these investigations together with recovery test data suggest that separation of the As species from soil solutions can be achieved in less than 5 min with detection limits of 0.50, 0.10, and 0.10 mg L−1 for As(III), As(V), and DMA, respectively.

The effect of dilution on the composition of soil solutions of 8 contrasting soils ranging in pH from 5·3 to 8·8 with reference to zinc (Zn) and copper (Cu) was studied. Soil samples were equilibrated with water in various water:soil ratios for 24 h. Equilibrium solutions were extracted and analysed for dissolved organic carbon (DOC), and major and minor elements. The separation of the soil solution at field capacity (FC) was carried out by a drainage method. Although the concentration of ions decreased upon dilution, the total quantity of sodium (Na), potassium (K), Zn, Cu, and DOC extracted per unit of soil weight increased. In contrast, the total quantity of Ca and Mg decreased in most soils. The ratio of Zn and Cu to Ca correlated to dilution level, whereas the ratio of Zn to monovalent cations decreased in low pH soils. The relationship between the quantity of Zn and Cu at different levels of the water : soil ratio in the soils studied showed that the concentration of these trace metals at FC soil moisture can be estimated from the soil extract. Increases in soil moisture content led to a marked change in the ion-pair, free hydrated metal concentrations, and complexation. Log Zn2+ was linearly related to solution pH. Zinc solubility was not consistent with published solubilities of any common minerals. Also, Zn solubility in alkaline soils tended to be higher than reported values in the literature, indicating that soluble metal–organic ligand complexation was underestimated in these soils. The relationship between pH and log Zn2+ was affected by dilution in several ways.

Alfisols, Entisols, Inceptisols, Ultisols, Vertisols, and Oxisols are all commonly found in tropical and subtropical regions receiving more than 500 mm mean annual rainfall. Landscapes throughout the tropics and subtropics are, however, dominated by Oxisols and Ultisols occupying extensive areas of potentially highly productive soils. The mineral fractions of these soils consist primarily of low-activity clays having variable surface charge that differs from high activity clays in the origin of that charge. Low activity clays are dominated by iron (Fe) and aluminium (Al) oxyhydroxides and 1:1 layer silicates (kaolin). Much research has been conducted on the effects of pH, organic matter (OM), and cation composition of the soil solution on the surface charge characteristics of variable charge soils from the tropics. In general, net negative surface charge increases with increasing soil pH and OM content. Adsorption of metal ions by variable charge soils and minerals also increases as their pH, clay, and OM contents increase. Although the precise mechanisms for the change in net negative charge of soil and mineral surfaces with increasing pH are not fully understood, the generation of negative charge either through dissociation of H+ ions from surfaces or consumption of OH- ions by soils is generally accepted. In soils dominated by permanent charge surfaces, heavy metals are not mobile but in variable charge soils, the low surface charge density creates conditions conducive to increased mobility. Consequently, the adsorption of heavy metals, in particular, cadmium (Cd) by strongly weathered soils in relation to the effects of inorganic and organic ligands and the implications for metal transport are reviewed.

Sodic soils cover over 28% of the total land area in Australia. It is clear from the papers delivered at the conference and workshop that sodic soils are beset with serious chemical, physical and nutritional problems. This report summarises these problems based on the discussions during the workshop sessions together with issues raised during group discussion on: (a) distribution, classification and mapping, (b) physical processes, (c) chemical processes, (d) nutrient constraints, (e) biology and organic matter, (f) environmental consequences and (g) management strategies for economically sustainable crop production on sodic soils. The most serious problems raised during the meeting concern inappropriate definition of sodic soils, soil water balance, nutritional requirements on sodic soils, chemical and mineralogical bases of structural instability in relation to amelioration strategies, farming systems to improve organic matter and biological activity, and information transfer. These issues are presented in detail.

The composition of the soil solution for a wide variety of Natal soils was determined after extraction of the solution by pressure filtration. The total cation concentration varied from approximately 0.001 M for the highly weathered soils representative of the Oxisol Order to about 0.003 M for the less weathered soils of the Vertisol Order. A good correlation was obtained between the activity ratio, aK/√aCa + Mg of the soil solution and the natural equilibrium activity ratio of the soil as determined by ionic equilibria studies.