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Soil micromorphology, using light microscopy and scanning electron microscopy (SEM), was used to describe detailed soil morphological and compositional changes and determine mineral weathering pathways in acid sulfate soils (ASS) from the following 2 contrasting coastal environments in Barker Inlet, South Australia: (i) a tidal mangrove forest with sulfidic material at St Kilda, and (ii) a former supratidal samphire area at Gillman that was drained in 1954 causing sulfuric material to form from sulfidic material. Pyrite framboids and cubes were identified in sulfidic material from both sites and are associated with sapric and hemic materials. Gypsum crystals, interpreted as a product of sulfide oxidation, were observed to have formed in lenticular voids within organic matter in the tidal mangrove soils at St Kilda. Sulfide oxidation was extensive in the drained soil at Gillman, evidenced by the formation of iron oxyhydroxide pseudomorphs (goethite crystallites and framboids) after pyrite and jarosite, and of gypsum crystals. Gypsum crystals occur where a local source of calcium such as shells or calcareous sand is present. Sporadic oxidation episodes are indicated by the formation of iron oxide and jarosite coatings around coarse biogenic voids. These observations indicate that mineral transformation pathways are strongly influenced by soil physico-chemical characteristics (i.e. oxidation rate, Eh, pH, soil solution chemistry, mineralogy, and spatial distribution of sulfides). This information has been used to illustrate the interrelationships of pyrite, carbonate, gypsum, jarosite, and organic matter and help predict soil evolution under changing hydro-geochemical, redoximorphic, and thermal conditions in soils from coastal environments.

The effects of pH and soil-applied Cd on Cd concentration in wheat grain were determined using 4 soils in a glasshouse study. Grain Cd concentrations increased significantly (P < 0·001) with increasing applications of Cd for all soils. Generally, for the Alfisols the Cd concentration in grain decreased with increasing soil pH for all Cd treatments. The Cd concentrations in grain from plants grown on the Haploxerert showed variable responses to pH, depending on the Cd treatment. Smaller decreases in grain Cd concentration with increasing pH were seen on soils with native Cd compared with grain from soils to which Cd had been added. Generally, Cd uptake (mg/pot) by grain grown in the Alfisols showed the same trends with increasing pH as seen with grain Cd concentrations, indicating no yield dilution effect. On the Bordertown soil (a Palexeralf) the ratio of Cd in shoot material (excluding grain) to Cd concentration in grain was found to increase significantly (P < 0·001) with increasing Cd treatments and decrease significantly (P < 0·001) with increasing pH. The ratio on the Bordertown soil did not remain constant across Cd or pH treatments, which suggests that the use of soil tests to predict Cd concentration in grain may be problematical. The transfer coefficients for Kapinnie, Freeling, and Bordertown soils were dependent on pH, whereas that for the Inman Valley soil was independent of pH. The relationship between the reciprocal of the distribution coefficient (K¡ 1 d) and the transfer coefficients (Cd grain/Cd soil) was generally good for all soils except the Vertisol from Inman Valley. distribution coefficient, EDTA-extractable Cd, transfer coefficient.