The effect of changing pH and electrolyte concentration on the dispersion and zeta potential of Naand Ca-forms of kaolinite, illite and smectite was investigated in relation to changes in their net negative charge.The percentage of dispersible Na-clay and the percentage increase in net negative charge was positively correlated with pH, but the slopes varied from clay to clay. In general, the net negative charge was the primary factor in clay dispersion, and the pH affected clay dispersion by changing the net charge on clay particles. Na-smectite had larger net charge at all pHs than Na-illite and Nakaolinite, and it always had larger flocculation values. The role of electrolyte concentration could be due to its effect both on flocculation and variable charge component of the clay minerals. The zeta potential at different pHs also reflected the same trend of clay dispersion with net particle charge.In Ca-clays the trends were similar to Na-clays up to pH 7.0. In more alkaline solution CaCO? formation led to charge reduction on clay particles, resulting in flocculation and reduction of zeta potential. At similar pHs the electrophoretic mobilities of all the clays showed constant potential behaviour. However, the zeta potentials of Ca-clays were always smaller than those of sodic clays because the clays were more aggregated. Net particle charge was the most important factor in controlling clay dispersion for the whole range of pH and ionic strength and for all types of cations.?Permanent address:

Low nutrient availability and biological activity are the main challenges in calcareous soils with low organic matter (OM) content. The purpose of the present study was to evaluate the responses of soil nutrient status and biological traits to addition of corn residue biochar produced at different pyrolysis temperature in a calcareous soil. Biochars were made at 200 (BC200), 350 (BC350), and 500°C (BC500), added to a calcareous soil at 1 and 2% (w/w) and were incubated for 90 days. The application of biochars increased soil organic carbon (SOC), electrical conductivity (EC), cation exchange capacity (CEC), total N [1.21-to 1.41-fold], available P [1.71-to 2.65-fold], K [1.53-to 2.60-fold], Mn [1.14-to 1.21-fold], microbial respiration [1.21-to 2.23fold], substrate-induced respiration [1.22-to 2.63-fold], microbial biomass carbon [1.20-to 2.24-fold], the activity of catalase [1.80-to 2.93-fold], and dehydrogenase [1.47-to 2.30-fold], which varied with the pyrolysis temperature and application rate. Generally, all the measured biological attributes were higher in BC200 than the other treatments. The BC200 biochar increased soil inorganic nitrogen [1.14-to 1.21-fold] and available Fe [1.12-to 1.17-fold], Zn [1.32-to 1.42-fold], and Cu [1.06-to 1.10fold]. In contrast, the BC500 at 2% rate decreased available Fe, Zn, and Cu. The findings revealed that the application of corn biochar obtained at 200°C to calcareous soil was more efficient for improving the nutrient availability and microbial activity.

A greenhouse experiment evaluated the chemical and physical changes of a Natrixeralf (with alkaline pH 9·4 and 5% CaCO3), as influenced by the changes in carbonate chemistry, pH, and particle charge following the application of gypsum, green manure, and glucose. Gypsum reduced the pH from 9·38 to 7·89, increased Ca2+ in soil solution, and decreased the sodium adsorption ratio (SAR1:5) from 11·6 to 1·2. Green manure, due to increased CO2 production, reduced the pH to 8·68 and SAR1:5 to 7·52. Green manure plus gypsum reduced pH to 7·67 and SAR1:5 to 0·91. The interactive effect of gypsum and green manure on all soil properties was highly significant as shown by ANOVA analysis. Reduction of soil pH was also reflected in the levels of carbonates in the soil solution. Addition of glucose increased the microbial activity and produced fatty acids. The drastic reduction in pH (<6·0) was related to the amount of glucose added. The concentrations of Ca 2+ and carbonates, and SAR1:5 values, were inversely related to the soil pH after glucose addition. The data on soluble Na2CO3 and NaHCO3, zeta potential, mechanical dispersion, aggregate stability, and saturated hydraulic conductivity confirm the effects of pH reduction and carbonate solubility as influenced by the amendments in alkaline sodic soil.

The effect of changing pH on the dispersion of clay from sodic soils was investigated in relation to changes in net charge on clay particles. A positive relationship was obtained between pH and the percentage of dispersible clay for each soil clay. The percentage increase in net negative charge was also positively correlated with pH. However, the slopes of these relationships varied between soil clays. In general, the net negative charge was the primary factor in clay dispersion and the pH affected clay dispersion by changing the net charge on clay particles. In comparing the values for pure clay minerals quoted in the literature with soil clays having similar mineralogy, it was found that soil clays had higher flocculation values. This is shown to be due to higher net negative charge on soil clays than the corresponding values for pure clay minerals found in the literature. The effect of soil organic matter in enhancing the net negative charge probably contributes to the higher charge on soil clays.The critical dispersion concentration for clay dispersion from soil aggregates was lower than the flocculation values observed for the separated soil clays. The separated soil clays had high negative charge due to exposure of surfaces which were originally bonded in the aggregates. The dispersive potential of a number of Alfisols, Oxisols, Aridisols (calcareous soils) and Vertisols collected from different parts of Australia was highly correlated with soil pH. The relationship with CEC was poor because CEC was estimated at a pH different to the natural pH of the soil. This study has brought out the importance of pH in the management of dispersive soils.