The rate of acidification under wheat in south-eastern Australia was examined by measuring the fluxes of protons entering and leaving the soil, using the theoretical framework of Helyar and Porter (1989). Monthly proton budgets were estimated for the root zone (0-90 cm layer) and for the 0-25 and 25-90 cm layers. After an annual cycle, the root zone was alkalinized by 0.5 to 3.1 kmol OH-ha-1. The alkalinity originated from the mineralization of the organic anions contained in the organic matter. The budget was near neutrality in the 0-25 cm layer (range: -1 . 0 to 1.4 kmol H + ha-l), whereas there was net alkalinization in the 25-90 cm layer (1.7 to 2.3 kmol O H -ha-a). In the 0-25 cm layer, the acidity produced in autumn by mineralization of organic nitrogen was counterbalanced by the alkalinity released from crop residues. The main acidifying factor in this layer was leaching of NO~-during early winter (2.4 kmol H + ha-1). Nitrate added through leaching was the main alkalinizing factor in the 25-90 cm layer, as added NO~-was taken up by the roots or denitrified in this layer. Urea fertilization had almost no effect on the rate of acidification, as little NO 3 was leached out of the root zone. The factors acidifying the soil under wheat were limited in this environment because of the small amount of NO 3 leached and the retention of the crop residues.

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The only way to increase the low CEC of sandy tropical soils over the long term is to apply high CEC materials such as 2:1 clay minerals. Acid activated bentonite is used in Thailand in the vegetable oil industry during the clarification process. The waste bentonite is discarded afterwards. The aim of the study was to compare the effects of the addition of these oil bentonites (OB) with the addition of cation beneficiated bentonite (BB) on soil properties and plant growth. Palm, rice and soybean OB, and bentonite beneficiated with calcium, magnesium, and potassium were applied at rates between 5 and 40 t ha −1 to an Arenic Acrisol. Three consecutive crops of sorghum were grown in pots. Biomass and plant nutrient content were determined at each growth phase, and selective soil properties were measured at the start and the end of the study. Beneficiated bentonite was not water repellent, but the addition of OB resulted in soil water repellency. The application of bentonite at the rate of 40 t ha −1 increased the cation exchange capacity (CEC) from 0.6 cmolc kg-1 in the control to 1.9 and 0.7 cmol c kg −1 in the BB and OB, respectiveley. The lower value of the CEC for OB compared to BB was probably due to the activation process and oil coating. OB applications at rates higher than 20 t ha −1 did not increase biomass, and biomass decreased with increasing water repellency. The other treatments produced a higher biomass than the control. However biomass was below potential because of widespread nitrogen deficiency. Exchangeable K was exhausted in two crops, whatever the initial level, stressing the issue of K management in this soil type. Soybean OB is a promising material for soil chemical properties and biomass production, probably because of its low oil content.

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