RATE OF SOIL ACIDIFICATION UNDER WHEAT IN A SEMIARID ENVIRONMENT

The rate of acidification under wheat in south-eastern Australia was e xamined by measuring the fluxes of protons entering and leaving the so il, using the theoretical framework of Helyar and Porter (1989). Month ly 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 zon e was alkalinized by 0.5 to 3.1 kmol OH- ha(-1). The alkalinity origin ated from the mineralization of the organic anions contained in the or ganic matter. The budget was near neutrality in the 0-25 cm layer (ran ge: -1.0 to 1.4 kmol H+ ha(-1)), whereas there was net alkalinization in the 25-90 cm layer (1.7 to 2.3 kmol OH- ha(-1)). In the 0-25 cm lay er, the acidity produced in autumn by mineralization of organic nitrog en was counterbalanced by the alkalinity released from crop residues. The main acidifying factor in this layer was leaching of NO3- during e arly winter (2.4 kmol H+ ha(-1)). Nitrate added through leaching was t he main alkalinizing factor in the 25-90 cm layer, as added NO3- was t aken up by the roots or denitrified in this layer. Urea fertilization had almost no effect on the rate of acidification, as little NO3- was leached out of the root zone. The factors acidifying the soil under wh eat were limited in this environment because of the small amount of NO 3- leached and the retention of the crop residues.