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.