EFFECT OF LUPINS AND LOCATION ON SOIL ACIDIFICATION RATES

The effect of years since clearing, frequency of lupin crops, and location on soil acidification rates was determined on a deep yellow sand in the northern wheatbelt of Western Australia. The study involved soil sampling of 87-89 sites at each of 2 locations (Mingenew, East Chapman) representing a range of years (3-40) since land clearing and frequency of lupin crops (0-11). The sites were sampled to a depth of 80 cm in 10-cm increments, and measurements included soil pH, pH buffering capacity, and bulk density. The rate of acidification for the profile at Mingenew (400-450 mm rainfall/year) in a rotation without lupins (3-4 years pasture and 1 year wheat) was 0.42 kmol H+/ha.year requiring 21 kg CaCO3/ha.year to neutralise. When a lupin-wheat rotation was grown at Mingenew, the net acidification for the profile was 0.62 kmol H+/ha.year (or 31 kg lime/ha.year). The main causes of acidification were organic anion removal and nitrate leaching. At East Chapman (325-375 mm rainfall/year), the acidification rate depended on years since clearing. For 8-14 years since clearing the acidification rate was negative, and for 15-40 years since clearing it was positive: in year 8 the rate for the profile was -0.39 kmol H+/ha.year; in year 15, 0.04 kmol H+/ha.year; and in year 40, 1.58 kmol H+/ha.year (79 kg CaCO3/ha.year). The main causes of acidification were organic anion removal, nitrate leaching, and ammonium-based nitrogen fertiliser use. There was a significant difference in initial pH (pH of the uncleared sites) of the soil samples from the 2 locations; at East Chapman the initial pH was 0.2-0.3 units higher than at Mingenew, and the rate of pH decline was greater at East Chapman.