EFFECT OF AN ACIDIFYING NITROGEN-FERTILIZER AND LIME ON SOIL-PH AND WHEAT YIELDS .1. SIDE-EFFECTS

Three long-term trials were commenced in 1980 at Merredin, Wongan Hill s, and Newdegate to investigate the effect of an acidifying fertiliser containing 17.5% nitrogen (N) and 7.6% phosphorus (P) (based on ammon ium sulfate and ammonium phosphate) on soil pH, soil acidity related p roblems, and wheat grain yields under continuous cropping. Treatments were 3 rates (kg/ha) of N + P applied with the cereal seed (nil; 17.5 N + 7.6 P; 35 N + 15.2 P), with or without 3 t/ha of ground limestone (with or without MgSO4, KCl, MoO3) applied in 1980. Two extra treatmen ts were 2 rates of limestone (70, 140 kg/ha) topdressed with the cerea l crop each year along with the low and high N + P fertiliser treatmen ts, respectively. This paper reports soil properties for the first 10 years of the trials. In the acidic Merredin soil (pH 4.3), there was m inimal effect of N + P fertiliser on soil pH. The pH was slightly redu ced at 0-10 cm depth. At Wongan Hills, soil pH at 0-10 cm depth was re duced over time by N + P application from 4.8 to 4.2. At Newdegate, on ly the high rate of N + P reduced pH over time, from 4.6 to 4.3 at 0-1 0 cm. Limestone at 3 t/ha in 1980 increased soil pH at 0-10 cm depth a t all 3 sites; however, in all cases pH fell with time. Limestone appl ied at 70 or 140 kg/ha with each N + P application increased pH at 0-1 0 cm depth by 0.1-0.4, 0.1-0.4, and 0.3-0.9 pH units at Merredin, Wong an Hills, and Newdegate. Soil aluminium (Al) concentrations (extracted in 0.01 mol CaCl2/L) were generally low at Wongan Hills and Newdegate in the absence of N + P fertiliser. These levels rose after N + P app lication to 4 and 2 mug/g at 0-10 cm depth at Wongan Hills and Newdega te. Soil Al concentrations at Merredin were high, particularly in the subsoil: 3-5, 9-13, and 23-29 mug/g in the 0-10, 10-20, and 20-40 cm d epths. With the high rate of N + P, Al concentration rose to 10 mug/g at 0-10 cm. Application of 3 t/ha of limestone reduced this to <1-2 mu g/g. Application of 140 kg/ha of limestone with the high N + P fertili ser rate lowered soil Al concentration at 0-10 cm. Extra acid that acc umulated in treated plots compared with control plots varied from -34. 7 kmol/ha (acid-neutralised) for the treatment at Merredin receiving o nly 3 t lime/ha in 1980 to 23.8 kmol/ha for the treatment at Wongan Hi lls receiving high N + P. With the treatments receiving lime only, the amounts of acid neutralised were only 82, 66, and 58% of those predic ted at Merredin, Wongan Hills, and Newdegate, respectively. Acid accum ulation in the 3 treatments receiving high N + P was within the predic ted range at Wongan Hills, as it was for the treatment receiving high N + P plus 3 t lime/ha in 1980 at both Merredin and Newdegate. However , for the treatments receiving only high N + P or high N + P plus 140 kg lime/ha.year at these 2 sites, the acid accumulation rates were les s than predicted. Levels of exchangeable cations in the soil were high est at Wongan Hills and lowest at Newdegate. Application of N + P decr eased Ca concentration at all sites and reduced the concentration of e xchangeable Mg at Wongan Hills. Lime applied at 3 t/ha increased the c oncentrations of exchangeable Ca and Mg at all sites. There were no ef fects of treatments on concentrations of exchangeable K or sodium.