Reacidification and reliming effects on soil properties and wheat yield

Summary. Farmers in southern New South Wales began lime application in the 1980s. Many have now limed most of the acidic soils on their properties, and are considering reliming. This is an important economic consideration as lime is a costly input. We accurately located an old lime experiment established in 1982 and applied and incorporated lime in 1992, to give factorial combinations of 5 rates of lime applied in 1982 and 5 rates applied in 1992. The plots were soil sampled and cropped to wheat (cv. Janz) in both 1992 and 1993. The rate of soil pHCa decline in the 0–10 cm soil from 1983 to 1993 following lime application in 1982 was dependent on the pHCa increase achieved 1 year after lime application (1983). The rates of decrease varied from 0.10 pHCa units/year, after 5000 kg/ha was applied, to 0.02 pHCa units/year following application of 500 kg/ha of lime. Evidence in 1992 and 1993 suggested that the pHCa effect of lime applied in 1982 had moved down the soil profile below 10 cm. Wheat yield in 1992 responded to lime applied in 1982 but not to lime applied in 1992. In the 1993 season, the 1982 and 1992 applied lime gave significant yield increases. The response in grain yield in 1993 to 1992 applied lime was greatest where no lime, or low rates of lime, had been applied in 1982. Grain yield in 1993 was described as a function of pHCa in the 0–10 cm and 10–20 cm layers in that season. Maximum yield of the aluminium sensitive cultivar Janz was obtained where the pHCa was about 5.5 in both layers. Where the soil pHCa was 5.0 in the 0–10 cm soil layer, grain yield increased with increasing pHCa in the 10–20 cm layer from 3.2 t/ha at pHCa 4.1 to 3.9 t/ha at pHCa 5.3. Reliming at 2000 kg/ha increased grain yield in 1993 by about the same amount as an initial application of the same lime rate. We suggest that the residual benefit in grain yield was due in part to movement of the lime effect to the subsurface soil. It appears that maximum yields may only be achieved with the amendment of the subsurface soil by a series of lime applications over several decades or by the combined use of shallow incorporated lime and plant tolerance of soil acidity.