Reacidification and reliming effects on soil properties and wheat yield

Farmers in southern New South Wales began lime application in the 1980s. Ma ny have now limed most of the acidic soils on their properties, and are con sidering 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 combinati ons of 5 rates of lime applied in 1982 and 5 rates applied in 1992. The plo ts were soil sampled and cropped to wheat (cv. Janz) in both 1992 and 1993. The rate of soil pH(Ca) decline in the 0-10 cm soil from 1983 to 1993 follo wing lime application in 1982 was dependent on the pH(Ca) increase achieved 1 year after lime application (1983). The rates of decrease varied from 0. 10 pH(Ca) units/year, after 5000 kg/ha was applied, to 0.02 pH(Ca) units/ye ar following application of 500 kg/ha of lime. Evidence in 1992 and 1993 su ggested that the pH(Ca) effect of lime applied in 1982 had moved down the s oil profile below 10 cm. Wheat yield in 1992 responded to lime applied in 1982 but not to lime appli ed in 1992. In the 1993 season, the 1982 and 1992 applied lime gave signifi cant yield increases. The response in grain yield in 1993 to 1992 applied l ime was greatest where no lime, or low rates of lime, had been applied in 1 982. 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 sensiti ve cultivar Janz was obtained where the pH(Ca) was about 5.5 in both layers . Where the soil pH(Ca) was 5.0 in the 0-10 cm soil layer, grain yield incr eased with increasing pH(Ca) in the 10-20 cm layer from 3.2 t/ha at pH(Ca) 4.1 to 3.9 t/ha at PHCa 5.3. Reliming at 2000 kg/ha increased grain yield i n 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 move ment of the lime effect to the subsurface soil. It appears that maximum yie lds may only be achieved with the amendment of the subsurface soil by a ser ies of lime applications over several decades or by the combined use of sha llow incorporated lime and plant tolerance of soil acidity.