Wheat grain-yield response to lime application: relationships with soil pH and aluminium in Western Australia

Soil acidity, or more specifically aluminium (Al) toxicity, is a major soil limitation to growing wheat (Triticum aestivum L.) in the south of Western Australia (SWA). Application of calcium carbonate (lime) is used to correct Al toxicity by increasing soil pH and decreasing soluble soil Al3+. Soil testing using a 0.01 m calcium chloride (CaCl2) solution can measure both soil pH (pH(CaCl2)) and soil Al (Al-CaCl2) for recommending rates of lime application. This study aimed to determine which combination of soil pH(CaCl2) or soil Al-CaCl2 and sampling depth best explains the wheat grain-yield increase (response) when lime is applied. A database of 31 historical lime experiments was compiled with wheat as the indicator crop. Wheat response to lime application was presented as relative yield percentage (grain yield for the no-lime treatment divided by the highest grain yield achieved for lime treatments x 100). Soil sampling depths were 0-10, 10-20 and 20-30 cm and various combinations of these depths. For evidence that lime application had altered soil pH(CaCl2), we selected the change in the lowest pH(CaCl2) value of the three soil layers to a depth of 30 cm as a result of the highest lime application (pH(min)). When pH(min) <0.3, the lack of grain-yield response to lime suggested that insufficient lime had leached into the 10-30 cm soil layer to remove the soil Al limitation for these observations. Also, under high fallow-season rainfall (228 and 320 mm) and low growing-season rainfall (GSR) (<140 mm), relative yield was lower for the measured level of soil Al-CaCl2 than in the other observations. Hence, after excluding observations with pH(min) <0.3 or GSR <140 mm (n = 19), soil Al-CaCl2 provided a better definition of the relationship between soil test and wheat response (r(2) range 0.48-0.74) than did soil pH(CaCl2) (highest r(2) 0.38). The critical value (defined at relative yield = 90%) ranged from 2.5 mg Al kg(-1) (for soil Al calculated according to root distribution by depth within the 0-30 cm layer) to 4.5 mg Al kg(-1) (calculated from the highest Al-CaCl2 value from the three soil layers to 30 cm depth). We conclude that 0.01 m CaCl2 extractable Al in the 0-30 cm layer will give the more accurate definition of the relationship between soil test and wheat response in SWA.