Determining wheat responses to copper in prairie soils

Abstract

Criteria for assessment of the Cu status of prairie soils were developed in the early 1980s and are currently in use with some modifications. Although there appears to be a reasonable agreement on the soil critical value of 0.4 mg kg-1 soil based on DTPA-extractable Cu, a marginal range that extends between 0.4 and 1.2 mg DTPA-Cu kg-1 soil is used to generate fertilizer recommendations. The objective of this study was to verify the existing soil critical value and identify the frequency of agronomic as well as economic response of spring wheat to Cu fertilization on soils with levels both below and above the critical value. We carried out a large number (52) of trials with spring wheat (Triticum aestivum L.) between 1991 and 2000 in all three Prairie Provinces and compiled a large number of spring wheat experiments conducted in western Canada (63) by a number of researchers. Fifty-nine trials carried out on soils containing marginal and sufficient Cu levels were evaluated and were compared to 56 experiments on soils containing deficient levels. A number of methods of placement and products were compared. Agronomic responses were obtained in 41 of the 115 experiments and only by broadcasting either 3 to 5.5 kg Cu ha-1 as CuSO4·5H2O or 0.5 to 2 kg Cu ha-1 as Cu-sulphonate and incorporating the product or by foliar application of 0.22 kg Cu ha-1. Calibration of the DTPA-extractable Cu test using a Mitscherlich type of function verified the soil critical value of 0.4 mg Cu kg-1 soil. Statistically significant grain yield responses were obtained in 87, 16 and 0% of trials carried out on soils containing <0.4, 0.4 to 0.8 and >0.8 mg DTPA-extractable Cu kg-1 soil. Economic responses, defined as those where Cu application yielded a 2:1 return on the cost of fertilizer in the year of application, varied depending on the price of wheat and ranged between 19 to 45% in deficient soils for a price range of $100 to $200 t-1 of wheat grain; economic responses on marginal and sufficient soils occurred in 6% and none of the cases, respectively. Hence, fertilization of soils with DTPA-extractable Cu levels >0.4 mg kg-1 is associated with very high risk. Key words: DTPA-extractable, calibration, soil critical value, deficient, marginal, agronomic, economic