Soil and plant potassium optima for maize on a kaolinitic clay soil

In South Africa, there is an extreme paucity of information relating maize (Zea mays L.) yields to soil and plant K levels and most K fertilizer recommendations are highly subjective. Controversy also surrounds the practice of relating K fertilizer recommendations to estimates of yield potential. The work reported here was conducted to address these shortcomings. The yield and plant compositional response of maize to a range of soil K levels was studied for eight seasons on a Metz clay soil (clayey, kaolinitic, thermic Plinthustalf). Relative yields were related to exchangeable soil K and the leaf parameters: K content, (Ca + Mg)/K and K DRIS index. Although grain yield maxima varied from 5.85 to 10.80 Mg ha−1, 91% of the relative yield variability was explainable in terms of exchangeable soil K. Leaf-based indices were also closely related to relative yield, but were sensitive to variations in topsoil moisture status prior to sampling and were not considered diagnostically superior to the K soil test. It is concluded that the optimum K level in this soil is c. 0.32 cmol L−1, that there is no justification for relating K requirement to estimates of yield potential and that leaf diagnostic criteria should be interpreted with circumspection.