Interactions between potassium, calcium and magnesium in sugarcane grown on two contrasting soils in South Africa

The correction of soil fertility constraints is necessary for the successful production of sugarcane. Inhibition of potassium (K) uptake by sugarcane plants in the presence of high concentrations of calcium (Ca) and magnesium (Mg) has been reported, with some sugarcane-growing countries adjusting K fertiliser recommendations accordingly. Although the depressive effect of K on Ca and Mg uptake is well documented for other crops, this phenomenon has not been widely recognised in sugarcane, and is not taken into account when making Ca and Mg recommendations. The interactions between K, Ca and Mg were therefore investigated on two contrasting soil types (Oxisol and Inceptisol) on the east coast of KwaZulu-Natal, South Africa, in factorial-designed field trials. Trials at each site included the plant crop (first crop after planting) and two ratoon crops (crops which regrow following harvest). This paper deals with the impact of the treatments on leaf nutrient levels and crop yields. Potassium treatments (0, 100, 200 and 300 kg K ha−1, as KCl) resulted in significant (P < .001) increases in leaf K concentrations, along with relatively consistent increases in sugarcane and sucrose yields with increasing leaf K. Increased leaf K concentrations led to decreases in leaf calcium (Ca) and magnesium (Mg). Calcium silicate products Calmasil® (a calcium silicate slag) and blast furnace cement, applied at rates to supply 0 and 300 kg Si ha−1, also supplied large amounts of Ca and Mg. Application of these products resulted, at times, in increased leaf Ca and Mg. Silicate application increased sugarcane and sucrose yields in three out of the four ratoon crops under study. Despite recorded increases in leaf Ca and Mg, leaf K was not decreased by silicate application on the Oxisol, and seldom so on the Inceptisol. It is proposed that the inhibitory effect of Ca and Mg on K uptake has historically been overestimated in sugarcane, and that the reverse effect - K suppression on Ca and Mg uptake - may have been underestimated.