Response of cassava, sunflower, and maize to potassium concentration in solution III. Interactions between potassium, calcium, and magnesium

Abstract

Abstract The effect of increasing solution potassium concentration from 0.5 to 8024 μM on the absorption, distribution, and utilization of calcium and magnesium by 12 cultivars of cassava, and one cultivar each of sunflower and maize, was studied at constant concentrations of calcium (753 μM) and magnesium (147 μM). Increasing the solution concentration from 0.5 to 6 μM K stimulated the rate of calcium absorption in five cassava cultivars, but had no effect on the remaining cassava cultivars, sunflower or maize. Thereafter, increasing solution potassium concentrations generally depressed rates of calcium absorption. Rates of magnesium absorption were strongly depressed by increasing solution potassium concentrations, particularly from 0.5 to 122 μM. The absorption of magnesium may involve two mechanisms, one which is highly specific, insensitive to potassium, and the other which is highly sensitive to potassium and operative at solution concentrations below 122 μM K. Similar, but less clear cut evidence, indicates the operation of more than one calcium absorption mechanism. The incidence of potassium-induced magnesium deficiency symptoms in cassava was associated with inherently lower magnesium absorption rates in that species, together with a greater retention of magnesium in the roots. Differences in distribution of calcium and magnesium within plant tops in the three species are discussed in relation to effects of potassium on absorption and remobilization from older tissues. The ecological adaptation of cassava to low fertility soils is considered and the hypothesis advanced that cassava should be more prone to magnesium deficiency than calcium deficiency when fertilizer potassium is applied.