Manganese toxicity in pasture legumes - I. Effects of calcium and phosphorus levels in the substrate

The effects of Ca and P levels in the substrate on Mn toxicity in several pasture legumes were studied. In solution culture, high Ca supply alleviated Mn toxicity by reducing the proportion of absorbed Mn moving to the shoots, whereas high P application intensified the toxicity by increasing uptake. In addition, there was an indication of a Mn × P interaction in the plant which could influence dry matter yield of some tropical legumes. In the soil, Mn toxicity was influenced by both availability and plant processes. Toxicity could be prevented by small applications of CaCO, effective largely by reducing the exchangeable Mn level. High Ca(HPO) supply aggravated toxicity by increasing the exchangeable Mn fraction of the soil. The influences of both CaCO and P on exchangeable manganese fraction were attributed to their effects on the pH of small soil pockets, without influencing general soil pH. In addition to its local effect on soil pH, high P supply could directly increase this fraction of soil Mn. Mn toxicity, either caused by Mn supply or induced by high P application, could be overcome by large Ca application in solution culture and by liming in the soil. Furthermore, the absolute levels of Ca and P in the substrate appeared to be more important than their ratios in affecting Mn toxicity. The implications of the results for fertilizer practice are discussed. Mn toxicity is suggested as an explanation of some cases of lack of response to P application, normally attributed to P fixation by the soil