Predicting toxic concentrations of manganese in acid soils

Abstract

One of the major problems of identifying manganese toxicity in plants is the absence of a critical concentration in plant tissue, above which plant growth is impaired. Consequently this prevents identification of limiting concentrations of manganese in the soil that require remedial treatments. Magnesium ions are known to modify the appearance of manganese toxicity in some plant species. We have studied the effect of the ratio (R_p) on plant growth. Critical values of the ratio were identified (R_pc) where plant growth was reduced relative to unstressed plants. Experiments in solution culture showed that R_pc is 20 for wheat (Triticum aestivum L. cv Mara) and two species of melon. The critical value for the appearance of visual symptoms of manganese toxicity in tomato (Lycopersicum esculentum L.) was between 3.4 and 6.5. Separating soil solution in a coarse-texture soil derived from quartzdiorite (pH of the soil in the field = 4.5) showed that R_pc was simply related to the ratio of Mg:Mn in the rooting medium (R_s) according to the equation: