The development of manganese toxicity in pasture legumes under extreme climatic conditions

Summary

Manganese levels and pH in soil were measured on limed and unlimed plots at bi-monthly intervals for two years at five field sites with lucerne (Medicago sativa) and subterranean clover (Trifolium subterraneum) and related to rainfall and temperature. Pot experiments with lucerne, subterranean clover and rape (Brassica campestris) were used to confirm the results of the field experiments.

Manganese toxicity developed in lucerne and subterranean clover under waterlogged conditions after heavy rain on the slightly acid soils (pH 4.7–5.5). Lucerne also showed manganese toxicity on the same soils in summer after extended hot, dry conditions. The maximum available manganese was 210 μg/g in the waterlogged soil (0–15 cm) and 128 μg/g in the heat affected soil.

Lime treatment of 2240 kg/ha reduced the maximum available manganese to 148 μg/g in waterlogged plots and to 47 μg/g in the heat affected plots but failed to correct manganese toxicity. However, lime corrected toxicity symptoms under less severe conditions.

In the pot experiments, available manganese reached 270 μg/g after 2 weeks artificial waterlogging and 68 μg/g after 2 weeks dry heat exposure.