1. Micronutrients are required in minute quantities by plants.
2. Their moderate decrease causes deficiency symptoms while their moderate increase causes toxicity.
3. The reduction in dry weight of a tissue by 10% by any mineral is known as toxicity.
4. It is not easy to identify toxicity symptoms.
5. Most of the time, the excess of an element inhibits the uptake of another element resulting in causing the deficiency symptom of that element.
6. Manganese inhibits calcium translocation towards apex of stem and exhibits symptoms of chlorosis with grey spots appearing on leaves.
7. This is because manganese competes with iron and magnesium for uptake.
8. Therefore what we see as symptoms of manganese toxicity, may be the deficiency symptoms of Fe, Mg and Ca.