Critical Concentration and Toxicity of Minerals

Knowing a mineral's critical concentration matters for two practical reasons: it tells a grower the minimum tissue level needed to avoid deficiency symptoms, and it warns against oversupplying a mineral to the point of toxicity. Below the critical concentration a plant shows deficiency symptoms; above the normal adequate range, the same mineral becomes toxic. By convention, the concentration at which tissue dry weight is reduced by 10% is taken as the toxic threshold. Calcium's role illustrates why fine control matters even within a single mineral's normal range: it is regulated intracellularly by calmodulin, a small, heat-stable calcium-modulating protein found in all eukaryotic cells that binds Ca2+ and uses the resulting shape change to fine-tune m …