Critical Concentration

Figure 12.2 plots growth rate (as a percentage of the maximum) against the concentration of a mineral nutrient in plant tissue, and the resulting curve has four zones. In the deficient zone, tissue concentration is below the critical nutrient concentration and growth increases sharply as more of the mineral becomes available. The critical nutrient concentration itself marks the boundary into the adequate zone, where growth is at or near its maximum and stays roughly flat across a fairly wide range of tissue concentration - this is the zone a grower aims to keep a crop within. Beyond the adequate zone lies a transition zone, and finally the toxic zone, where further increases in tissue concentration actively reduce growth; the toxic concentration is conventionally defined as the point at which growth (or dry weight) has fallen by 10% from its maximum. The practical value of th …