On limiting supply of essential elements, the growth of plants is inhibited. The concentration of essential elements whose deficiency inhibits the growth of plants is called critical concentration. An element becomes limiting below its critical concentration. It is termed as deficient element. Growth declines and deficiency symptoms become visible if element concentration is above the level of critical concentration, it is termed as adequate. The growth is not increased if concentration is increased. Toxicity appears of sufficiently high element level. Any mineral ion concentration in tissues that reduces the dry weight of tissues by about 10 percent is considered toxic.