Potassium (K) - Macronutrients
The main source of K+ for plants comes from withering of K containing minerals. Potassium released by withering dissolves in the soil solution. It can then be taken up by plants or adsorbed onto soil colloids. In plants, K+ is an important cation. It is taken up by the plant at high rates through K+ channels present in the membranes. The concentration of K+ in the cytoplasm is about 100 mM which is 5-10,times higher than K+ concentration in the vacuole. The phloem sap is rich in K+. It is the most abundant cation present in phloem with the concentration approaching that in the cytoplasm.
As the solutes of the phloem sap can be translocated both upwards and downwards in the plant, K+ movement is bidirectional. Potassium is necessary for meristematic growth. It is involved in controlling water status of plants and maintains cell turgor. There is less water loss from plants supplied with K+ due to a reduction in transpiration rate. As you know K+ plays an important role in opening and closing of stomata. Plants inadequately supplied with K+ have impaired stomata1 activity. It is also involved in the translocation of photosynthates. The main biochemical function of K+ is the activation of various enzymes. Potassium deficiency does not immediately result in visib1.e symptoms. At first there is only a reduction in growth rate. Chlorosis and necrosis appear later.
These symptoms generally appear on the margins and tips of older leaves. Plants suffering from K+ deficiency show a decrease in turgor. They easily become flaccid under water stress. Resistance to drought is, therefore, poor. Inadequate soil K+ levels can be corrected by the use of K+ fertilisers. The most widely used and cheapest potash fertiliser is potassium chloride (KCl) which is known commercially as muriate of potash.