Potassium (K) ranks seventh in order of copiousness in the earths crust and an essential major nutrient element for plants and animals together with nitrogen and phosphorus. Total K contents in soils range between 3000 and 100,000 kg ha-1 in the upper 0.2 m of the soil profile. The availability of Potassium is governed by the equilibrium among its soluble, non-exchangeable, and fixed forms. Of these forms, soluble and exchangeable K is readily available to plants whereas non-exchangeable K is slowly available. It is known that when fertilizer K is applied to a soil, some of its portion is adsorbed and becomes non-exchangeable, thus non-available for immediate uptake by plants. Due to low addition of K fertilizer and removal of straw from the fields, soil K depletion has been going on at a faster rate. The nutrient balance sheet for Pakistan shows an annual deficit of 265,000 tones K2O. About 98% is bound in the mineral form, whereas 2% is in soil solution and exchangeable phases. Potassium has been reported as one of the most important element responsible for low yields in soils of Pakistan especially under intensive potato-maize cultivation system. On sandy irrigated soils crop responses to K are fairly good and even clayey soils respond well to add K (NFDC, 2003). Although it does not become a part of plant body yet it performs many essential functions. Potassium acts as catalyst for chemical reactions, it activate at least 80 different enzymes, involved in plant growth. Starch synthetase is the key enzyme controlling the rate of incorporation of glucose into long chain starch molecules. Thus k-fertilizer application increases the tuber size of potatoes and the sucrose content of sugarcane. It acts synergistically with plant growth regulators (PGRs) like Auxin and Gibberellins .Some times K+ acts as growth promoter. Photosynthesis is the process by which plants harvest the energy of sunlight to produce sugars. At the site of ATP production in photosynthesis, the electrical charge balance is maintained by K ions .When plants are deficient in K the rate of photosynthesis and the rate of ATP production are reduced and ultimately all processes dependent on ATP are inhibited is the chemical police-man or little brother of the plant cell, because it is a major osmotica involved in stomatal movement. The tiny openings are surrounded by ?guard cells? and it is only while they are turgid that the stomata remain open and carbon dioxide can pass through into the leaf. But most of the water transpired by the plant is lost through the stomata when they are open. Thus, if there is a water deficit, the plant needs to close the stomata to conserve water. The plant controls the opening /closing of the stomata by regulating the concentration of potassium in the guard cells. A large concentration of potassium ensures turgid cells and open stomata. When the potassium in the guard cells is lowered, they become limp and the stomata close. Even plant deficient in K is less able to absorb water because its accumulation in plant roots produces a gradient of osmotic pressure that draws water into the roots. Potassium not only involved in sugar loading, unloading an translocation but also transport water and nutrients (nitrates, phosphates, calcium, magnesium & amino acid) throughout the plant in the xylem.As quality is becoming an increasingly important market factor, especially for export, So Potassium is known as a quality element .High levels of available K improve the physical quality and nutritional value of grain, forager, fruits ,vegetables and other crops particularly important late in the growing season in fruit and flower formation and boll-filling in cotton crop. Potassium deficiencies greatly reduce crop yields. In fact, serious yield reduction may occur without the appearance of deficiency symptoms. Decreased resistance to pests, diseases, salinity, excesses of temperature and drought are also associated with K deficiency. In the global cycling of potassium, it is first mined from the naturally occurring deposits and is refined by simple processes. As potassium fertilizer, it is applied to agricultural soils, taken up by plants, and enters the human food chain. It is then transferred to sewage treatment works and, because it is almost entirely soluble in water, is discharged in the effluent to water courses and from there back to the sea. So, using potassium fertilizers is not destroying a natural resource. Rather the use of potassium in agriculture is part of a very long global cycle. Today, some 95% of the total potassium output is used as fertilizer. Plant and wood ashes containing potassium carbonate were used for making soap and glass in antiquity and small amounts of potassium salts are still used for this purpose today. In glass making, potassium gives a hard, heat resistant glass. There is a range of other minor industrial applications. Potential uses include (i) water softening, to replace calcium and magnesium salts that make water ?hard? and (ii) as a de-icer. Using potassium salts for these two purposes would be more expensive than the alternatives, but the wastewater would contain a valuable plant nutrient. Potassium chloride (Muriate of potash, MOP) accounts for about 95% of all potassium fertilizers used in agriculture because it is the cheapest per ton and most widely obtainable. As fine crystals it can be readily incorporated into granular compound fertilizers or it can be compacted into suitable sized particles to be spread by machine or used in blends. Potassium Sulphate (Sulphate of potash, SOP) is more expensive per ton than muriate of potash, as it contains two nutrients, potassium and sulphur. It tends to be used for high value crops and those where it can be shown to improve the quality of the crop. It improves the burning quality of tobacco and it increases starch levels in potato tubers. Potassium Sulphate can also be used to advantage for all crops grown on saline soils, which occur in arid and semi-arid areas. Potassium nitrate also contains two nutrients, nitrogen as nitrate which is readily available to crops, and potassium. Both potassium Sulphate and potassium nitrate are used in fertigation systems, where nutrients are added to, and applied with, the irrigation water. A number of fertilizer manufacturers produce specialty fertilizers in which the ratio of potassium to other plant nutrients is adjusted to meet the specific needs of certain crops or the fertilizer has certain desirable physical features. In human body, potassium is involved in nerve function, muscle control an blood pressure. A diet low in K and high in sodium may be one of the factors that lead to high blood pressure. Athletes require larger quantities of potassium-rich foods. Potassium is found in bananas, apricots, oranges, dates, water melons, potatoes, sweet potato, spinach, carrots mushrooms, peas, lentils, peanuts, milk, yogurt, lean meat.
By : Rashad M. Balal, Dr. M. Mumtaz Khan, M. Adnan Shahid and Dr. M. A. Pervez
Institute of Horticultural Sciences, University of Agriculture Faisalabad