Abstract

Potassium chloride (KCl), muriate of potash, is the most commonly used K fertilizer in the world providing crops with two nutrients, K and Cl, essential for both plants and animals. On application to most soils, both these nutrients are readily available to crops. In arable cropping, KCl is applied with other N and P containing fertilizers frequently increasing the efficiency of their utilization. The physiological and biochemical roles played by K in crop plants are mostly well understood and have been extensively investigated. Potassium is known to activate more than 60 enzymes, has a direct function in the synthesis of protein, exerts a major influence on plant water relations, cell turgor and is essential in the process of growth and development of cells. Potassium also plays a key role in photosynthesis and the transport of resulting sugars together with amino N compound to developing fruits and roots. During recent years it has become increasingly clear that K has a major function in crop production in mitigating effects of abiotic and biotic stresses including salinity, cold, frost, waterlogging, and drought as well as insects, pests and various diseases. The detailed molecular and biochemical controlling mechanisms involved still require further elucidation. Potassium and nitrogen are the two nutrients that are taken up in the largest amounts by crops. There is an extensive literature showing these two nutrients act in partnership to have an enormous impact in determining crop yield and quality. When K supply is inadequate (i.e. the ratio of K/N supplied to crops is too low), low molecular weight sugars and amino acids accumulate rather than the formation of high molecular weight compounds including proteins, starch, polysaccharides and cellulose present in high quality crops. Crop yields and quality are correspondingly affected. The role of chlorine as an essential nutrient in various aspects of crop nutrition is also becoming increasingly evident. The chlorine content of the lithosphere is about 500 mg kg^-1 and occurs almost exclusively as the anion chloride (Cl^-). In general, irrigation water contains less than 150 Cl^- mg L^-1, which makes it suitable, for most crops providing that leaching can take place. Recommended amounts of KCl fertilizer applied to crops in field practice are in the range of 75-150 kg ha^-1 for field crops and 300-500 kg ha^-1 for horticultural crops. KCl is the only suitable form of Cl supply for irrigation and fertigation because of its high solubility. Chlorine is an unusual micronutrient in that Cl concentration in plants can vary enormously. It is required in the water splitting reaction of photosynthesis in the evolution of oxygen, charge compensation, and osmoregulation of the whole plant, as well as regulating movement of the stomatal guard cells of some crop species. Deficiency in crops usually only occurs in areas at great distance from the sea where the input from the atmosphere does not meet the demand, which for dryland wheat production is 4-8 kg ha^-1. It can also be a problem in crops such as kiwi fruit and palm trees that have a particularly high demand for Cl. Chloride toxicity occurs worldwide and is a general stress factor limiting crop growth in arid and semi arid areas. Crops differ markedly in sensitivity to Cl. Sensitive crops include: pepper, cabbage, lettuce, rape, tobacco, potato and sweet potato whereas a number of the major staple world crops are insensitive: rice, wheat, corn, sorghum, cotton, tomato, eggplant, banana and peach. It is well recognized that Cl can suppress a wide variety of plant pathogens in different crops. These include rust disease of barley (Puccinia hordei), stem blight disease of corn (Aspergillus niger), brown heart disease of potato (Cephalotrichum stemonitis), barley root rot (Cochliobolus sativus, Fusarium culmorum), and corn stalk rot (Fusarium spp). The soil borne fungus take-all in cereal crops (Gaeumannomyces graminis) is particularly sensitive to Cl. The reason for this appears to relate to its enhancing effect on Mn uptake and in increasing cellular osmotic pressure. In the context of human nutrition, the benefits of a balanced diet supplied with adequate amounts of K and Cl is discussed as well as the detrimental effects of excess NaCl.