UNDERSTANDING POTASSIUM AND ITS USE IN A

1 European Fertilizer Manufacturers AssociationUNDERSTANDINGPOTASSIUMAND ITS USE JOHNSTON cover POTASSIUM new 21/02/03 11:03 Page 2 The plants grown in our diverse European landscape need a variety of nutrients. One of theessential nutrients is POTASSIUM . UNDERSTANDING POTASSIUM and its Use in Agriculture explains the importance of POTASSIUM inplant nutrition and the diet of humans and animals, and describes the role of POTASSIUM basedfertilizers in European IN AGRICULTUREE uropean Fertilizer Manufacturers AssociationEuropean Fertilizer Manufacturers' AssociationAvenue E. van Nieuwenhuyse 4B-1160 BrusselsBelgiumTel + 32 2 675 35 50 Fax + 32 2 675 39 61E-mail Potash Development AssociationBrixtarwLaugharneCarmarthen SA33 4 QPWales, United KingdomTel and fax +44 1994 427 443E-mail byWith the support 21/02/03 11:00 Page 1 Alchemist symbol for was first isolated in 1807 by Sir Humphrey Davy.

2 It is a soft, silver white metal that reacts so violently with waterthat the metal does not occur in nature. The chemical symbol forthe element, K, derives from kalium, the Latin version of the Arabicword for alkali. In agriculture, POTASSIUM is often referred to aspotash. This name derives from the ancient practice of obtainingpotassium salts by burning wood, extracting the ash with water andevaporating the resulting solution in iron pots hence pot-ashes .The resulting solid would be a mixture of POTASSIUM salts, mainlypotassium carbonate, chloride and IN AGRICULTUREC hemical symbol for 21/02/03 11:00 Page 2 IntroductionPOTASSIUM in PLANTS, ANIMALS and HUMANSThe importance of POTASSIUM to in the human in drinking in NATURE and its USE TO MANR eserves and resources of main uses of in AGRICULTUREI nputs of POTASSIUM to which contain of POTASSIUM POTASSIUM and SOILP lant physical properties and root behaviour of POTASSIUM in POTASSIUM and its availability to soil for readily available values for readily available soil and SOME ISSUES RELATED to its USE in AGRICULTUREM ovement of POTASSIUM in and from and organic in AGRICULTURE and the ROLE of the FERTILIZER INDUSTRYThe nutrient requirement of concepts in the management of POTASSIUM in plant nutrient use of IN 21/02/03 11.

3 00 Page 3 Acknowledgements, Sources and ReferencesEdited by Jackie Burton (EFMA) with scientific assistance from John Hollies (PDA), Adolf Krauss(IPI), Ingrid Ste n (Kemira Agro Oy), International Fertilizer Industry Association fromAmazone, BASF AG, Cleveland Potash, EFMA, Hydro Agri, Institute of Grassland andEnvironmental Research (IGER), Kemira, K+S Aktiengesellschaft, Lemken, Macaulay Institute,Potash Development Association (PDA), and Rothamsted IN 21/02/03 11:00 Page 45 POTASSIUM IN AGRICULTUREAs a major constituent within all living cells, POTASSIUM is an essential nutrient and isrequired in large amounts by both plants,animals and humans. Humans obtain themajority of their POTASSIUM either directly fromplants or indirectly through the animal productsin their ranks seventh in order of abundancein the earth s crust. As rocks slowly disin-tegrate, POTASSIUM is released, but the rate ofrelease is frequently too slow to provide thelarge amounts of this essential nutrientrequired by crops.

4 POTASSIUM fertilizers, minedand refined from naturally occurring deposits,are available to supplement soil potassiumsupplies so that crops can produce econom-ically viable yields and soil fertility can plants, animals and humans, potassiumalways occurs as a positive ion (K+) in the liquidin every living cell. It is also found as the K+ionin soils, many rocks and seawater. Potassiumions are always balanced by an equal numberof negative ions. For example, solid potassiumchloride, KCl, when dissolved in water,dissociates (breaks apart) into equal numbersof K+and Cl-(a negative ion).In this booklet POTASSIUM will be used whenreferring to the element, and potash (K2O,the oxide of POTASSIUM ), when reference ismade to the POTASSIUM content of quick conversion, 1 kg K is equivalent kg 21/02/03 11:00 Page 56 POTASSIUM IN 21/02/03 11:00 Page 67 POTASSIUM IN AGRICULTUREThe importance of POTASSIUM to plantsPotassium has two roles in the functioning ofplant cells.

5 First, it has an irreplaceable part toplay in the activation of enzymes which arefundamental to metabolic processes, especiallythe production of proteins and sugars. Onlysmall amounts of POTASSIUM are required forthis biochemical , POTASSIUM is the plant-preferred ionfor maintaining the water content and hencethe turgor (rigidity) of each cell, a biophysicalrole. A large concentration of POTASSIUM in thecell sap ( the liquid inside the cell) createsconditions that cause water to move into thecell (osmosis) through the porous cell wall(Box 1). Turgid cells maintain the leaf s vigour so thatphotosynthesis proceeds in plants, animals and humansOsmosis in plantsOsmosis is defined as the passage of waterfrom a region where the concentration ofsalts is low through a semi-permeablemembrane to a region where theconcentration of salts is higher.

6 In plants,water moves from cells with a low, to thosewith a higher, concentration of salts. Thisprocess is responsible for the movement ofwater within the plant and also the uptakeof water from the soil by the roots. Theseconditions that lead to water movement aretermed the osmotic potential . Much of thepotassium in the plant is in the liquid withinthe cell and its presence increases the saltconcentration and thus controls themovement of plants take up water by osmosis theircells become gradually more turgid (swollen)until no more water can enter the is very important to plants becauseit maintains the rigid structure of mostannual crops which do not have a woodystructure of trunks and branches. In allplants, turgid leaves have a large surfacearea and this optimises the process ofphotosynthesis in the green chloroplasts inthe 1 POTASSIUM is essential for growth and 21/02/03 11:00 Page 78 POTASSIUM IN AGRICULTUREP hotosynthesis is the process by which plantsharvest the energy of sunlight to producesugars.

7 These sugars contain carbon derivedfrom the carbon dioxide in the air that hasentered the leaf through the stomata, tinyopenings mainly on the underside of the tiny openings are surrounded by guardcells and it is only while they are turgid thatthe stomata remain open and carbon dioxidecan pass through into the leaf. But most of thewater transpired by the plant is lost throughthe stomata when they are open. Thus, if thereis a water deficit, the plant needs to close thestomata to conserve water. The plant controlsthe opening /closing of the stomata byregulating the concentration of POTASSIUM inthe guard cells. A large concentration ofpotassium ensures turgid cells and openstomata. When the POTASSIUM in the guardcells is lowered, they become limp and thestomata close. A high osmotic potential in plant cells is alsoneeded to ensure the movement through theplant of nutrients required for growth, and thesugars produced by photosynthesis, forexample, the transport of sugar to grains, beetroots, tubers, and fruit.

8 By maintaining the saltconcentration in the cell sap, POTASSIUM helpsplants combat the adverse effects of droughtand frost damage and insect and diseaseattack. It also improves fruit quality (Box 2)and the oil content of many oil-producingcrops. Balanced fertilisation ensures that the plant hasaccess to an adequate amount of each nutrientand is essential to optimise yields and, whereappropriate, minimise environmental POTASSIUM also ensures that otherinputs required to achieve optimum economicyields are used efficiently. This applies especially to the use of nitrogen(Box 3). POTASSIUM ensures the turgor, or rigidity of plant the guard cells surrounding the stomata are rigidthe stomata remain open, allowing carbon dioxide to passinto the leaf where the carbon is converted to beet well supplied with POTASSIUM has a large leafarea to produce sugars.

9 Lack of POTASSIUM leads to severewilting (as below).Stomata (plant leaf pores) natural size mmK+ 21/02/03 11:00 Page 89 POTASSIUM IN AGRICULTUREWith an adequate POTASSIUM supply, increasedyields with nitrogen are accompanied by largeramounts of nitrogen in the crops and thussmaller residues of nitrate in the soil at harvestat risk to loss. Plant roots take up POTASSIUM from the waterin the soil (the soil solution) when growingvigorously. A rapidly growing cereal crop willtake up as much as 6 kg K per hectare per day(ha, 10,000 m2) (Box 4), and sugar beeteven more, up to 8 kg K per ha. If this rate ofuptake is to be maintained, the POTASSIUM inthe soil solution has to be replenished quickly,which is only possible if the soil containssufficient readily plant available reserves ofpotassium. Most of these reserves haveaccumulated from past applications offertilizers and manures and they must bemaintained by applying fertilizers or manurescontaining of POTASSIUM on yield and quality of orangesK2 OFruit Yield Juice TSS 1 Acidity Vitamin Cappliedweightg/tree g kg/tree % % % mg/100ml0 total soluble solids (personal communication from Desai et al.)

10 , 1986)Box 2 Plant available POTASSIUM and the uptake of nitrogenAdequate amounts of POTASSIUM must be readily available in the soil for plant uptake tomaintain cell turgor and efficient photosynthesis. If there is not sufficient POTASSIUM , nitrogenwill be used inefficiently, as shown by the yields of spring barley. With too little readily plant available potassiumin the soil, it was only justified to apply 50 kgnitrogen per hectare (kg N/ha) but withadequate POTASSIUM , 100 kg N/ha gave theoptimum yield. When nitrogen does not increase yield becauseof lack of POTASSIUM , the excess nitrogenremains in the soil after harvest as nitrate, atrisk to 3050100150543201 Grain t/haNitrogen applied, kg/haReadily available soil POTASSIUM statusAdequate 21/02/03 11:00 Page 910 POTASSIUM IN AGRICULTUREP otassium in the human dietHumans and animals need to obtain anadequate supply of POTASSIUM from their foodand feed to be healthy and grow normally.