Potash for Sugar Beet

Function of potassium, sodium and magnesium in crops

Potassium has several roles in plants. It is essential for the activation of some enzymes within the cell. Only a small proportion of the total K in plants is required for this, perhaps 20% in cereals for example, but in these roles it cannot be replaced by any other element. Much more K is required to fulfil other major functions in plants. In these roles the plant prefers to use K rather than other elements like sodium and calcium. For this reason the plant must have a sufficient supply of K available to it because it will take up large quantities of K from the soil during growth.

Potassium is essential for growth and is the main element used to maintain cell turgor (rigidity) and to regulate the water content of the plant. Potassium is also involved in controlling the opening and closing of the stomata (the openings in leaves through which water is lost by transpiration and carbon dioxide enters the leaf to be converted to plant dry matter and sugar).

The importance of cell turgor cannot be over-emphasised. For maximum sugar yield it is essential to optimise the interception of solar radiation (sunlight) to provide the energy for the conversion of carbon dioxide to sugars. This requires a rapidly expanding leaf canopy. Nitrogen (N) supply drives the rapid production and expansion of cells. These rapidly expanding cells require K to obtain water and maintain their turgor. Green plants often contain more K than N.

Potassium also has another equally important role in the transfer of sugars produced in the leaves to the storage root. In its passage from leaf to storage root each molecule of sugar has to pass through innumerable cell membranes, and K+ ions are an essential component of the 'molecular pump' within the cell membranes that facilitate this passage.

	Leaves of sugar beet showing the sympotms of potassium deficiency
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The role of sodium (Na; common salt is sodium chloride, NaCl) in sugar beet nutrition is interesting. At one time it was thought that K and Na were totally interchangeable in their functions in the nutrition of beet, but recent work shows that this is not entirely correct and that there are important differences. Sugar beet has a specific, irreplaceable requirement for Na and usually takes up about 50 kg/ha on a soil well supplied with K. Most of this Na is retained in the tops where it is used to mainly to sustain leaf growth and canopy development. A rapidly expanding leaf canopy is essential to intercept incoming radiation effectively and to drive sugar production.

click to enlarge

The other role for Na in beet relates to it being descended from wild beet that grew in maritime environments where there were large amounts of Na and little K. In this environment the plant adapted to use Na as well as K to maintain cell turgor and would take up proportionally more Na than K. Modern cultivars of sugar beet descended from these wild forms have retained the ability to use Na as well as K in regulating cell turgor. Thus today on K Index 0 and 1 soils that contain too little K for maximum yield the plant will take up Na early in growth to make up for the limited supply of K. This will enable the regulation of cell turgor and the water content of the crop to optimise the rapid development of the leaf canopy. Where there is certainty that the plant-available supply of K is adequate throughout the depth of soil explored by roots then this additional Na is not needed by the crop as a replacement for K. However if it is present in soil then, like K, it has no adverse environmental impact.

Magnesium (Mg) is an essential constituent of chlorophyll, which gives leaves their green colour. Chlorophyll is involved in the conversion of carbon dioxide to sugars and Mg also has a role in protein synthesis. Soils at Mg Index 2 and above usually contain sufficient plant-available Mg to meet the needs of the crop. On soils of Mg Index 0 and 1, Mg should be applied. When adequately supplied with Mg there is about twice as much in the tops as in the roots, so most of the Mg taken up by the crop is returned to the soil when tops are ploughed in.