Potash for sugar beet & root crop
Summary
- Potassium (K, potash: K2O) is essential for all crops and sugar beet requires large amounts.
- Soils should be maintained above the critical level for plant-available K, P and Mg to achieve optimum yields and quality of all the crops grown on the farm.
- For sugar beet, the critical level for potash is K Index 2-. Below this level, yield will be lost. Yields are not increased when soils are at K Index 3 and above.
- Crops need K where the roots can get it, i.e. throughout the soil explored by the roots. Sugar beet only very occasionally responds to freshly applied potash, and then only on soils with very low levels of available K (K Index 0). It is wise to regard applications of potash as part of the longer-term strategy to raise or maintain soil K at a suitable level rather than as a means of immediately increasing beet yields.
- Potassium in the ploughed-in tops is returned to the soil K reserve.
- To maintain adequate levels of soil K, the K removed in the harvested beet needs to be replaced on all soils below K Index 3. The quantity removed can be seen for individual contracts from British Sugar On-line.
- Sugar beet requires a small amount of sodium (Na). In addition sodium can substitute for some of the functions of potash; applying Na will be of benefit on K Index 0 and 1 soils and on soils low in available sodium.
REMEMBER: Once soil K has been adjusted to above the critical level it needs to be maintained at that level. This can be done by estimating the amount likely to be removed in the harvested beet before the sugar beet is sown or replacing the amount removed before the next crop is sown. Either way, the actual K status of the soil needs to be checked by soil sampling and analysis every 4-5 years. Because the K status of the soil can vary widely within a field, growers may wish to maintain their average soil K index closer to 2+.
The function of Potassium, Magnesium and Sodium in root crops
Potassium (K, usually referred to as potash: K2O) is an essential nutrient for all living organisms. For sugar beet, adequate amounts of K must be available in the soil for the uptake by roots to enable crops to achieve large yields of acceptable quality. Potassium also enhances the ability of plants to resist stress from diseases and pests, and cold and drought. Potassium performs these roles in all crops, so it is important to ensure adequate K for all crops grown on the farm.
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.
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.
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.
Use of Sodium (salt)
Sugar beet has a specific requirement for sodium (Na) in addition to K, for growth. The crop usually takes up about 50 kg Na/ha on a soil well supplied with K, an amount much less than that of K. Most of the Na is retained in the tops where it is mainly used to sustain the growth of the leaf canopy. On soils very low in K (Index 0-1), Na can replace K in its role of maintaining cell turgor. Sodium applied using Johnson’s K-Salt supplies both K and Na, and blended Mg and P if requested. JSE-Systems can also supply just straight salt if that’s all thats required.
It has been thought by some that adding salt to soil might damage soil structure, a view which stems from seeing the adverse effects on soil following prolonged flooding with seawater. There is no evidence that where salt is applied to soils at the appropriate recommended rate, it has any adverse effect, even on soils with little structural stability.
The BBRO Growers Guide & RB 209 Recommendations for Sodium
Potassium uptake & offtake by Sugar Beet
Potassium uptake (into the crop)
Sugar beet has a large requirement for K. Trials indicate that where maximum yields are achieved at soil K Index 2- (i.e. 120-180 mg K/litre) the corresponding uptake into the crop is 420-480 kg K2O/ha of which 90-100 kg K2O/ha is in the harvested beet and therefore removed from the field. On soils with plentiful K, the uptakes and offtakes of K can be much larger than this optimal requirement (i.e. up to 600 kg/ha in the crop and 150 kg K2O/ha removed, respectively). Extra K in the roots, however, does not increase clean beet or sugar yields; it serves only to decrease beet quality.
Potassium offtake (removal from the field)
Much of the K needed by the crop is taken up during the early months of growth. However, more than 70% of it remains in the tops at harvest, and will be returned to the soil if the tops are subsequently ploughed in. Although the amount of K removed in harvested beet is comparatively small, it still has to be replaced to maintain the soil at an appropriate K Index.
Therefore, current fertiliser policy should be based on the long-term aim of achieving and maintaining the soil at least at K Index 2-. For soils at Index 2- (120-180 mg K/litre) and 2+ (180-240 mg K/litre) only maintenance dressings need be given to replace the K removed in the beet. Where soils are at Index 0 or 1, extra K needs to be applied in addition to the maintenance dressing to build up soil reserves. On light textured soils, such as sands, that have too little clay to hold much K, the K will need to be applied for each crop in the rotation. On soils of K Index 3 and above, no maintenance or build-up dressings of K need be applied.
In the past the quantity of K removed in the harvested beet, which is needed to calculate the maintenance dressing (M), had to be estimated approximately, from the guideline that each tonne of beet harvested from the field contains 1.7 kg K2O. A new service is now available at British Sugar On-line that calculates how much K2O is present in beet delivered to the factory. The calculation normally relates to the contract as a whole but can be related to specific fields if growers schedule their deliveries on a field by field basis. This is a unique service for sugar beet growers which should help them ensure that the correct amounts of potash are returned to each field.
Fertiliser recommendations for Potash, Magnesium and Sodium
BBRO’s Growers Guide and RB 209 give the same recommendations for potash and magnesium applications, both of which are related to soil Index. The maintenance dressings (M) for K at soil Index 2- in those publications relate to replacement of offtake at a yield of 60 t/ha:
*This is a general estimate for the maintenance dressing. As indicated, individual growers can obtain more accurate offtakes from British Sugar On-line.^ Only is available sodium in the soil >20kg/litre
Timing of applications
K-Salt can be applied at anytime of the year prior to planting.
For soils at K Index 2- there are several potential benefits to be gained by applying the necessary K, Na and Mg in the late autumn and early winter prior to ploughing the land destined for the beet or root crop. Sugar beet has active roots at depth, but the main advantage of such early spreading is that the absence of newly applied nutrients in the seedbed is helpful to germination and the establishment of a good plant population. The fact that there will be no wheel-marks in the seedbed from the spreading and incorporation of these nutrients is another significant benefit.
JSE-Systems operate a fleet of high capcaity belt spreaders suitable for spreading K-Salt and can deliver up to 10t directly in the spreader; with in a 50km radius of Ancaster, Lincolnshire.
Typical analysis of Johnson's K-Salt & application rates
Example grades of Johnson's K-Salt blends
Nutrient values expressed as oxides