Potash for Sugar Beet

Potassium uptake (into the crop)

Sugar beet has a large requirement for K. Chart 3 shows the total crop uptakes and offtakes of K in the harvested beet of the crops whose yields are shown in Chart 2. Where maximum yields were achieved at soil K Index 2- (i.e. 120-180 mg K/litre) the corresponding uptake into the crop was 420-480 kg K₂O/ha of which 90-100 kg K₂O/ha was 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.

Chart 3: Potassium uptakes and offtakes vs. soil K status on a silty clay loam at Rothamsted (solid symbols) and a sandy loam at Woburn (open symbols).

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Photograph of a K- deficient plot on the left compared with a correctly fertilised plot, taken during the recent trial work at Rothamsted

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Potassium offtake (removal from the field)

Much of the K needed by the crop is taken up during the early months of growth (Chart 4). 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.

Chart 4: The uptakes of potassium into the sugar beet crop during growth.

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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:

Recommendations		Soil K or Mg Index
		0	1	2-	2+	3	4	5
Potash	kg K2O/ha:	150	125	100M*	75	0	0	0
Magnesium	kg MgO/ha:	150	75	0	0	0	0	0

* This is a general estimate for the maintenance dressing. As indicated, individual growers can obtain more accurate offtakes from British Sugar On-line.

The BBRO Growers Guide and RB 209 also give the following recommendations for sodium:

Recommendation		Available sodium in soil, mg/litre
		less than 20	20 - 40	over 40
Sodium	kg Na2O/ha:	200	100	0