Potash for Sugar Beet

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+.

In the experiments at Rothamsted and Woburn, yields of clean beet and sugar increased as the availability of soil K (K Index) increased. Maximum sugar yield was achieved on plots which were at K Index 2- or above in all six years, both on the silty clay and the sandy loam soil. Sugar yields were greatly decreased when the K Index was below this critical value because the lack of plant available K decreased both the yield of clean beet and the sugar content (% sugar).

The responses of sugar beet to soil K (K Index) in these recent, highly definitive experiments are very similar to those from experiments done in the 1970s. They show that the existing recommendations on K fertiliser use in RB 209 and the BBRO Growers Guide do not need to be changed greatly for modern, higher yielding sugar beet crops.

In these experiments, as well as British Sugar's more extensive trials on soils of widely ranging K Index, there were no yield responses to freshly-applied fertiliser K at any level of soil K, even on low Index soils on which responses might have been expected. In part this is because it is difficult to mix freshly applied K evenly into the top soil by cultivation.

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Potash for Sugar Beet