Potash for Sugar Beet

Potash and sugar beet

During the last 20 years yields of clean beet and sugar in the UK have been increasing linearly, at an average annual rate of 0.48 tonnes per hectare of beet and 0.01 t/ha sugar. Now the aim is to increase the average yield from just over 50 t/ha to 70 t/ha beet (adjusted tonnes). This raises the question of whether the currently recommended amounts of potash are adequate to achieve this target, especially as the recommendations are largely based on results of experiments that are 30-40 years old. In this revised leaflet, we use data from recent experiments, in which yields exceeding 70 t/ha adjusted beet were obtained, to re-examine the recommendations for potash.

Chart 1: UK Yields of beet and sugar, 1980-2003

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Potassium is essential

An aerial view of the recent sugar beet trial plots at Rothamsted

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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.

It is now well established that plant-available reserves of potassium and phosphorus (P, phosphate: P₂O₅) can accumulate in most UK soils. Applications of both fertilisers and organic manures containing P and K contribute to these reserves. Maintaining an appropriate level of plant-available P and K in the top-soil is important because it ensures that growing roots can access sufficient P and K to meet the changing needs of a crop during the growing season. These reserves can be estimated by soil analysis and a soil can be assigned to a P and K Index, as in RB 209. The Index relates to the soil and not to the crop. Its value indicates the level of plant-available K in the topsoil with the number increasing as the level of plantavailable nutrient increases (see leaflet 24 Soil Analysis).

The needs of sugar beet for potash and its responses to this nutrient are summarised on the next page. Each of these points is discussed in more detail in the following sections while the role of K in plants is addressed at the end of the leaflet.

Some of the recent data used here are from six years of trials on two sites which have soils with a wide range of long-established K Index values within a single field. One is on the silty clay loam at Rothamsted, the other on the sandy loam at Woburn. The aim was to check whether modern high-yielding varieties of sugar beet require more K to achieve maximum yields of beet and sugar. The experiments were funded by the British Beet Research Organisation (BBRO) and done by British Sugar plc and Rothamsted Research.