Potash for Cereals

Introduction

There is tremendous pressure on cereal farmers to reduce costs. Expenditure on fertilisers ranges between £60-100 per hectare and this represents around a third of total cereal variable costs making this an important area for potential cost cutting. Basal P and K are particular targets because reductions in use may have little or no visible effect in the short term. This leaflet provides a comprehensive review of phosphate and potash nutrition for cereals and gives detailed recommendations for optimum economic fertiliser usage.

National balance sheet for basal nutrients

Since around 1997, average potash usage in England and Wales has been less than potash removed by grain and straw resulting in an inevitable reduction of soil potash reserves. A similar imbalance has also developed for some other main arable crops and this also applies to phosphate. These trends have continued and will lead to serious consequences for yield and quality. The length of time before financial penalties occur will depend upon soil type and the P and K reserves of individual fields.

The area of cereals receiving no P or K at all has also increased significantly in recent years - in 2002/03, 40% of the winter wheat area sown received no P or K. As only around 25% of the soils growing cereals are at an index where potash can safely be omitted, this is further evidence of imprudent cost cutting.

It must not be assumed that because cereals are known to be efficient scavengers of P and K and because no visual symptoms are seen, that such cost cutting is without penalty. Serious economic consequences can occur with out any visible symptoms of deficiency. Soil P and K values may decline only slowly but as they fall below accepted threshold levels, the risk of yield and quality losses increases. Such risks are not worth taking because of the essential requirement of these nutrients for the maintenance of yield and quality.

Why potash is important

Potash affects both yield and quality of grain as well as the general health and vigour of the plant. Cereal crops need at least as much, if not more, potash than any other nutrient including nitrogen. Potash is needed in such large amounts because it is the major regulator of solution concentrations throughout the plant. It controls cell sap content to maintain the turgor of the plant and supports the movement of all materials within the plant. Potash supply is thus essential for all nutrient uptake by the roots and movement to the leaves for photosynthesis, and for the distribution of sugars and proteins made by the green tissue for plant growth and grain fill.

The practical implications of shortage of potash are summarised in the following table: