Introduction

There is always pressure on cereal farmers to reduce costs. Typical expenditure on fertilisers for cereals is usually around a half of total cereal variable costs making this a common target for cost cutting. Basal phosphate and potash are particular targets because reductions in use that damage yield 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.

Potash application is not matching offtake

Since the mid - 1990s, 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 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, manure use and the phosphate and potash reserves of individual fields.

Overall annual potash balance in England & Wales for cereals, oilseed rape, potatoes and sugarbeet (excluding manure inputs).

click to enlarge

The area of cereals receiving no annual phosphate or potash at all has also increased significantly in recent years with over 50% of the winter wheat area sown receiving no fertiliser phosphate or potash. As only around 20% of arable soils are at an Index where potash can safely be omitted for cereals, this is further evidence of imprudent cost cutting.

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

Potash affects grain yield and quality, crop vigour and drought resistance

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:

Deficient K			Satisfactory K
Low yield			No yield restriction
Inefficient N response			Full N response
Increased risk N loss			Minimum N loss
Reduced 1000 grain and specific weights			Full 1000 grain and specific weights
Reduced grain ripening period			Maximum grain ripening period
Fewer grains per ear			Full grain number per ear
Poorer grain sample			Normal grain sample
Weaker straw			Normal straw strength for variety
Increased risk of lodging			Lodging risk normal for variety
Increased susceptibility to drought			Normal drought resistance
Increased disease susceptibility			Normal disease susceptibility