Potash and a new season

Nutrient uptake pattern of a typical 8 t/ha wheat crop, showing the uptake of potash to be considerably greater than for nitrogen or phosphate.

'Oilseed Rape and Potash'

The revised version of PDA Leaflet 13 'Oilseed Rape and Potash' includes the useful illustration of the pattern of the high uptake of potash (250-300 kg K₂O/ha) by winter rape during the growing season. Also the new cover illustration shows a large proportion of sky, as a reminder that all which is written concerning the use of potash for oilseed rape also applies equally well for rape grown for biofuels and other industrial (non-food) uses!

Soil sampling to check P, K and Mg reserves, and pH

The soil sampling season is upon us again; the importance of adequate potash reserves in soil cannot be overstated. While a standard maintenance application of potash (K₂O) should be enough to replace the potash removed in the harvested crop (offtake, see right), the quantity of K which must be available to the growing crop (uptake) is much greater. As can be seen in the chart below, for an 8-tonne wheat crop the peak potash content in the crop is about 300 kg K₂O/ha. This large amount of potash has to be available to the growing crop from soil reserves, and unless these are adequate and capable of supplying this potash - maybe at 10 kg K₂O/ha/day - the crop will not perform to its full potential. Average soil P and K reserves for the field should not really be allowed to fall below the mid point of Index 2, allowing for the variability within the field. Indeed where crop yields are known to vary within a field, it is important to check that the soil P and K status is adequate in the high-yielding areas, because higher than average yields remove proportionally more nutrients and soil reserves can become more depleted in these areas.

This principle does not just apply to wheat of course, but is relevant for all crops; soil P and K reserves must not be allowed to fall below Index 2. Among other functions, potash is critical for efficient nitrogen utilisation, and the chart (above left) illustrates the effect of a deficiency of potash on nitrogen use efficiency.

See PDA Leaflet 24: 'Soil analysis: key to nutrient management planning'

Crop offtakes of phosphate and potash, i.e. removals

The information in the chart reproduced below was sent out in a PDA leaflet last year 'Phosphate and Potash Removal by Crops' which can also be printed from the PDA website. These values form the basis of the maintenance (M) fertiliser recommendations in RB209, and represent an average removal rate of nutrient per tonne of harvested crop.

For further information please contact:
PDA info@pda.org.uk
Potash Development Association

update: June 2006