The application of potash and phosphate fertilisers when planting a winter cereal enables these nutrients to be worked into seedbeds in an effective manner.
In turn, this helps distribute the nutrients through the cultivated layer of the soil .
As a consequence, they are available to the establishing crop (if autumn drilled) and available through the spring when the uptake is greatest.
The excellent planting conditions of last autumn may well have allowed many tillage farmers to include potash (K) and phosphate (P) fertilisers to the newly established seed beds.
However, according to the Potash Development Association (PDA), this does not mean that the value of these inputs is reduced if applied during the early spring period. .
Only modest supplies of nutrients are needed by any for establishment and overwintering.
Uptake then rises during vegetative growth in spring and is particularly large for potash with levels peaking in the plant around flowering and then declining as older leaves are lost and as the plant matures towards harvest.
Peak uptake of potash by high yielding cereals is around 250kg/ha: for oilseed rape crops it is 250-300kg/ha.
Field trials have indicated that as a generalisation potash supply will be adequate from soils at Index 2.
Soils at the lower end of this index contain 120mg/l of available K, which is equivalent to 145Â kg/ha of potassium oxide in the top 10cm.
This falls well short of the total needed by high yielding crops and has to be supplemented by potash from greater depth and on heavier soils also from release of non-exchangeable soil K.
On lighter, shallower soils it may not be practical to raise soil K to this target level and soil supplies may thus not be adequate to fulfil the total need.
The lower the level of soil K, the greater is the need for fertiliser supplementation. Potash top dressing has an obvious place in such circumstances.
With a big increase in demand in the spring, applications of all nutrients, including phosphate and sulphur could also be beneficial where there is a demand.
Although phosphate requirements are relatively modest in comparison to nitrogen and potash, its low soil mobility may improve the value of its application, particularly on low index soils.
In contrast, the high mobility of sulphur in the soil, combined with the well documented reduction in atmospheric sulphur deposition, has led to most soils benefiting from spring applications.