The Potash Development Association (PDA) is confirming that many farmers did not get fertiliser spread last autumn, due to the heavy rain at the time and the severe deterioration in ground conditions.
However, recent dry weather and good field conditions will prove an opportunity to catch up on any missed applications of potash.
Autumn dressings of phosphate (P) and potash (K) enable these nutrients to be worked into seedbeds to help distribute the nutrients through the cultivated layer, so they are available to the establishing crop (if autumn drilled) and available through the spring, when the uptake is greatest.
This does not mean, however, that if not applied in the autumn, the value of these inputs is reduced.
Potash applications
Only modest supplies of nutrient are needed for establishment and overwintering, according to the PDA.
Uptake then rises during vegetative growth in spring and is particularly large for K, 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 K 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 145kg/ha of potassium oxide (K2O) in the top 10cm.
This obviously 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 fulfill the total need.
The lower the level of soil K, the greater the need for fertiliser supplementation. Potash top dressing has an obvious place in such circumstances.
With a big increase in demand seen by all nutrients in the spring, applications of all nutrients, including phosphate and sulphur could also be beneficial where there is a demand.
Although P requirements are relatively modest in comparison to nitrogen (N) and K, its low soil mobility may improve the value of its application now, particularly on low index soils.
In contrast, the high mobility of sulphur (S) in the soil, combined with the well documented reduction in atmospheric sulphur deposition, has led to most soils benefiting from spring applications.
There are a multitude of product options for these important nutrients, however, outside of NS or NKS products, polyhalite provides an important source of sulphur, along with K, magnesium (Mg) and calcium (Ca).
