NK Arable Top Dressing

The general importance of potassium

Inadequate potassium supply to crops will result in serious penalties. Unfortunately low soil K cannot be "seen" and crop deficiency symptoms may not be identifiable as K shortage or not be evident until too late to remedy.

Penalties of low K

• Serious yield loss especially in adverse growing conditions eg dry summers.
• Weaker growth resulting in higher lodging risk.
• Lower crop vigour giving greater susceptibility to disease, pests and weed competition.
• Poor grain sample - low grain size, poor specific and 1000 grain weights, more tailings.
• Inefficient N uptake and utilisation leading to poor N response and less protein synthesis.
• Adverse effect on marketability.
• Lower tolerance to physiological stress - wet, drought, frost, wind.

Individually these effects may be slight but overall the result is lower crop value and higher costs per tonne.

Potash top dressing

Potash top dressing can be a useful strategy for a number of practical and technical reasons, particularly for winter cereals and oilseed rape.

 1  Timing of nutrient uptake

Only modest supplies of nutrient are needed 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 250 kg/ha; for oilseed rape crops it is 300-350 kg/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 120 mg/l of available K which is equivalent to 145 kg/ha of K₂O in the top 10 cm. 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 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.

 2  Rate of uptake

Apart from the total peak requirement the rate of crop demand for potash is also important and may be as high as 5 or even 10 kg/ha/day for cereals and oilseed rape during the period of rapid growth. Low K soils and particularly lighter soils may not be able to supply potash at these rates. Provision of extra potash ahead of this period by top dressing obviously helps to fill this need.