NK Arable Top Dressing

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 3  Potash offtake

Potash removal is often under-estimated as crop yields increase and straw is frequently removed, A typical 8.5 t/ha crop of winter wheat with straw baled removes 100 kg/ha of potash.

Standard figures for K₂O removal are:

	kg/t grain
CEREALS
Grain only
All cereals	5.6
Grain + Straw
Winter wheat & barley	11.8
Spring wheat & barley	13.7
Winter & spring oats	17.3
OILSEED RAPE
Seed only	11.0
Seed + Straw	17.5

 4  Low soil K

Around 25% of all arable soils are below the target level of index 2 (Representative Soil Sampling Survey data). This is often the result of management rather than of the inherent fertility and results from the under-estimation of removal referred to above.

Low potash is most likely on lighter soils and fertiliser applications need to provide more than is expected to be removed in the crop so that the extra helps to improve soil reserves. It is more practical to split the larger amounts required between basal applications and top dressing.

 5  Potash leaching

Small quantities of K (1-10 kg/ha) naturally occur in drainage from most soils but fertiliser potash loss by leaching is not significant from most arable soils. Only the K in soil solution is at risk of being lost from top soils and even then it may be held in sub-soil, depending upon clay content, from where deeper roots will retrieve it. Light shallow soils with low clay content (under 5%), especially under conditions of high and irregular rainfall, are most susceptible to leaching. Such soils are often naturally deficient in K and have greater fertiliser potash requirement. Potash is best applied “little and often” on these soils and single large dressings should be avoided.

"On such soils it is better to apply both N & K together rather than at separate times"
extract from a paper by A E Johnston & K W T Goulding , Rothamsted Research (See PDA leaflet 4 Potash Manuring for Arable Crops)