NK Arable Top Dressing

 6  N & K partnership

High yielding crops need large supplies of both nitrogen and potash and it is no accident that the addition of N stimulates the need for and uptake of K. Nitrogen is mostly taken up as the negatively charged anion nitrate (NO₃^-) and to maintain neutrality the plant needs to take up equal amounts of a positively charged cation. Potassium as K⁺ is the preferred cation. Where K⁺ may be in short supply from the soil there is obvious benefit from top dressing with nitrogen plus potash.

Within the plant, synthesis of N to simple amino compounds and ultimately to protein depends upon an adequate supply of K thus providing further support for ensuring N and K supply by top dressing.

Full response to nitrogen will thus not be obtained where potash is limiting and this will result in lower yields and poorer crop performance.

 7  P & K balance

For the great majority of cereal crops, potash removal is considerably greater than that of phosphate and this should be reflected in fertiliser use. Potash top dressing offers a convenient way of maintaining the correct replacement balance in a number of cases:

	On light soils where soil P levels are satisfactory but K levels need to be improved:
		Use basal PK to provide necessary phosphate (most beneficial at establishment) and some potash. Plus NK top dressing to make-up potash needs.
	On soils of good phosphate and potash fertility:
		Use NK top dressing to replace some of the potash removed. Omit phosphate altogether if soil index is sufficiently high.
	On well established crops where no basal fertiliser was used because of weather etc:
		Use NK top dressing for speed of work and to minimise number of different products. Make-good phosphate need in seedbed of following crop.

		Action
			Identify the potash needs for each field according to soil analysis and expected removal.
			Plan autumn seedbed fertiliser on basis of soil P and K levels, soil texture/depth and workload.
			Top dress with N and K before the end of March for cereals and rape as necessary.
			Apply 35-70 kg/ha potash depending upon the specific need.