Cutting back on P and K fertiliser affects next year’s yields

A combination of this year’s high yields and a continued neglect of potash risks next year’s crop yields, as well as getting the most from nitrogen fertilisers.

That’s the warning from a nutrient specialist, who urges growers to think again before cutting baseline plant nutrients such as potash.

According to K+S UK & Eire, managing director Richard Pinner, industry figures released this summer, suggest that any further cut in potash use will render applied nitrogen less efficient and continue to affect soil fertility.

See also: Earlier nitrogen application bumps up winter barley yields

He adds that this year’s high yields will have further removed soil reserves, especially where straw has been carted away.

Declining use

Mr Pinner points out that the recently published Professional Nutrient Management Group’s report on nutrient management planning shows that both phosphate and potash use is falling behind that of nitrogen.

The report states the gap between N and P+K is “too wide to be sustainable in the long term.”

The data reports surveys carried out by Defra in 2014 of 6,000 farms shows that while N-use has remained fairly constant at an average of 150kg/ha across arable land over the last five years, peak use of potash and phosphate has decreased significantly.

The figures reveal that potash use has halved since the early 1980s with the result that only 10% of arable soils are at target indices for both phosphate and potash.

It shows that 31% of arable soils are, for example, below the target 2 minimum K index.

Nitrogen efficiency

In addition, while in the 1970s, the average N:K ratio applied to arable crops in England and Wales was 1.67:1, now, over three times as much N is applied compared with K. 

“Given that many crops require similar levels of potassium as they do nitrogen, these inputs are clearly out of sync and failure to redress this balance will continue to cut bottom-line margins considerably,” says Mr Pinner.

He points out that the yield penalties associated with nitrogen use in the absence of sufficient potash are well documented and have been widely reported by researchers in the past. 

Trials at Rothamsted demonstrated that around 80% of all applied nitrogen can be used efficiently by a winter wheat crop. 

But where potassium is deficient at index 1, N utilisation fell to 70% and where the K was at index 0, less than half of applied nitrogen was used by the crop.

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