Arable farmers looking to maintain profits by cutting costs, should think again before cutting baseline plant nutrients such as potash, suggests a leading fertilizer manufacturer.

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 denude soil fertility.

He warns those that think that nitrogen alone can drive yield are fooling themselves and that this year’s high yields will have further removed soil reserves, especially where straw has been carted away.

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 and that the gap between N and P+K is, as the report states: ‘too wide to be sustainable in the long term’.

The data reports surveys carried out by Defra in 2014 of 6000 farms shows that while N-use has remained fairly constant at a mean 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.

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 3 times as much N is applied compared to 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.

GrowHow trials at Rothamsted demonstrated that around 80% of all applied nitrogen can be utilised 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 50% of applied nitrogen was used by the crop.

In these trials, the difference between the best N use efficiency and the worst was worth almost 100 kg N/ha resulting in obvious environmental and financial losses. Factor in the lost yield potential of over 2 tonnes of cereals or a tonne of OSR that it could generate, and the financial consequences are still greater.

Mr Pinner’s advice this autumn is to look long and hard at other major nutrient supply issues. “Get soil indices tested and ensure that P, Mg and in particular K are not limiting and armed with this, identify which nutrients you require to correct soil fertility issues and select a product or blend of products to closely match the fields requirements.

“The costs of applying balanced base fertilisers targeted to individual fields is a small price to pay for better N-utilisation to drive forward yields and the large returns likely this season,” he says.

“While yields have gradually increased since the 1970’s through better variety choice and crop husbandry, the continued ‘mining’ of nutrients from the soil from these larger crops isn’t sustainable and growers must redress this imbalance.”