Arable News

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Long term trial suggests RB209 may be undercooking barleys K needs

Results from long term fertiliser trials suggest that current recommendations for potash on winter barley may be restricting yields.

The research – having just completed its eighth season – is examining the effects of potash alongside magnesium on a range of crops in a typical light land rotation in Norfolk.

In the previous seven years, the most consistent performance has come from winter barley crops, which are grown in rotation alongside sugar beet and vining peas.

Across all treatments annually applied potash and magnesium delivered mean yield responses ranging from 0.4-0.9t/ha in every one of the five years barley was grown.

Last harvest though, the trials picked out a clear response from higher rates of applied potash of up to 1t/ha and also delivered a bolder grain sample.

While a 75kg/ha rate – as typically recommended under RB209 guidelines for the soil and crop – gave a disappointing 0.1t/ha yield response, 125kg/ha of potash pushed yields up by 1.03t/ha compared to untreated controls.

This, says the company that sponsored the work, is a clear indication that long term use of potash to build soil fertility, combined with today’s higher potential varieties, can reap dividends.

The trials are part of an ongoing long term rotational study conducted for fertiliser business K+S UK & Eire, by independent trialling specialist Armstrong Fisher.

Each year, in similar fashion to the classical Broadbalk experiment at Rothamsted, the same plots received the same treatment. This helps build a picture of longer term fertility, compared to crops grown on soils where no potash or magnesium has been applied throughout the trial period.

Over the previous five years the trial was cropped with barley, 22 out of 25 fertiliser treatments gave a significant yield benefit over the zero K+Mg control of up to 0.9t/ha.

In 2015, however, a broader range of potash rates were applied for the first time and the use of 100 and 125kg/ha potash resulted in a £44 to £62/ha return over the fertilizer input cost in yield terms alone.

Grain quality was also improved with the higher potash rates lifting specific weights by 1kg/hl and TGW from 52 to 56g, at the same time as cutting the levels of screenings by 6%.

According to K+S UK and Eire managing director, Richard Pinner, the results suggest that the rate of potash applied can have an enormous effect on yield, quality and financial return.

“We know – from previous work by Rothamsted – that when potash levels are below target indices, around 30-50% of applied nitrogen is not utilised and in this trial, the higher K rates used in 2015 in this trial may have helped improve N utilisation.

“In addition, the value of higher levels of potash and magnesium is well documented in terms of driving forward growth and grain-fill, at the same time as counteracting abiotic stress such as disease and drought.”

“Looking at the wider picture, this data shows that on soils that don’t have the ability to hold either nutrient, that annual applications that counteract these issues help boost yield in barley and the same will almost certainly hold true for wheat as well,” says Mr Pinner.

Coming at a time when soil analysis data from NRM Laboratories suggests that one third of all arable soils sampled every year in the UK are deficient in potash and one in every five short of magnesium, yields are almost certainly being curtailed.

“Our advice going forward on light soils such as those used in this experiment is to use annual applications to keep crops on track. As a rule of thumb it would take 450kg/ha of potash (750kg/ha MOP) to raise the soil Index from 1 or 2-. Using RB-209 build-up application rates, this would take 15 years to achieve,” he says.

  • Written by: Farmers Guide
  • Posted:
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