One-third of UK oilseed rape crops magnesium deficient

Oilseed rape growers could boost their yields by 0.2t/ha by using magnesium fertiliser, while treated urea fertiliser is shown to be just as good as ammonium nitrate, Richard Allison takes a look

Applying magnesium fertiliser to oilseed rape crops on soils low in the nutrient can increase yields by 0.2t/ha, according to new results from an extensive six-year trial.

Magnesium is usually included in soil analysis reports, but is often ignored. Although it’s two years since DEFRA published its RB209 Fertiliser Manual, many growers are unaware of the increases in the magnesium threshold.

The nutrient is a key component of chlorophyll and, therefore, is crucial for photosynthesis. It is also required for seed filling and in high risk situations including dry and cold conditions, sandy soils, and soils with excessive levels of potash and high pH.

Many view magnesium as a secondary or a micro nutrient, but it really is a major nutrient, says Jerry McHoul, technical manager at fertiliser group K+S UK & Eire.

“An oilseed rape crop typically takes up 30-50kg/ha MgO in a year,” he says.

“Previously, only growers on index 0 soils were recommended to apply 50-100kg/ha every three to four years. But this now also applies to index 1 soils,” he adds.

The Professional Agricultural Analysis Group (PAAG) survey of soils in 2011 showed that one-quarter of the UK arable area is index 1 or less. Therefore, Mr McHoul estimates that about 160,000-170,000ha of oilseed rape could be potentially underperforming.

To test this, he commissioned independent trials consultants Armstrong Fisher to carry out trials across the key oilseed rape growing areas of the UK on index 1 soils that were not receiving any magnesium fertiliser.

All crops received a generous rate of potash (DEFRA recommendations plus at least 50%) to ensure it was not limiting.

The mean response to modest base applications of solid magnesium fertiliser as kieserite applied to match crop offtake across the six years was 0.13t/ha averaged over the 10 sites.

Foliar applications were made during times of stress with 25kg/ha split into two doses. This pushed the response up to 0.21t/ha.

“Responses to foliar magnesium tend to be greater when root volume and expansion are being limited by soil compaction or water logging. So it could have a key role in wet conditions, such as those being currently experienced in crops,” he says.

Austrian data

  • Foliar applications of magnesium can even give yield responses in magnesium-rich soils when crops are under stressed conditions.
  • Mr McHoul highlighted data from one Austrian farmer, who saw an average response of 0.7t/ha over the last 15 years with magnesium sulphate. “Responses tended to be greater in drier years, because stress limits root uptake.”

More on this topic

Is magnesium getting the attention it deserves?

Richard Allison on G+