Despite oilseed rape being deemed less susceptible to magnesium deficiency, experience from the Yield Enhancement Network (YEN) indicate good magnesium nutrition could be critical for achieving high yields.
Reviewing oilseed rape yield data from more than 100 YEN entrants over two years, to determine factors associated with high yields, Dr Pete Berry, head of crop physiology at ADAS, says there was an association between yield and magnesium levels.
He says: “Both magnesium index of the soil and the concentration of magnesium in the seed of the crop showed positive associations that were consistent in two seasons, and we think could be important."
Dr Berry says growers perhaps should be looking at foliar applications during the season during stem extension, and into flowering to make sure the crop does not suffer any transient deficiency.
“The current RB209 guidance is that soils with Mg Index 0 should have 50-100 kg MgO/ha every three to four years,” Dr Berry says. “Current questions are whether crops will respond additionally to foliar magnesium delivered via a different means rather than solid fertilisers rotationally, which is often the current method, and whether OSR crops need more magnesium than is currently recommended.”
David Booty, technical development manager at Omex, says he is not surprised to hear this because magnesium plays a part in a lot of different vital systems within the plant.
He says: “Magnesium is important for canopy and seed formation, so the demand is going to tend to be later in the season.
“In a year like last year, where it was very dry for a long period during seed formation, even if there is enough magnesium in the soil and you put enough fertiliser down, there might be a problem with the plant accessing that.”
Magnesium deficiency in oilseed rape can be hard to spot, and by the time such symptoms are visible, the plant will be have suffered, says Mr Booty.
He adds: “It is fairly unusual to see actual symptoms of magnesium deficiency. It tends to affect the older leaves first – it is an important ingredient in chlorophyll for photosynthesis, so you get pale areas appearing on the leaves.
“Generally speaking the veins of the leaves stay dark green, but in between the veins and leaves you get yellowing which will eventually turn into brown spots which are dead cells.
“If you are putting on a treatment to correct signs of deficiency, then you are doing a rescue job which is not very effective. It would be better to topically apply some magnesium during the period of peak demand of the crop just to make sure it does not run out.”
If the crop’s magnesium requirements are not met by the soil index, then foliar applications may be necessary says Mr Booty. Tissue or SAP testing can give a more precise diagnosis.
“Because the demand tends to be later in the season during seed formation in the case of OSR, then applying it in the base might not be the most effective way of delivering it anyway, since peak demand is at a time of the year when the weather could mean it will not be able to take it out of the soil,” he says.
“Foliar application of magnesium late season is a good way of doing it, which is already something we do routinely in cereal and potato crops.”