Apply enough potash before OSR drilling

Growers are being urged to apply sufficient potash before drilling oilseed rape this autumn to boost winter crop survival and maximise yields.

Applying potash before planting rape can increase yields and help protect the crop against a harsh winter, says Jerry McHoul, technical director of fertiliser firm K + S Eire.

Maximising the potassium (K) content of a plant’s sap helps lower the rape plant’s freezing points. “In effect, potassium acts almost like an anti-freeze.”

This year, yields recovered thanks to a prolonged flowering/pod fill period, but some damage was done when temperatures plunged to -15C, without any snow cover or significant leaf waxing, he reckons.

Plenty of attention is given to pigeons, adjusting seed rates and monitoring green area index (GAI) to create crops which will overwinter successfully.

However, little thought is given to the nutrient demand, which starts in the autumn and explodes in early spring, he says.

“Growers can and should do more to maximise potential by setting the crop up prior to the winter shutdown,” he stresses.

“This includes ensuring adequate potash is available to the plant to help reduce stress effects and sufficient phosphate to boost early root development and strong natural growth.”

In addition, soils that are rich in potash are able to retain their water for longer periods, which can help in a dry season.

Most growers in continental Europe apply potash rotationally before the rape crop, recognising its high demand for the nutrient.

“That’s why our continental neighbours aim for early establishment, pay close attention to soil structure and plant more vigorous hybrids,” he says.

Understanding the difference between nutrient uptake and offtake is crucial, Mr McHoul notes.

“A modest 4t/ha crop removes less than 50kg/K2O from the field in the seed fraction, but the quantity of potash that the green growing crop takes up from the soil is closer to 300-400kg/ha of potash. This needs to be found from the soil reserves.”

Where K indices are below optimum levels, potash should be applied on to cereal stubbles prior to drilling rape on all soil types excepts sandy soils, he says.

“This way, plants which already typically contain around 70kg/ha of K20 come out of the winter benefiting from P & K induced rooting and having reduced winter stress.

“The nutrients have had time to work into the soil and are ready for the massive uptake phase which kicks off in the spring.”

Any unused potash will then still be available in the soil for the following cereal crop, he says.

“It’s by far the best route on medium and heavy soils where the K index is below 2 and while some growers may be concerned over leaching risks on light land, here, a split application, holding some fertiliser back, will ensure availability over the season.”

Richard Overthrow, an agronomist at NIAB TAG, agrees that it is important to apply potash – but only if levels are deficient.

“If the soils suggest that you are lacking in potash and are not at the ideal index of 2 then you do need to remedy it to ensure your oilseed rape crop has enough potash going into the winter,” he says.

“If your potash is already at the right level, there will be no benefit to adding anything after that.”

Ideally, P & K should be applied before drilling, but it can be applied later in the season, says Mr Overthrow.

“Many growers apply their P & K in April when they apply their nitrogen fertiliser, accepting that it is not going to be available until the following crop. That way, the soil is not depleted for the following year.”

Oilseed rape growers should ensure crops receive the right nutrition from the moment they are sown or risk losing yield, warns Ian Robertson, from The Glenside Group.

“Oilseeds need more than just nitrogen, phosphate, potash and sulphur to achieve their full yield potential, but often that’s all they get,” he says.

Nutrients like calcium, magnesium, zinc, boron, molybdenum, copper and manganese are all important, he insists.

“They help the plant become established quickly and help develop a healthy canopy and a good, deep tap root, so it can fully access the nutrients and moisture available in the soil.”

The best way to assess what the crop needs is to complete a soil test ahead of sowing, such as the Albrecht Soil Survey, and amend the fertiliser programme accordingly, he says.

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