Potash boosts hungry oilseed rape crops

German oilseed rape growers tend to apply high levels of potash to their crops to push up yields. Farmers Weekly reports on whether this approach might work for growers over here


British oilseed rape growers could benefit by taking a leaf out of their German counterparts’ books and applying more potash to their crops.


While the continental approach is to give potash-hungry oilseed rape crops the nutrients they require from the word go, growers in Britain often fail to feed the crop sufficiently for high yields.


That’s the view of Tim Kerr, fertiliser manager at distributor Hutchinsons, who says that a switch to the German practice could help drive up yields.


He says a modest 4t/ha oilseed rape crop removes less than 50kg of potash from the field in the seed but the quantity of potash the crop needs for vegetative growth is closer to 300-400kg/ha.


“This potash comes from the soil reserves and from applied fertilisers and if it simply isn’t available then yields will suffer as a consequence,” he says.


Mr Kerr stresses that growers should aim at helping crops survive harsh winters by promoting strong rooting and good early establishment.


He points out that maximising the potash content of an oilseed rape plant’s sap also helps lower its freezing point as, in effect, potassium acts like antifreeze.


“Furthermore, potash also has a direct effect on soil moisture availability and soils with adequate levels of potash could hold as much as two weeks more water supply during periods of drought,” he says.


In Germany, common practice is to apply about 140kg/ha potash to oilseed rape – that is about three times that of British colleagues – recognising that most is then left in the soil for a following cereal crop.


Agronomist Stephan Deike advises 40 growers in northern Germany for agronomy group Landberatung, where many farmers grow oilseed rape one year in every three and see 4.5-5.5t/ha yields on heavier soils and 4t/ha on lighter ground.


Most use reduced tillage cultivations to work the soil to 20-25cm and incorporate straw and focus on conventional varieties drilled from 15 August to the end of the month before switching to hybrids sown up to 10 September.


“The key is to get the crop to the 6-leaf stage as soon as possible – at this stage lateral shoots are present in the leaf axils and these are very important for yield,” he says.


“We normally use 50kg of nitrogen in the seedbed, this helps in the min-till system to break down the straw and promote crop establishment.


Then, if four leaves are not present by the end of September we’ll go in with further nitrogen to accelerate growth,” he adds.


On lighter land, Dr Deike advocates 40kg/ha potash in the autumn followed by 100kg/ha in the spring. On heavy soils, half if not all of the potash can go on in the autumn as these soils can effectively “hold” the nutrient due to their higher clay and organic matter contents.


He points out that the German crop is probably more at risk from frost damage or drought than in Britain.


“It really is critical to get the crop established in order to survive some of our winters, and in this respect potash is a key component of our growers’ autumn fertiliser programme,” Dr Deike adds.


Mr Kerr points out that with British use of potash just half of what it was 15 years ago this could explain why average oilseed rape yields have seen little improvement in the past decade.


His advice is to apply potash this autumn to oilseed rape on all soil types where indices are below optimum, except light sandy soils where a split application in the autumn and spring will ensure availability over the season.


“This way plants which already typically contain around 70kg potash/ha when coming out of the winter would benefit from P & K induced rooting and reduced winter stress,” Mr Kerr says.


He adds that the nutrients will have had time to work into the soil and will be ready for the massive uptake phase which kicks off in the spring.


More on this issue


One third of UK oilseed rape crops are magnesium deficient

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