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Applying Farmyard manure? Take account of the potassium content

28 February 2012

There is no pressing need to change potato potash fertiliser recommendations.  But many more growers applying farmyard manure must take account of its potassium content; and all, but especially those in the north, should keep a close watch on soil magnesium levels. 

 “This latest review, conducted with Stuart Wale and Alex Sinclair of SAC, looked at existing evidence to make sure that the advice is in farmers’ best interests for profitable potato growing,” says ADAS’s Peter Dampney.
Potassium is an essential potato nutrient needed in larger amounts than nitrogen. Indeed, potatoes require more potash than all major arable crops, the review notes.
“Enough must be applied for optimum tuber yield and quality, but the trick is to make best use of all sources - not just those from the bag,” urges Mr Dampney. “These include soil reserves and relatively cheap livestock manures.”
The British Survey of Fertiliser Practice shows that about 35% of the potato area receives farmyard manure, typically applying 300kg K2O/ha, he notes.
“That may be enough on its own to replace offtake and is worth over £150/ha. Yet the survey found that farmers allow for only about 10% of the potash supplied from manures when calculating the amount of potash applied as fertiliser. So there’s lots of scope for savings.”
AHDB Potatoes Gary Collins, who has been involved in promoting the NIRS method of manure analysis, says:  “This new method is faster and more accurate than traditional wet chemistry, and will give a more accurate indication than book values for the manure going onto individual fields. This will help to increase the accuracy of fertiliser application in a bid to reduce input costs whilst maintaining crop requirements.” 
“Various studies have shown a range of offtake values,” continues Mr Dampney. “But our review concluded that there’s no evidence to change from the current 5.8kg K2O/ tonne of tubers, which is equivalent to 290kg K20 for a 50t/ha crop.”
To maintain soil fertility the potash removed in tubers must be replaced, he points out.
Yield responses to potash, if any, are usually small and hard to predict, he says. “They’re more likely on low to average K soils (index 1or 2), common on the sandy/medium land where many potatoes are grown.
“At these responsive sites, optimum potash rates are usually significantly less than offtake. So growers using rented land may be tempted to apply less K than is actually removed.”
That may not affect tuber yield but can reduce soil K status – a factor which should be reflected in growing contracts, the review suggests.
Too much magnesium may reduce potassium uptake, and the review states ‘There are strong anecdotal suspicions that potassium shortages in potatoes can be induced where soil magnesium levels are unnecessarily high.’
Such naturally acid soils, mainly in Co Durham, Yorkshire, Nottinghamshire, and Scotland, where magnesian limestone is used for liming, account for 10-15% of the UK potato area, says Mr Dampney.
“One 5t/ha application of lime containing 10% Mg will apply 500kg Mg/ha which is way above offtake of about 20kg Mg/ha.
“Soil Mg indices of 5 in those areas are common. Farmers should check their soil Mg levels and choose calcium-based limes rather than magnesian lime. It’s important to avoid the soil K:Mg ratio falling below 0.5:1 (as Mg/litre).
“Unless practices change, soil Mg indices will continue to rise and potentially aggravate this potash imbalance problem.”
Overall, the research evidence suggests that potash levels for optimum yield are also sufficient to minimise tuber quality defects, adds Mr Dampney. “But growers should always comply with any contract requirements.” 
Printable Version Grower Gateway - Issue 2, 2012
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