LITTLE LINK BETWEEN K AND YIELDS
POTASSIUM nutrition for sugar beet is under the spotlight after fertiliser response trials and grower surveys by British Sugar.
The key message, says George Milford, research consultant at BSs Holmewood Hall research station, is that unlike other crops there is little relationship between yield and potassium offtake by sugar beet. So growers applying hefty potash dressings to obtain heavier beet crops are wasting money.
"Equally important, from the BS point of view, they could be increasing impurity levels in their crops," says Mr Milford. "Potassium is one of the most important factors affecting factory processing lines."
The findings run counter to the assumption that the concentration of potassium in harvested beet stays constant, and that todays higher yields therefore merit increased dressings.
Quite why sugar beet should differ from other crops remains unclear. But a series of response trials from 1993-97 and grower surveys confirms the fact, he maintains.
27 different experiments on soils from sands to silty clays with K indices from 0 to 5 were carried out. They showed that although sugar output ranged from 5 to 15t/ha (2 to 6t/acre) on both high and low potash land, the amount of K20 removed was independent of it.
Not unexpectedly offtake tended to be higher on high K soils, which suggests there is an element of luxury uptake by the crop, says Mr Milford.
"The biggest fertiliser dressings – 300kg/ha – boosted beet potassium by only 30kg/ha. That was small compared to the difference of 90kg/ha between the low and high K index soils."
A comprehensive survey of 8000 contracts in 1997 found beet potassium concentrations lay in a relatively tight range and distinct pattern, he adds. "It was quite a remarkable feature."
Mr Milford believes the findings should allow more grower and field-specific potash recommendations to be made in future. "What we are trying to do is identify those crops which still get too little for optimum yield, but at the same time pinpoint those with excessive K."
The bottom line is that in terms of root quality everything that growers can do to boost yields for a given potash application, even on high K soils, will be of value at the factories. "It is a win/win situation for growers and processor alike." *
• K conc at harvest not constant.
• High yields dont need high K.
• Excess K leads to impurities.
• More specific grower advice.