21 January 2000

K care helps safeguard cereal yield and quality

By Charles Abel

DO not milk your potash reserves too hard, cereal quality and yield could both suffer

Autumn sales of P/K products were only one-third of normal, suggesting up to two-thirds of cereal growers may be considering a P and K holiday this season. But for top yields and quality soils must be able to meet a crops peak potash uptake, not just its total off-take, says Terra Fertilisers Mike Slater.

"There is nothing wrong with taking a P/K holiday, provided you know why you are doing it. But many farmers think all they need to do is replace what they take off. That just is not the case with potash, the plant needs up to two-and-a-half times more potash to support spring growth than it finally removes."

Failing to supply that can render nitrogen use less efficient, harming yield and grain protein content.

His advice is to maintain potash input levels in spring or autumn, unless soil levels are medium index two or above. Where indices are lower or where a usual autumn dose was omitted use tissue testing before mid-April to check levels and apply a corrective dose before late April.

Light, stony and poorly struct-ured soils where rooting struggles are most vulnerable to potash shortfall.

Tissue testing costs £14-£20 a sample, depending on the range of other nutrients being checked.

"Last year we saw very high yields, which will have depleted soil potash levels in many areas. To sustain the high yield potential afforded by earlier sowing, strobilurin fungicides and the efficient use of other inputs crops must have enough potash," says Mr Slater. &#42

K boosts N use

Fairly heavy loam at Aubourn Farming, Lincs, growing Consort winter wheat in 1998/99 after beans. Although no autumn P/K was applied spring sampling showed the crop took up plenty of K from high soil reserves, accumulating over 300kg/ha of potash as expected. Nitrogen supply was more limited, the crop accumulating 260-270kg/ha. Despite this the crop yielded 11t/ha, thanks to extremely efficient nitrogen use, stemming from the adequate K supply, Mr Slater suggests.

Low K, low quality

Chaucer winter wheat on chalk in Dorset. April N and K tissue contents acceptable, but by May K content was only 160kg/ha after a spring K dressing omitted. With strob fungicide and timely rains an 11.3t/ha yield was still achieved. But poor K level restricted transfer of N despite healthy 350kg/ha crop N content, leaving grain protein content of 10.9-11%, losing £12/t premium. "There was enough N available, it was just that the low K level led to awful transfer into the grain," says Mr Slater.

KADVICE

&#8226 Low K hits N utilisation.

&#8226 Yield and quality suffer.

&#8226 Think peak demand, not just final off-take.

&#8226 Tissue test to check levels.

Consider crop effect first if planning a potash break, urges Terra.