Straw from light land may not be as useful a source of crop nutrients as some growers believe, says a Suffolk contractor.

With fertiliser prices soaring, Derek Keeble says he is becoming concerned for the supply of straw to livestock producers and other users such as Ely power station.

Despite straw making good money in the swath many more farmers in his Woodbridge area are chopping and incorporating, believing it contains useful amounts of potash, says Mr Keeble.

“Arable farmers are increasingly concerned at the rising price of fertiliser and are saying that the straw price should go up. So we said that if the price has to go up let’s do some research to find out what’s really in it.”

He approached Richard Garnham of Farm Monitor, Woodbridge, to check. Mr Garnham found samples of harvest 2007 straw from light soils in his area contained much less potash than official figures suggest (see table) – and indeed, no phosphorus at all.

Straw nutrient content (kg/t)

          Farm Monitor

Potash Development Association

 

Potash

Phosphate

Potash

Phosphate

Wheat

1.4

0

6.2*

0.8*

Barley

1.2**

0

8.1*

1.0*

* Assumes straw yield is 65% of grain yield
** Spring crop expected to have slightly higher level

“We thought that we may have interpreted the results wrongly. So we had them re-tested by another lab, which found them very similar. We also thought that it might just be a peculiarity of 2007, so we also checked some straw from the 2006 harvest that Derek still had, and the figures were very much in line.

“There is a correlation between soil potash level and what is in the straw. But there are lots of other factors that can be involved,” admits Mr Garnham.

Many cereals in the area are treated more as breaks between high value vegetable crops and may not receive as much care and attention as on all-combinable crop farms, he suggests.

Heavy rain can leach potash from standing crops and straw in the swath, notes Chris Dawson of the Potash Development Association. “But I’m surprised the figures are as low as they are. Straw really needs sufficient K in it or the crop won’t grow.”

Mr Garnham acknowledges that he is unaware of the yields from the crops whose straw he tested.

Value worth recalculating, says adviser

As fertiliser prices have rocketed the value of straw has increased, arable farmers may need to consider the extra phosphate and potash fertiliser they need to apply if they sell the straw off farm, says Peter Hoey of N Yorks-based Fieldfare Training.
Mr Hoey’s calculations, using RB209, show that at today’s fertiliser prices, with triple superphosphate at £410/t and muriate of potash at £325/t, the value of the phosphate and potash removed in straw is £33/ha (£13/acre) for average yielding – 8t/ha (3.2t/acre) – winter wheat and barley crops.
“Put another way, the value is £6/t of straw sold. When I did this calculation on 2001, the value was £2/t.
Mr Hoey says potash in straw is the more significant nutrient, with 50kg/ha (40 units/acre) removed by the straw of an average-yielding winter cereal crop. “Phosphate removal is only about 6kg/ha (5 units/acre).”