10 April 1998

Planning spring rates is easier with N testing

By Charles Abel

NITROGEN testing to check soil reserves is getting the thumbs-up as a strategic tool for planning spring inputs.

At CWS Agricultures Stough-ton Estate, near Leicester, farm manager, Alastair Leake, has no doubt it helped him fine-tune nitrogen rates this spring. And fen farmer Mike Wakeham has used the approach to optimise N inputs at 400 Farm, Benwick, Ramsey, Cambs, for several seasons.

"We get a Deep N test done by Levington on four representative fields and the results really do help us tailor our N input," says Mr Wakeham.

This years results show the impact of both previous cropping and cultivation regime. Land after sugar beet, which had been ploughed and power harrowed twice, contained just 60kg/ha (48 units/acre) of nitrogen. By contrast pea and linseed land, which was disced twice and drilled early, contained 190kg/ha (152 units/ acre).

Minimal cultivations clearly help retain nitrogen, notes Mr Wakeham. That is partly due to less microbial nitrogen release, but also a result of reduced water movement through the profile, he suggests.

Rates also take account of spring plant populations. The wheat after peas and linseed had 600-700 tillers a sq m in early March, a population which the moisture-retentive soil can support. But the crop following beet was only just tillering, with two to three a plant and 270-280 plants a sq m.

Big nitrogen saving

The net result is a big nitrogen saving on the 120ha (300-acre) mainly marine silt farm. Total input to wheat after peas and linseed is just 250kg/ha (two bags/acre) of a 34% N product, compared with 550kg/ha (4.4 bags/acre) on land ploughed after beet.

The £130 cost of testing is easily paid for by saving a single tonne of nitrogen, or harvesting 3t more wheat, Mr Wakeham notes.

In Leicestershire Mr Leake has been recording rotational N patterns for CWS Agriculture since 1993. He, too, uses the Levington Deep N service, which takes cores to 90cm (35in) depth.

Criticisms that the test is costly to use routinely where soil types vary across the farm are partly justified, he admits. That is why he is building up a picture, so fewer test results can be extrapolated across the whole farm in future.

"We know soil N is related to winter rainfall, prior cropping and cultivations. If we can build up data from our own site we will be better able to forecast availability in future," he says.

Last year tests showed residual N supply was high, varying from 120kg to 190kg/ha (96-152 units/ acre). The initial concern this year was that unused nitrogen left by lower yielding crops would mean high reserves in the spring. But tests have shown subsequent rains cut reserves to more usual levels.

Mr Leake goes on to stress the importance of understanding what happens on each farms own soils. "On our farm tests show reserves are closer to normal this spring, despite expectations that they might be higher, so we have gone with more normal rates, matched to plant populations as the best indicators of final yield."

Could a soil mineral N test have helped you tailor your N inputs more accurately this spring?