24 April 1998

Nitrogen losses need watching after wash-out

THE fate of fertiliser applied just before the recent wash-out continues to exercise the minds of specialists.

East Anglian supplier Mark Law believes heavy rain has caused massive N losses and could have a severe impact on arable crop potential unless growers replace it.

ADAS is more cautious but warns that losses from sugar beet and potato land could be significant. British Sugar expects to be able to issue advice soon after further tests to determine how much leaching has taken place. But the British Potato Council says it is too early to be sure whether extra N will be required.

Missing nitrogen

In many winter cereals at least half the nitrogen applied in the two weeks up to Easter will have been lost either through leaching or run-off and need replacing, maintains Mr Law.

ADAS soil scientist Brian Chambers is less pessimistic.

"You need an awful lot of water to wash applied nitrogen beyond the active rooting zone of winter wheat and barley. You might have lost 20kg/ha but no more." But he acknowledges the difficulty of assessing losses on land which has been flooded. "We just havent got a model for surface run-off."

In such cases soil mineral N and tissue tests should be useful but are economically questionable, he suggests. "They are fairly expensive – £60-100 a field for both tests."

Mr Law believes yield potential may be gone before such test results are known. "The problem is the time lapse," he warns. "I would advise growers to replenish at least some of the lost N . They need to be pro-active." The picture is particularly worrying in sugar beet and potatoes where, on light land, loss of potash as well as N could bring both yield and quality penalties, he adds.

Much basal fertiliser went onto bare exposed land which has since had up to 180mm (7in) of rain, he points out. "For potatoes not yet planted it will be six weeks before they want it." By then much of it will be no longer there, he argues.

ADAS says it may be worth re-applying seed-bed N where sugar beet is obviously pale and struggling, but unless there is more heavy rain the overall amount should not be increased. The crop is deep-rooted and excess N reduces sugar yield, it warns.

Where land flooded, much depends on whether the surface structure collapsed allowing more fertiliser run-off, says Hydro Agris Jim Lewis. &#42