23 January 1998

Nitrogen levels higher than seasonal average

DESPITE recent heavy rain, ADAS is urging growers to allow for higher-than-normal soil mineral nitrogen levels when planning spring fertiliser inputs.  That advice is based on surveys showing higher reserves than last year. Even after rain-induced leaching is allowed for good uptake from soil reserves can be expected – particularly on heavy land.

The tests, which were conducted last autumn, show reserves after heavy land cereals are slightly up on 1996 levels and about 20kg/ha (16 units/acre) higher than the long-term average.

The abnormally high levels can be explained partly by last seasons unusually high temperatures and dry spring and summer, interspersed with a very wet June, says senior consultant David McKnight. "June stimulated more mineralisation of N than usual, just when a crops ability to recover it was falling."

Additionally, yields, and hence nitrogen off-take, were generally lower than expected, he explains.

Autumn reserves after oilseed rape on heavy land were particularly high – 30kg/ha (24 units/acre) up on the previous two seasons. Even on light land after cereals they were 16kg/ha (13 units/acre) more than the long-term mean, he notes.

Clearly recent rains may have leached some of the available N. "But I reckon heavy land levels will still be higher than normal despite the rain. Most crops are still very lush and dont look starved of nitrogen."

First split applications to winter oilseed rape, which has a higher economic N demand than cereals, should be applied as normal, he suggests. "I would be tempted to treat it as usual to make sure you dont inadvertently starve it. Then when the February results come through you can adjust the second split as necessary." &#42

NITROGEN RESERVES

&#8226 Well above long-term average.

&#8226 Increased mineralisation.

&#8226 Heavy land cereals well fed.

&#8226 Leaching unknown until February.