Soil N reserves at five-year low after rain

16 March 2001

Soil N reserves at five-year low after rain

HEAVY rain means soil mineral nitrogen reserves are at the lowest for five years, according to one national laboratory which has seen a 20% surge in demand for soil nitrogen-testing, partly due to the launch of MAFFs revised RB209 advisory booklet.

"We have seen thousands of samples this year and soil reserves are the lowest for the past five years," says Iain James of Natural Resource Management. The trend is confirmed by ADAS.

Last season total SMN in the top 90cm (35in) of soils tested averaged about 60kg/ha (48 units/acre), says Mr James. "This year, only on dark, peaty organic soils are we seeing any significant reserves."

Last year slightly higher N levels at depth (ie 60-90cm) indicated nitrogen had moved down the profile but had not all been lost.

"We are not seeing that this year. Overall reserves are much lower and more uniform."

A typical analysis shows reserves of 11.9, 12.4 and 12.2kg/ha, respectively, at 0-30cm, 30-60cm and 60-90cm, he says. "They are all in the teens and suggest the N has been stripped from top to bottom."

There is little evidence that mineralisation of organic matter in earlier warmer weather has offset leaching very much, he adds. "It has not been showing up in our samples."

Samples taken in mid-late February from a range of soil types in the south by precision farming company SOYL and analysed by NRM highlight the low reserves.

All had indices of zero, with total available Ns of 21-56kg/ha (17-45units/acre), says the firms Nick Hall. "We had intended to re-test every 14 days in the same spots to get a good indication of what is really going on." Unfortunately foot-and-mouth ended that plan.

Results from three chalkland sites reinforce the view that cultivation boosts N supply, he adds. The first sample, to 60cm (24in) from undisturbed soil ploughed the previous autumn, recorded 27.6kg/ha. The other two sown with barley but no fertiliser, registered 48.4 and 55.8.

"That suggests that once you start drilling you can get some very rapid mineralisation." Experience of the farm means that the apparently good N release will not sway his fertiliser recommendations greatly.

"But it does show that once you get some warmth into the soil available N changes quite quickly." When soil testing, growers can use the simple RB209 guide (see table) to assess what crops have already taken up, says ADASs Peter Dampney. Measuring Soil Nitrogen Supply – what remains in the soil and crops and what should become available from organic matter – will provide the best steer on N top-dressing. &#42

Crop N contents


Shoots/sq m kgN/ha

500 5-15

1000 15-30

1500 25-50

Oilseed rape

Crop height (cm) kgN/ha

10 35-40

15 55-65

20 75-85

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