Nitrogen levels leave scope to adjust inputs
C LAIMS from ADAS that soil mineral nitrogen is at record levels and fertiliser rates need cutting are being challenged.
Average figures are very misleading and site-specific information is needed before adjusting rates, say other samplers.
"There is a lot of nitrogen floating about which must be good news," says ADAS Gleadthorpe-based soil scientist Brian Chambers. With due allowance this should help cut input bills, he explains.
But fertiliser sellers say dressings must be based on individual field needs, measured directly or through an assessment of weather, soil, cultivation and crop history.
ADASs MAFF-funded survey of reserves at 28 sites in February shows average mineral N levels in the top 90cm (35in) on light soils are at a record 74kg/ha (59 units/acre). That is 16kg/ha (13 units/acre) more than the long-term mean.
Although January was wet, winter drainage and hence leaching has been less than normal, says Dr Chambers. "And average temperature has been 1C up, so there has been a lot of mineralisation."
Available reserves averaging 100kg/ha (80 units/acre) on heavy land are among the highest recorded, he adds. After oilseed rape there is a mean of 130kg/ha (104 units/acre) left, 25kg/ha (20 units/acre) more than in the past two years.
The implications are clear, says Dr Chambers. Although individual sites vary, lodging risk is high, making wheat management trickier than normal.
How much N rates should be adjusted depends on expected grain prices. At £80/t it should be 10-20kg/ha (8-16 units/acre), suggests Dr Chambers. At £90-95/t cuts should be half that.
Anna Budden of Bracknell-based soil analyst Natural Resource Management agrees that soil N is tending to be quite high. "But it is very variable. Taking an average and saying it is higher all over is very dangerous."
Levington Agricultures Ian Richards believes heavy January rains led to significant leaching, meaning growers may need to use an average 20-30kg/ha more nitrogen than last year. However, the firms monitoring on behalf of Hydro Agri is confined to just six sites, he acknowledges.
"You have got to be site specific," stresses Hydros Rosie Carne. The firms PrecisionNPlan aims to help growers do just that.
Cargills Paul Antcliff claims a dozen farm mineral nitrogen tests show most available N is further down the soil profile than last year. "This means growers are likely to need to apply more N this year, and especially early on." *
SOIL N TESTS
• Mean levels up on 1997.
• Potential input savings.
• Malting Ns may rise.
• N deeper than normal?
• Beware of averages.
• Be site specific.