Given the unusually warm and wet winter combinable crop growers are being urged to delay soil sampling for available nitrogen reserves until nearer the time that fields need top-dressing.

Assessing soil N supply is increasingly important to ensure fertiliser applications are as near optimum as possible to be cost-effective and environmentally acceptable.

This month’s mild conditions are likely to be encouraging an abnormally high release of N from soil reserves through mineralisation, notes Tony Morgan, analyst at Natural Resource Management in Berkshire.

Add in recent heavy rain, which could be washing much of that N down the profile, and tests made now could be less valuable than in more normal seasons.

“I don’t think it is worth doing any sampling just yet. Our advice would be to hold off for about a month. Sampling only ever provides a snapshot of what’s available and we always say it should be done as close as possible to when you want to apply fertiliser.

“With temperatures as high as they are there’s plenty of mineralised N available to keep crops going for now.”

There is always a compromise between taking samples early enough to be able to get them analysed in time and late enough to be useful, says ADAS‘s Peter Dampney.

ADAS expects to publish more information on winter rainfall and likely soil N reserves in Farmers Weekly at the start of next month.

Soil mineral N levels are likely to be higher than normal after the dry autumn, says Mr Dampney.

“I’d anticipate that the rain we’ve had is going to have had a significant effect, but not everything will be at rock-bottom level. And unless we get a deluge from now on the impact will decrease as crops start to grow and take up N.”

Laurence Blake of Gloucestershire-based Hill Court Farm Research reckons the unusual conditions mean considerably more N than usual is likely to have been taken up by crops.

“So determining crop N by leaf area index will be particularly important this year. Soil N variability between the time of SMN sampling and fertiliser application may well be offset by increased N uptake.”

andrew.blake@rbi.co.uk

  • Use soil samples only as a guide
  • Delay sampling – warm weather means higher-than usual soil mineralisation + heavy rainfall washing N through profile
  • Sample close to fertilising date – but allow time for analysis
  • Use leaf area index to assess N uptake