Atmospheric pollution is now so bad that practically every farm in Britain, whether organic or intensive, benefits from huge quantities of free artificial fertiliser deposited from above.
Scientists calculate that a typical organic farm receives an extra helping of nitrogen fertiliser from the atmosphere equivalent to about 8,000 cowpats falling on each hectare of land every year. This is equivalent to 40-45kg - about a quarter of the amount of chemical fertiliser typically used on intensive farms.
'It really is quite significant,' said Keith Goulding, a soil chemist at the Rothamsted Experimental Station in Hertfordshire, where the oldest field experiment in the world has helped to estimate how much airborne nitrogen is falling on Britain.
A field at Rothamsted has been cultivated with cereals every year since 1843, and the amount of added nitrogen fertiliser carefully monitored on individual plots. Water running off in a drain is monitored for nitrogen.
Rothamsted's scientists have also kept records of nitrogen in rainwater since 1853. They have found that over the past 150 years the nitrogen content of rainwater has increased by about three times as a result of pollution from cars, power stations, farm animals and the increased use of chemical fertilisers that can be blown away.
Dr Goulding said that Rothamsted, being in central England and near to major motorways and urban areas, will receive more atmospheric nitrogen than, say, parts of rural Wales or Scotland where the winds blow in from the Atlantic. Farms further east, however, are receiving even more, with some woodlands receiving twice as much free nitrogen fertiliser from the atmosphere.
He said the amount of nitrogen fall-out was undoubtedly benefiting organic farmers, who rely on farmyard manure and leaving fields fallow to improve the nitrogen content of soils. Rothamsted scientists are sceptical that organic methods on their own can provide enough nitrates to fertilise crops and prevent the soil from becoming depleted.
Patrick Holden, policy adviser to the Soil Association, which assigns certificates of approval to organic farms, said airborne nitrogen fertiliser does not affect the status of organic food: 'You cannot guarantee that any food is free of residues in today's polluted world.' Organic farming was defined by what the farmer did: 'It is not defined in terms of the absence or presence of residues . . . The whole basis of organic farming is to try to move away from intensive inputs of chemical fertilisers.'
A scientific committee set up by the Department of Environment to investigate the effects of nitrogen pollution is due to report within the next few weeks. One of the areas it is expected to highlight is the danger of increased fertilisation to rare British plants in areas where the natural flora is perfectly adapted to poor soils.
The committee has collated evidence showing that the number of plant species growing in these areas has diminished significantly in the past few decades. Up to three quarters of certain plant species in north-west England, for instance, have disappeared because they have received too much chemical fertiliser from atmospheric fall-out. Pristine parts of the Pennines are particularly badly affected.
In the Brecklands of East Anglia - where scientists have monitored some of the highest levels of nitrogen fall-out - the area of heather-covered land has declined by as much as 70 per cent in the past 20 years, largely because over-fertilised heather is more appetising to animals.
In the South East, a type of grass called Brachypodium pinnatum, an 'aggressive invader', has taken over nature reserves in the southern chalk grasslands, forcing out more delicate species.
Again, scientists hold airborne nitrogen pollutants responsible.
David Fowler, professor of environmental physics at the Institute of Terrestial Ecology in Edinburgh and a member of the scientific committee, said measurements of nitrogen deposition in other parts of Europe support the British findings: 'If we continue to contaminate Europe on the scale we are doing then species diversity will decline.'
Professor Fowler said that more than half of the deposited nitrogen is in the form of ammonia or ammonium, which comes from the use of chemical fertilisers on farms and the waste products of domestic livestock. Urine from farm animals is especially rich in ammonia, a nitrogen-containing gas. The rest of the deposited nitrogen is in the form of oxides of nitrogen.
Some of the nitrogen compounds dissolve in rainwater but others are deposited in a dry form and are difficult to measure.
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