By Alex Kirby, News Online Environment Correspondent and presenter of Costing the Earth

A prominent UK agricultural scientist says some soils are now too fragile to grow arable crops.

His comments come as concern grows over the impoverishment of soil in many parts of the UK.

The scientist, Professor David Poulson, is the head of soil research at the Institute of Arable Crop Research in Hertfordshire. He was speaking to BBC Radio Four's programme Costing the Earth.

Professor Poulson told the programme there were specific problems of soil health associated with intensive agriculture. Under continuous arable farming, he said, the organic matter in the soil tended to decline, and this changed its physical properties, making it more susceptible to erosion.

Areas at particular risk include the North and South Downs in southern England, parts of the West Midlands, and the Welsh borders.

"I think perhaps in the worst areas probably the only sensible alternative is not to do arable farming. In the long run, we have to look at taking areas that are prone to severe physical damage out of, certainly, arable production."

Professor Poulson is also concerned at the accumulation in soil of heavy metals like zinc, cadmium and copper, from the dumping of commercial and industrial wastes, notably sewage sludge.

"We know that quite low levels of these metals can have negative effects on soil microbes, and on nitrogen-fixing bacteria. The metals stay in the soil for many, many years, and only very tiny amounts get washed out or taken up by plants.

"We need to be concerned about those. But 90% or more of the metals remain in the soil, virtually for ever.

"The concentrations will build up, and at some stage in the future they could have a bad effect on the microbes. We certainly need to worry about those."

Another scientist, Dick Thompson, of the Soil Survey and Land Research Centre at Silsoe in Bedfordshire, told the programme that some farming soils were beginning to lose their productivity.

"They are simply organo-mineral mixes of sand and silt and clay, and seem to have no life in them. They are now producing very much lower yields than other soils.

"They contain very little organic matter. If you take the living component out of soil, it's a bit like switching the lights off in a factory. Everything comes to a grinding halt."

Dick Thompson is also concerned at the small amount being spent on monitoring what is happening to the soil - less than a million pounds a year, compared with several tens of millions on monitoring air and water quality.

"At the moment, we live in ignorance. We don't actually know where we are in terms of soil quality."