The soil in city allotments is much healthier than that on our farmland, according to scientists who see urban farming as the answer.

They say that intensive agriculture has so depleted the nutrients needed to grow crops in the fields that it may only be able to sustain another 100 harvests.

Scientists at Sheffield University have found that at the same time, lack of bio-diversity is causing a dramatic fall in wildlife populations in this country.

They argue that in order to ensure future generations are able to grow fruit and vegetables, we should start to see our towns and cities as potential farmyards. Growing crops and wildflowers in urban areas would also boost bio-diversity.

A recent study by Dr Jill Edmondson from the univesity's Department of Animal and Plant Sciences, found that soils under Britain's allotments are significantly healthier than soils that have been intensively farmed.

Dr Edmondson took soil samples from 27 plots on 15 allotment sites in urban areas. She also sampled soils from local parks, gardens and surrounding agricultural land. Compared with arable fields, the allotment soil was found to be significantly healthier.

Allotment soil had 32 per cent more organic carbon, 36 per cent higher carbon to nitrogen ratios, 25 per cent higher nitrogen and was significantly less compacted.

A scheme has been proposed by Professor Nigel Dunnett, also of Sheffield University who plans to transform a piece of wasteland in the centre of Sheffield into an eco-park filled with UK native wildflowers.

The university says that it is intended to act as a template for other cities on both sides of the border, and represents a new approach to urban growing.