Seven years after GM soya was introduced to Argentina as an economic miracle for poor farmers, researchers claim it is causing an environmental crisis, damaging soil bacteria and allowing herbicide-resistant weeds to grow out of control.

Soya has become the cash crop for half of Argentina's arable land, more than 11m hectares (27m acres), most situated on fragile pampas lands on the vast plains. After Argentina's economic collapse, soya became a vital cash export providing cattle feed for Europe and elsewhere.

Now researchers fear that the heavy reliance on one crop may bring economic ruin.

The GM soya, grown and sold by Monsanto, is the company's great success story. Programmed to be resistant to Roundup Ready, Monsanto's patented glyphosate herbicide, soya's production increased by 75% over five years to 2002 and yields increased by 173%, raising £3bn profits for farmers hard-hit financially.

However, a report in New Scientist magazine says that because of problems with the crops, farmers are now using twice as much herbicide as in conventional systems.

Soya is so successful it can be viewed as a weed itself: soya "volunteer" plants, from seed split during harvesting, appear in the wrong place and at the wrong time and need to be controlled with powerful herbicides since they are already resistant to glyphosate.

The control of rogue soya has led to a number of disasters for neighbouring small farmers who have lost their own crops and livestock to the drift of herbicide spray.

So keen have big farmers been to cash in on the soya bonanza that 150,000 small farmers have been driven off the land so that more soya can be grown. Production of many staples such as milk, rice, maize, potatoes and lentils has fallen.

Monsanto says the crop is the victim of its own success. Colin Merritt, Monsanto's biotechnology manager in Britain, said that any problems with GM soya were to do with the crop as a monoculture, not because it was GM. "If you grow any crop to the exclusion of any other you are bound to get problems. What would be sensible would be to grow soya in rotation with corn or some other crop so the ground and the environment have time to recover," he said.

One of the problems in Argentina is the rapid spread of weeds with natural resistance to Roundup Ready. Such weeds, say opponents of GM, could develop into a generation of "superweeds" impossible to control. The chief of these is equisetum, known as marestail or horsetail, a plant which rapidly chokes fields of soya if not controlled.

But Mr Merritt said horsetail could be a troublesome weed in any crop. "I reject the notion that this is a superweed or that it will confer genetic resistance on other weeds and make them superweeds. It always has been a troublesome weed."

The soya was originally welcomed in Argentina partly because it helped to solve a problem of soil erosion on the pampas which had been caused by ploughing. Soya is planted by direct drilling into the soil.