The ambition of Manuel Santos Uribelarrea is written in big black letters on the side of machines reaping the plains of South America: MSU. It is harvest time and the state-of-the-art behemoths bearing his initials have a mission to revolutionise agriculture, change the world's eating habits and make their owner very, very wealthy.

At 28, Santos is on his way to achieving those goals, making him a lord of the pampas, master of all he surveys, and one of Argentina's most eligible bachelors. His company owns more than 100,000 hectares (247,000 acres) of farmland in Argentina and Uruguay, is expanding into Brazil and has plans for Ukraine.

The empire, however, is controversial - it is built on soya. Fast-expanding soya plantations are blamed for the destruction of forests across South America, posing an even graver threat than logging. The outcry has led to the tabling of a 'forestry emergency' bill in Argentina's lower house of congress. It would usher in a one-year moratorium on deforestation and oblige all 23 provinces to control and protect the region's biggest and most diverse eco-system outside Brazil.

Most soya producers shun the limelight and any possible association with the bulldozers. Santos, long-haired and fizzing with energy, is different. Speaking to The Observer at Villa Canas, four hours west of Buenos Aires, the founder and president of MSU said his company's drive for efficiency was helping to feed the world. 'The environmentalists are extremists who want to leave everything as it is,' he said. 'But soya is a great crop. It is an important part of sustainable development. We are contributing to Argentina and a better world.'

Argentina's reliance on cattle and grain changed in the Nineties when US biotech giant Monsanto turned the pampas into a springboard for genetic modification. New herbicide-tolerant soya turned the countryside brown, with 10 million hectares sown in the past decade. All of it was genetically modified, said Santos. The controversy over GM was an irrelevance. 'A political thing between the US and Europe,' he shrugged.

Six years ago Santos persuaded his father, a wealthy traditional farmer, to join the revolution. The young tycoon reinvests the profits to buy more land.

The company, which concentrates on producing and leaves the processing to others, considers itself one of the top five growers in Argentina. Soya is now by far the country's most valuable export and a driver of Argentina's recovery from the 2001 economic crash. A hefty 27.5 per cent tax on exports - worth £2.6bn in the first quarter of last year - has become a significant source of revenue.

Food for the world, dashing innovators, national economic saviours - there is some truth to that. There is, however, a dark side to many soya barons. 'They are destroying our forest. These large companies leave nothing but smoke and ashes,' said Oswaldo Maldonado, 48, who lives in a rural corner of Chaco, in northern Argentina, and regularly sees what the soya bulldozers have wrought: splintered tree trunks and flattened vegetation. Traditionally, soya cultivation was concentrated in the three central provinces of Buenos Aires, Cordoba and Santa Fe, but demand is driving the plantations into the northern forests.

If deforestation continues at its present rate, environmentalists predict that the lower forest ranges of the Yungas will disappear by 2010. The bush savannah of the Chaco, which covers a quarter of northern and central Argentina, is also threatened. More than 2.3m hectares of dry and humid vegetation have been cleared for soya since 1995.

Agronomists warn that the Chaco's dry bush is unsuitable for intensive agriculture. Yet small farmers recently took the opportunity of a new highway in the arid, western Chaco to try their hand at growing soya. Miles of now disused scrubland bear testimony to their lack of success. 'Now the land is useless for forestry or agriculture,' says Rolando Nunez, co-ordinator of a regional campaign organisation. 'Although we do have a nice, new trunk road'.

As soya advances, the rural population retreats to cities. In Chaco province, three in every four people now live in urban areas, many in slums.

Last year leading European supermarkets, food manufacturers and fast-food chains pledged not to use soya illegally grown in Brazil's Amazon. Argentina, however, remains vulnerable. Campaigners accuse the government of turning a blind eye. In the Chaco savannah, for example, there is just one inspector to monitor deforestation.

Out in the field, however, the air is thick with dust from harvesting, Santos gazed with almost childlike delight at the fleet of vehicles traversing the terrain. 'Look at them. Beautiful.'