The counter-drugs strategy of the United States is clearly failing. UN figures cited in the Guardian this week show that the cultivation of coca, the plant from which cocaine is derived, has surged in the Andes. The most dramatic rise has been in Colombia, the only country in the region that allows the use of pesticides to eradicate coca leaf - a policy promoted and funded by the US.

I recently received a disturbing email from southern Colombia warning that the fragile Amazonian soil could "soon be turned to desert". They were the words of a Catholic priest, so I rang a church worker whose parish lies deep in the Amazonian state of Caquetá. Military planes targeting coca farms, funded by the US, had been spraying mists of pesticides over food crops, grazing animals and even areas where children were playing, she said: locals were complaining of breathing problems and rashes; "strips of skin" have been peeling off cows, and chickens have died; and maize, yucca, plantain and cacao crops have wilted and shrivelled. "We fear there will soon be a very serious food shortage in the region," she said. The local parish has issued an urgent appeal.

The US has been funding the spraying campaign for more than two decades, but 70% of the world's coca leaf is grown in Colombia. Glyphosate is the most frequently used pesticide; its biggest selling commercial formulation is Roundup, made by Monsanto. The company acknowledges that contact with glyphosate may cause mild eye or skin irritation. But independent studies have suggested a far greater range of symptoms, including facial numbness and swelling, rapid heart rate, raised blood pressure, chest pains, nausea and congestion.

In Colombia, glyphosate is mixed with other chemicals, and because the exact composition has not been made public it has been impossible to test its toxicity. One addition, a surfactant, makes the corrosive liquid stick to the surface - leaf or skin - on which it is sprayed. The pesticide is used at higher concentrations than stipulated in the US, and is sprayed from above the recommended height of 10 metres. Farm workers in the US are advised to keep clear of weedkillers, yet in Colombia aerial spraying takes place with no warning, showering humans and animals with chemicals.

All Colombia's neighbours - Ecuador, Bolivia, Peru, Venezuela and Brazil - oppose the "fumigation" policy. The Andean and European parliaments have called for its suspension, as have numerous environmentalists, scientists and politicians in Colombia. But spraying has intensified since the launch in 2000 of Plan Colombia, the US-funded counter-narcotics strategy.

It was in that year that I first went to meet coca growers in Caquetá. One woman told me a familiar story. Sara's parents were landless, and had travelled south to set up a farm. In this remote region, with no paved roads, they found that coca was the only crop from which they could make a living.

Sara showed me the weather-beaten wooden press she uses to grind the coca leaves. Peasants here turn the coca leaves into a paste, which is then sold on to a middleman who takes it to a jungle laboratory to refine it into cocaine.

Sara also grows maize, yucca, sugar cane and tropical fruit, but these products don't make much money. It would take days to transport them along rivers or dirt paths to the nearest big market. In contrast, coca paste is easy to transport and, crucially, always in demand. But the peasants here are not rich. They receive just 0.1% of the final street price of cocaine.

The US focuses on one element of the trafficking chain, the poverty-stricken peasant. But the policy is not even effective. When their land is poisoned, peasants migrate and start growing coca again. They have no alternative. Spraying simply displaces the problem. Despite decades of spraying, coca cultivation in Colombia has grown by 500% since the 1980s, according to US state department figures. US politicians heralded a drop in cultivation after the launch of Plan Colombia, but the area of land covered by coca crops is now larger than when the plan was launched. Perhaps the clearest indication that the policy is failing is the falling price of cocaine, suggesting more, not less, of the drug is entering the US market.

Back in Caquetá, the church worker described how pesticides have run into rivers and streams, killing fish. Locals wait days before they dare drink the water. One of the most fragile ecosystems in the world "is being poisoned".