It is a marvellous thing, the soya plant. It grows as a little bush, a couple of feet high, and as it ripens many pods form, each containing two or three white beans the size of petits pois. By harvest time, the foliage has withered away; all that remains is a stalk, laden with pods.
The beans are rich in protein, much more so than, say, peas or rapeseed, so it makes a tremendously potent bodybuilder with which to feed cattle or pigs. You can feed it to human beings, too, as oil or tofu or any number of things - the plant comes from China, and the Chinese have been ingenious with it. And it has one more remarkable and rare property: it can extract nitrogen from the air and bury it in the ground - in other words, it can fertilise itself.
So clever and useful is the soya plant that it is threatening to swallow the Amazon rainforest. Where, until a few years ago, we had to worry about the encroachment of loggers and cattle ranchers on the vast forest that serves as a lung to the planet, now soya is becoming the driver of deforestation. You can see it from the air: the great, pale rectangular patches in the rippling, fluffy sea of dark green that is the forest. Or from the ground: the extraordinary prairies of fading green bushes, the brown stalks and pods showing that the harvest is near.
And no one is around, because that is another handy thing about soya: once you have your flat prairie, it is all very low-maintenance. Plant it using big machines and harvest it using big machines. Not much labour is involved.
The people resisting the destruction of the forest talk of it all the time. The producers insist theirs is just a follow-up crop, using land abandoned once loggers have taken the timber and the ranch cattle have had the best of the grazing on the cleared ground. But government officials and NGO activists say it is worse than that.
In some cases, soya farmers are moving ahead of the ranchers; their techniques have got better and they can plant the land soon after the last trees have been burned off. At the same time, on the shifting margins of the forest, there are land wars. Established smallholders are being forced out by bigger operators with big machinery, and from these patches of land more great squares grow. It can be a vicious business: a Catholic priest I spoke to not far from the Amazon port city of Santarém said he and his colleagues were sheltering a dozen families who feared for their lives. He knew three priests who had received death threats over the internet.
Global demand pushes the process along. In the case of China, as the country gets richer it eats more meat, and the meat is fed and fattened on soya beans. Europe, too, is eating more meat; and since BSE we are picky about what our livestock is fed on. Soya seems a healthy option.
None the less the Brazilian government, and regional governments in Amazonia, have been active on the deforestation problem, and are proud of the progress that has been made. Between August 2003 and August 2004, they say, 27,000 square kilometres of forest were lost; but in 2004-2005 the loss was down to 19,000 square kilometres. And the country's charismatic environment minister, Marina Silva, predicted there would be further progress this year.
The governments are managing to track down culprits more effectively - beefing up the traffic police so that cargoes of illegal timber or soya are spotted in transit; setting more land aside as protected; and re-educating public servants and the public about the environmental threat.
But punishing or bullying Brazilians will only get them so far, and they know it. For one thing, the president, Luiz Ignácio Lula da Silva, faces elections this October and needs to be popular. In Amazonas, the state that is the heartland of Amazonia, which is also the most forested and best preserved region in Brazil, the state government has come up with a long menu of positive ways to protect the forest. Making life better for those living there is the most important way forward, Virgilio Viana, the top Amazonas environment official, told me last week. That way, they will be less vulnerable to the predators.
So, for the first time, these very poor people have access to credit and tax breaks. And efforts are being made to increase the quality and thus the value of their produce. Health and administrative services are coming to them - by boat - so they don't have to trek into the towns.
This is imaginative and promising, but Viana was quick to add that it would probably not be enough to stop the onward march of soya. The power of the little plant is too great, especially if, as happened two years ago, China's soya farmers have a bad harvest.
In the end, everybody fighting the loggers, all the way up to President Lula, knows that Brazil can't solve this alone, and should not have to. Brazil, they say boldly, should be paid "environmental com pensation" by the rest of the world for looking after the Amazon. This would amount to billions of dollars for keeping deforestation at bay.
We benefit so much from this great global asset - the air, the water, the biodiversity - that losing it, even by salami slices, will eventually cost us more than we can afford. Better pay now, the argument goes, when the bill is easier to pay. Or would we rather just eat less meat, which would knock a hole in the soya market?