Actually, for us earthy types it never left, but more glitzy urban pseudo-environmentalists disappeared up their own bungs looking for a paler shade of green. They found brown of course. Fortunately, others followed a more fruitful course.

In 1879, the explorer Herbert Smith regaled the readers of Scribner's Monthly with tales of the Amazon, covering everything from the tastiness of tapirs to the extraordinary fecundity of the sugar plantations. . . The secret, he went on, was "the rich terra preta, 'black land', the best on the Amazons. It is a fine, dark loam, a foot, and often two feet thick."

Last month, the heirs to Smith's enthusiasm met in a hotel room in Philadelphia, Pennsylvania, during the World Congress of Soil Science. Their agenda was to take terra preta from the annals of history and the backwaters of the Amazon into the twenty-first century world of carbon sequestration and biofuels.

They want to follow what the green revolution did for the developing world's plants with a black revolution for the world's soils. They are aware that this is a tough sell, not least because hardly anyone outside the room has heard of their product. But that does not dissuade them: more than one eye in the room had a distinctly evangelical gleam.

The soil scientists, archaeologists, geographers, agronomists, and anthropologists who study terra preta now agree that the Amazon's dark earths, terra preta do índio, were made by the river basin's original human residents, who were much more numerous than formerly supposed. The darkest patches correspond to the middens of settlements and are cluttered with crescents of broken pottery. The larger patches were once agricultural areas that the farmers enriched with charred trash of all sorts. Some soils are thought to be 7,000 years old. Compared with the surrounding soil, terra preta can contain three times as much phosphorus and nitrogen. And as its colour indicates, it contains far more carbon. In samples taken in Brazil by William Woods, an expert in abandoned settlements at the University of Kansas in Lawrence, the terra preta was up to 9% carbon, compared with 0.5% for plain soil from places nearby.

From Smith's time onwards, the sparse scholarly discussion of terra preta was focused mainly on the question of whether 'savages' could have been so clever as to enhance their land's fertility. But Woods' comprehensive bibliography on the subject now doubles in size every decade. About 40% of the papers it contains were published in the past six years.

This warms my cockles for a couple of reasons. It's great soil science and it's great anthropology.

Everyone agrees that the explanation lies in large part with the char (or biochar) that gives the soil its darkness. This char is made when organic matter smoulders in an oxygen-poor environment, rather than burns. The particles of char produced this way are somehow able to gather up nutrients and water that might otherwise be washed down below the reach of roots. They become homes for populations of microorganisms that turn the soil into that spongy, fragrant, dark material that gardeners everywhere love to plunge their hands into. The char is not the only good stuff in terra preta — additions such as excrement and bone probably play a role too — but it is the most important factor.

Earlier posts here dealt with terra preta and bio-char. The explanation for its abundant phosphorus and nitrogen is that bio-char catches and holds these nutrients through a combination of chemical and physical properties, and releases them in the presence of enzymes secreted by plants. I'd like to see more research on this, but that's what I've read so far.

It makes sense because phosphorus and nitrogen are the two nutrients that get away most easily. They are repelled by most soils, unlike potassium which is attracted. Raising PH in acid soils and increasing organic matter can reduce this problem and the resultant leaching. Another problem with nitrogen is that soil bacteria are just as hungry for it as plants, perhaps more so. They "eat it" and release the nitrogen back to the atmosphere as nitrogen gas. It seems as if terra preta's "locking mechanism" foils these bacteria too.

What's even more exciting in the climate hysteria climate of today is . . .

Leaving aside the subtleties of how char particles improve fertility, the sheer amount of carbon they can stash away is phenomenal. In 1992, Sombroek published his first work on the potential of terra preta as a tool for carbon sequestration. According to Glaser's research, a hectare of metre-deep terra preta can contain 250 tonnes of carbon, as opposed to 100 tonnes in unimproved soils from similar parent material. The extra carbon is not just in the char — it's also in the organic carbon and enhanced bacterial biomass that the char sustains.

That difference of 150 tonnes is greater than the amount of carbon in a hectare's worth of plants. That means turning unimproved soil into terra preta can store away more carbon than growing a tropical forest from scratch on the same piece of land, before you even start to make use of its enhanced fertility. Johannes Lehmann of Cornell University in Ithaca, New York, has studied with Glaser and worked with Sombroek. He estimates that by the end of this century terra preta schemes, in combination with biofuel programmes, could store up to 9.5 billion tonnes of carbon a year — more than is emitted by all today's fossil-fuel use.

Are you a little damp yet? Still, the end of the century is almost a century away. Unless this can be accelerated some how we'll need other sequestration schemes as well in the near term.

Lehmann and his colleagues don't see biofuel as an alternative to char — they see the two developing hand in hand. Take the work of Danny Day, the founder of Eprida. This "for-profit social-purpose enterprise" in Athens, Georgia, builds contraptions that farmers can use to turn farm waste into biofuel while making char. Farm waste (or a crop designed for biofuel use) is smouldered — pyrolysed, in the jargon — and this process gives off volatile organic molecules, which can be used as a basis for biodiesel or turned into hydrogen with the help of steam. After the pyrolysation, half of the starting material will be used up and half will be char. That can then be put back on the fields, where it will sequester carbon and help grow the next crop.

I want one. I read about this stuff last fall, including Eprida's work, and made some charcoal this winter to try it. I can't tell if it works by eyeballing it. The problem is that my pastures are so wonderful at this point that it's all good. It's mid-August and I still have growth like the spring flush. My neighbors are lining up to get some of this stuff and give me leases for free. I don't give them squat for the land until the third year since I put in a lot of time and material, including seed at $2.00 per pound. The third year I give them normal irrigated pasture fees, even though these are dairy quality pastures. After the third year we renegotiate and they get their proppers.

But I've been saving up squaw wood, mulched black berry bushes, weeds and such all year to make more charcoal this winter when I can get another burn permit. These new properties I've acquired are a mess and will provide lots and lots of bio-trash in the next two months of killing labor cleaning them up for fall planting. If I live through it I'll try it on these nasty pastures and have a better idea of just what it can do.

I also use ashes from a local co-gen plant made from green waste and saw mill trash. That's high temperature pyrolysis in an oxygen environment so it's not the same stuff. It's useful but not bio-char. It seems that outfits like that would be much better off if they used something like the Eprida process. If I had a farm scale unit I could charge folks for disposing of their woody trash and make all the biodiesel and nitrogen doped bio-char I wanted. I can't find a flaw in this idea. I want one.