How Garbage Fueled Ancient Agriculture

When a couple of archaeologists whose friendship dated back to college days decided to use their vacation to explore the Amazon River basin a few years ago, they had no reason to think they would find much of interest.

Scholars had ruled out the likelihood that prehistoric Indians had made much use of that region because of soil that is so deprived of nutrients that one expert had labeled the Amazon and its many tributaries a "counterfeit paradise."

But what James Petersen and Michael Heckenberger found was so startling that it has helped galvanize researchers around the world and it has refocused attention on that region.

They found traces of a great city that had thrived on the banks of the Amazon more than 1,000 years ago. It may well have been one of the largest cities in the world at that time, with a population of at least 200,000 persons, but it disappeared in the wake of the Spanish conquest. Its many inhabitants became what Science magazine recently called "the forgotten people of Amazonia."

Turning Sand Into Paradise

But what is even more startling is the fact that somehow those early settlers had managed to enrich the soil with nutrients that persisted even to this day, turning what some have called a wet desert into a garden paradise. How they did it, and why the soil remained fertile for centuries despite the punishing heat and torrential rains that normally wash nutrients away, has stumped modern scientists.

They did something that nobody has been able to do since, and if experts can figure out exactly how they did it, vast regions of the planet that are not now suitable for agriculture might spring to life.

Like so many archeological discoveries, parts of this story have been known for years. The rich pockets of soil, known as terra preta do Indio (Indian dark earth) have been used by local farmers for many years because they can grow crops with virtually no fertilizer. And the fact that the hillsides were littered with ancient pottery shards was documented more than a century ago.

But nobody had put it all together before, and bit by bit a fuzzy story of a lost civilization is coming into focus. Petersen, now chair of anthropology at the University of Vermont, and Heckenberger, now at the University of Florida, began kicking around the idea of exploring the Amazon back in the 1980s. Archaeology was not exactly booming in the Amazon, partly because working conditions can be miserable and language barriers abound.

But Heckenberger persevered, becoming fluent in Portuguese and establishing contact with the few archaeologists in Brazil, especially at the University of São Paulo. After completing his doctoral research in the southern part of the Amazon, Heckenberger turned once again to his old friend.

"He said let's get a project going in the heart of the Amazon," Petersen says. "We went there on holiday in 1994 and found an archaeological site just by talking with local people."

Packed City

The people told them of pottery shards on a riverbank near the village of Acutuba, near the confluence of the Amazon and the Rio Negro, but when the two Americans began poking in the soil they were astonished. They found bits of pottery everywhere.

"It's just unbelievable," Petersen says. "Unbelievable."

Petersen and Heckenberger, who is back in Brazil now, have since excavated four major sites and explored 30 others with the help of Eduardo Goes Neves, an archaeologist at the University of São Paulo, who brought along 20 of his students to help out in the digging.

Petersen, who specializes in ceramics, was blown away by the scale of the finding.

"There's literally hundreds of millions of ceramic shards at some of the larger settlements," he says.

It takes a lot of people to make that much pottery. So unlike the small villages found in the region today, Petersen thinks there were huge settlements, with possibly as many as 400,000 residents at a time when even major cities around the world were much smaller.

The shards contain carbon and thus can be carbon dated, and that's a blessing because most pottery does not have carbon. Petersen says they show that the Acutuba sites were occupied for about two millennia beginning around 450 B.C. Other evidence indicates that those early settlers were farming in the rich soils that they apparently helped create.

There is little doubt among scholars that the people somehow enriched the soil, but there is much debate over how they did it. An international team of experts met in Brazil recently to compare notes in an effort to figure that out.

Rich Garbage

The early settlers did not resort to the slash and burn tactics that have devastated much of Amazonia in recent decades. Burning enriches the soil for a few years, making agriculture possible for awhile, but it doesn't take long for nature to wash away the nutrients.

But researchers have found that returning some carbon to the soil is a critical part of the formula. It helps retain nutrients, like decomposing vegetation and even human wastes. So there was probably some burning going on, but whether it was deliberate or accidental is unknown.

In an effort to figure out the formula, Brazilian researchers added carbon to soil to see if it would stimulate growth, but they had no success. Then they tried just adding fertilizer, and still had no luck. But when they added carbon and fertilizer together, they saw an astonishing 880 percent increase in the growth of the rice and sorghum planted in the test plot.

So somehow, those early settlers must have added both carbon and fertilizer to their plots, in which they grew mostly maize, but did they really know what they were doing?

Probably not. The rich soils that have persisted to this day most likely came about by accident.

Petersen thinks those fertile plots are probably yesterday's garbage pits. Stir in a little charcoal, toss in the remains of fish harvested from the rivers, and over the years the dirt became better than gold.

While they may not have done all that on purpose, once the soil was there, it probably stimulated the growth of villages into cities, Petersen says. And that added more garbage, and more plots of enriched soil.

Those cities are long gone now. But the soil, and the legacy, remains.

Lee Dye’s column appears weekly on A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.