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Two ideas about the areas of terra preta<\/em> soon came to be widely accepted. First, that they were man-made, as Wikipedia<\/a> makes clear: terra preta<\/em> \u2018was made by adding a mixture of charcoal, bones, broken pottery, compost<\/a> and manure to the low fertility Amazonian soil\u2019 and was \u2018created by farming communities between 450 BCE and 950 CE.\u2019 Second, that the high fertility is at least partly explained by the charcoal, which has since led to the marketing of a range of charcoal (or \u2018biochar<\/a>\u2019) based soil and compost additives, claimed to improve everything from soil structure to water retention and aeration.<\/p>Terra preta, which can be used for gardening – \u00a9 GeorgeVieiraSilva\/Getty<\/figcaption><\/figure>But what is terra preta<\/em> and can you make it?<\/h2>It\u2019s all about the charcoal<\/strong><\/p>That first idea \u2013 that terra preta<\/em> is man-made \u2013 had never been properly tested until recently. But an international team of researchers has looked at one of the best-studied terra preta<\/em> sites, at the Brazilian Agroforestry Research Station, and published their results in the journal Nature Communications<\/em>. They began by taking a close look at the charcoal that gives the soil its black colour, and quickly discovered two very suspicious things about it.<\/p>It wasn\u2019t, and in fact couldn\u2019t have been, put there by humans<\/p><\/blockquote>
But the wrong kind of charcoal<\/strong><\/p>In the first place, it\u2019s far too old \u2013 carbon dating shows it to be thousands of years older than the earliest evidence of human activity and plant cultivation in the region. In other words, it wasn\u2019t, and in fact couldn\u2019t have been, put there by humans. The second problem is its isotopic composition. This is complicated, but briefly, the carbon could not have come from the local vegetation. Terra preta<\/em> carbon shows the characteristic isotopic signature of the special kind of photosynthesis used by tropical grasses, which don\u2019t grow naturally in the Amazon. Maize cultivation could account for this, but that can be ruled out by other archaeological evidence.<\/p>One explanation is that the soils, and the charcoal and nutrients in them, are alluvial, washed down after erosion<\/p><\/blockquote>
Nutrients from upstream<\/strong><\/p>The simplest conclusion is that the charcoal not only pre-dates human activity, but also came from somewhere else. But where? Washed down from upstream looks most likely. Other evidence, including the sandy nature of terra preta<\/em> soils (in contrast to the very clayey surrounding soils) and the trace elements in them, all points in the same direction. It\u2019s also been noted that terra preta <\/em>soils are all within about 10 km of major rivers, and usually linked to bluffs that would have arrested both floodwaters and the sediment in them. Supporters of their man-made origin assume this is because that\u2019s where people lived, but a simpler explanation is that the soils, and the charcoal and nutrients in them, are alluvial, washed down after erosion linked to fires in grassy<\/a> savannas upstream.<\/p>![\"Aerial](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/40\/2024\/02\/GettyImages-1235067851-scaled.jpg\")
Aerial view of the rainforest and the Amazon river. – \u00a9 Anna Chaplygina\/Getty images<\/figcaption><\/figure>Too many nutrients<\/strong><\/p>A final difficulty for the man-made hypothesis is the sheer fertility of terra preta<\/em>: levels of important plant nutrients like phosphorus and calcium are several orders of magnitude greater than in normal Amazon soils. Some simple calculations show that this could only have been achieved by large human populations actively managing the soils continuously for thousands of years longer than the sites are known to have been occupied.<\/p>So it looks like terra preta<\/em> is less exciting, and certainly less unusual, than we thought; after all, alluvial soils are noted for their high fertility the world over. Of course the archaeology doesn\u2019t lie, and all that pottery, bone and other rubbish is certainly there, but the simplest explanation is that indigenous peoples recognised, preferentially settled and probably even maintained existing areas of high fertility \u2013 they just didn\u2019t create them in the first place.<\/p>But most crucially, what all this shows is that terra preta<\/em> owes its fertility, like all alluvial soils, to all that river-borne sand, silt and nutrients, rather than to the biochar<\/a>, which was just washed down from upstream along with everything else. Which maybe helps to explain why it doesn\u2019t seem to work miracles in the garden either.<\/p>
Two ideas about the areas of terra preta<\/em> soon came to be widely accepted. First, that they were man-made, as Wikipedia<\/a> makes clear: terra preta<\/em> \u2018was made by adding a mixture of charcoal, bones, broken pottery, compost<\/a> and manure to the low fertility Amazonian soil\u2019 and was \u2018created by farming communities between 450 BCE and 950 CE.\u2019 Second, that the high fertility is at least partly explained by the charcoal, which has since led to the marketing of a range of charcoal (or \u2018biochar<\/a>\u2019) based soil and compost additives, claimed to improve everything from soil structure to water retention and aeration.<\/p> It\u2019s all about the charcoal<\/strong><\/p> That first idea \u2013 that terra preta<\/em> is man-made \u2013 had never been properly tested until recently. But an international team of researchers has looked at one of the best-studied terra preta<\/em> sites, at the Brazilian Agroforestry Research Station, and published their results in the journal Nature Communications<\/em>. They began by taking a close look at the charcoal that gives the soil its black colour, and quickly discovered two very suspicious things about it.<\/p> It wasn\u2019t, and in fact couldn\u2019t have been, put there by humans<\/p><\/blockquote> But the wrong kind of charcoal<\/strong><\/p> In the first place, it\u2019s far too old \u2013 carbon dating shows it to be thousands of years older than the earliest evidence of human activity and plant cultivation in the region. In other words, it wasn\u2019t, and in fact couldn\u2019t have been, put there by humans. The second problem is its isotopic composition. This is complicated, but briefly, the carbon could not have come from the local vegetation. Terra preta<\/em> carbon shows the characteristic isotopic signature of the special kind of photosynthesis used by tropical grasses, which don\u2019t grow naturally in the Amazon. Maize cultivation could account for this, but that can be ruled out by other archaeological evidence.<\/p> One explanation is that the soils, and the charcoal and nutrients in them, are alluvial, washed down after erosion<\/p><\/blockquote> Nutrients from upstream<\/strong><\/p> The simplest conclusion is that the charcoal not only pre-dates human activity, but also came from somewhere else. But where? Washed down from upstream looks most likely. Other evidence, including the sandy nature of terra preta<\/em> soils (in contrast to the very clayey surrounding soils) and the trace elements in them, all points in the same direction. It\u2019s also been noted that terra preta <\/em>soils are all within about 10 km of major rivers, and usually linked to bluffs that would have arrested both floodwaters and the sediment in them. Supporters of their man-made origin assume this is because that\u2019s where people lived, but a simpler explanation is that the soils, and the charcoal and nutrients in them, are alluvial, washed down after erosion linked to fires in grassy<\/a> savannas upstream.<\/p> Too many nutrients<\/strong><\/p> A final difficulty for the man-made hypothesis is the sheer fertility of terra preta<\/em>: levels of important plant nutrients like phosphorus and calcium are several orders of magnitude greater than in normal Amazon soils. Some simple calculations show that this could only have been achieved by large human populations actively managing the soils continuously for thousands of years longer than the sites are known to have been occupied.<\/p> So it looks like terra preta<\/em> is less exciting, and certainly less unusual, than we thought; after all, alluvial soils are noted for their high fertility the world over. Of course the archaeology doesn\u2019t lie, and all that pottery, bone and other rubbish is certainly there, but the simplest explanation is that indigenous peoples recognised, preferentially settled and probably even maintained existing areas of high fertility \u2013 they just didn\u2019t create them in the first place.<\/p> But most crucially, what all this shows is that terra preta<\/em> owes its fertility, like all alluvial soils, to all that river-borne sand, silt and nutrients, rather than to the biochar<\/a>, which was just washed down from upstream along with everything else. Which maybe helps to explain why it doesn\u2019t seem to work miracles in the garden either.<\/p>But what is terra preta<\/em> and can you make it?<\/h2>