Terra preta can help recover forests – 06/05/2023 – Reinaldo José Lopes

The Amazon’s deep past is a jigsaw puzzle filled with fascinating pieces, many of which seem to have started to fall into place in recent decades. Some of them draw immediate attention, such as the splendid pottery or the presence of huge villages and roads many centuries before the arrival of Europeans. In complexity, however, I suspect that few surpass something apparently much less glamorous: a very peculiar type of soil.

I am referring to what the region’s residents call “Indian black soil”. The visual contrast of terra preta with the predominantly reddish soil of the Amazon region is so obvious that it ended up designating this type of soil. Also becoming increasingly evident is the fact that there is an association between terra preta and archaeological sites in the Amazon, in practically all areas, from Peru to the mouth of the Amazon River.

Estimates of the geographic distribution of terra preta vary. There are studies that speak of 0.2% of the forest area in the Amazon, which would be ten times the size of the municipality of São Paulo. Others point to something around 3% of the forest — three-quarters of the state of Paraná, if you will. The only certainty is that there is a portentous amount of terra preta out there.

We also know that this substrate is much more fertile than most soils in the Amazon. It is a soil that does not degrade easily with the weather and that carries large amounts of organic matter and minerals such as phosphorus, calcium, zinc and magnesium (although it is relatively low in potassium).

Another certainty: it is an anthropogenic soil, that is, produced by human action. Much more uncertain is the level of intentionality behind its emergence. We know that an important component of terra preta is the presence of numerous very small fragments of charcoal, the result of slow and incomplete burning of plant matter. There are also fragments of fish bones, pieces of pottery and chemical compounds that carry a chemical signature associated with human feces.

All of this seems to match some management system for plant and animal waste that, over time, may have become a process of deliberate production of more fertile soil, perhaps boosting the agricultural advance detected in the Amazon from about 2,000 years ago. back.

And there are good reasons to believe that the terra preta “recipe” can also be an ally of reforestation in the present. In a study that has just been published in the specialized journal Frontiers in Soil Science, Brazilian researchers have just shown that terra preta boosts the growth of native trees in soil where there was pasture before.

In their work, specialists from USP in Piracicaba, from Embrapa Western Amazon and from the National Institute for Research in the Amazon simulated reforestation in the laboratory. First they planted grass in pots with 3 kg of soil and, after 60 days, they cut it. Afterwards, they planted seeds of three native trees (embaúba, canafístula and pink cedar) in them, corresponding to the different phases of regeneration of a forest.

The detail is that some pots in the experiment had normal soil, while others had 20% and 100% black soil in their composition. The result: tree seedlings were up to five times taller in pots with a mixture of normal soil and terra preta, and six times taller in pots containing only anthropogenic soil.

The study team, whose first author is Anderson de Freitas, from USP, estimates that the differential of terra preta has to do not only with the soil’s nutrients, but also with the specific community of microorganisms that flourish in it. The lessons of terra preta, therefore, could be reproduced to shape other soils through the transfer of these microbes, without the need to exploit it commercially.

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