Terra Preta: Miracle soil allows forests to grow – and makes soil more fertile
Gardeners and organic farmers recreate the miracle soil from the Amazon themselves for more growth
Source: Getty Images
Forests that have been cut down are often not so easy to restore. For the Amazon region, researchers have produced amazing results with the ancient soil mix Terra Preta – and explain what can be derived from this for good soil in the garden.
Vor more than 2,000 years ago, the inhabitants of the Amazon began to transform what was originally poor soil. A kind of miracle soil was created, called “Terra Preta” and today, in imitation form, popular with many gardeners and organic farmers. A research team has now tested its potential for reforestation. Terra Preta, also known as Amazonian dark earth (ADE), can be of use here – by far not only in the Amazon, according to the result presented in the journal “Frontiers in Soil Science”.
Between about 450 B.C. Between 1000 BC and 950 AD, the inhabitants of what is now the Amazon region had significantly altered the region’s soils. For tens of generations, they have been enriched with charcoal from cooking and burning waste, animal bones, pottery shards, compost, faeces and dung, the researchers explain. The result, known today as Terra Preta (black earth), is particularly fertile because it is rich in nutrients and organic material. Typical is the dark coloring that comes from the charcoal.
With their experiments, the scientists led by Luís Felipe Zagatto from the University of São Paulo in Piracicaba now wanted to test whether the dark earth can help to reforest impoverished pastureland soils in deforested areas. They used ADE samples from the Brazilian state of Amazonas and, as a control, agricultural soil from the Luiz de Queiróz Superior School of Agriculture in the state of São Paulo.
36 pots, each filled with three kilograms of soil, were placed in a greenhouse with an average temperature of 34 degrees. One-third received control soil only, one-third received a 4:1 mixture of control soil and ADE, and another third received 100 percent Terra Preta. To first imitate a pasture, the grass species Urochloa brizantha, a common fodder in Brazil, was sown. After 60 days the grass was cut off, the roots remained in the pot.
Seeds of three tree species were then planted in the pots prepared in this way for small-scale reforestation: Cecropia pachystachya, Peltophorum dubium or Cedrela fissilis. After 90 days of growth, the size of the plants, their dry matter and the extent of the roots were recorded. In addition, measured values for the pH value, soil conditions and the concentration of certain substances such as potassium, magnesium, iron and zinc were measured over the course of the experiment. Molecular methods were used to analyze changes in microbial diversity in the soil.
The ADE soils therefore initially had larger amounts of nutrients than the control soils – for example 30 times more phosphorus. At the end of the experiment, the nutrient content was significantly lower, which reflects the uptake by the plants. 100% ADE soils continued to be richer in nutrients than the control soils, with nutrient levels in 20% ADE soils intermediate. ADE soils also showed a greater variety of bacteria and archaea than the control soils throughout the course of the experiment.
Microorganisms are at work
According to Anderson Santos de Freitas from the University of São Paulo, microbes are important for converting material into nutrients that can be consumed by plants. “Our data show that ADE contains microorganisms that are better at this conversion of soil and thus provide more resources for the development of plants.”
The dry matter of the grass was 3.4 times at 20 percent ADE and 8.1 times at 100 percent ADE compared to the control soil. The growth of the three tree species was also promoted: the seedlings of Cedrela fissilis and Peltophorum dubium were 2.1-fold/5.2-fold higher in 20 percent ADE and 3.2-fold in 100 percent ADE compared to those in control soils. times/6.3 times.
On the other hand, Cecropia pachystachya, a common pioneer tree in the region, only grew a little at all in 100 percent ADE. Cecropia species usually grow in clearings and set the forest’s natural recovery processes in motion. Below the temperature selected for the experiment, based on the values expected in the course of climate change, this potential is possibly only available to a limited extent.
In general, however, the mixture of soil nutrients and special microorganisms in Terra Preta makes it easier for trees to resettle, the researchers conclude. However, Siu Mui Tsai from the University of São Paulo emphasizes that this should not be taken as an invitation to mine and use the Amazon black earth near old settlements for such projects. “ADE has taken thousands of years to accumulate and it would take just as long to regenerate in nature if used.” The recommendation is therefore to copy the properties of the soil in a targeted manner.
Gardeners and organic farmers have long recreated the miracle soil from the Amazon themselves. A key component of the mixture with organic residues such as compost, manure or leaves is biochar from deliberately charred plant parts.
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