History of the Idea: From the Amazon Rain Forest to the Georgia Piedmont

In 1974, a group of scientists from Venezuela, Germany, and the U.S. began a study of the ecology of the rain forest in the Amazon Territory of  Venezuela.  The site was at San Carlos de Rio Negro, close to the border of Brazil and Colombia.  The U.S. portion of the project was to study the nutrient cycle of the undisturbed forest and the changes in the cycle due to slash and burn agriculture as practiced by the indigenous people. The first step was to characterize the undisturbed soil, and the nutrient movement through the soil.

Soils in the lowland Amazon Rain Forest are very old and very low in nutrient content, due to the millions of years of weathering.  Most of the soils are red clay composed of iron and aluminum oxide, which hardens into lateritic stone (brick) when dried.  Near San Carlos, the upper mineral soil (20-50 cm in depth) is reddish lateritic particles embedded in fine sand. Above the mineral soil ( 0-25 cm) is a mat of organic matter (decomposing leaf litter) penetrated by roots.  We poured a radioactive tracer (phosphorus – 32) over several square meters of the root mat, and in subsequent weeks, sampled the mineral soil below, and vegetation surrounding the experimental plot. 99.9 percent of the phosphorus was taken up by microorganisms in the soil or transferred by the roots into surrounding vegetation.  Only 0.1 percent leached down into the mineral soil.  This showed the importance of soil organic matter and the microbial communities that it supports in recycling nutrients to the vegetation and preventing leaching losses through the mineral soil.

Pictured at right: Soil profile near San Carlos de Rio Negro, Venezuela

Then we determined the changes in the ecosystem due to slash and burn agriculture. The hypothesis was that the slash and burn agriculture releases the nutrients stored in the forest by turning the wood into ash which then is quickly leached away by the heavy rains. This was to explain the rapid loss of crop growth after the first crop cycle.

To test the hypothesis, we established a one-hectare experimental plot, and an adjacent control that was not treated.   The first step was to cut the forest.  After several months of dry season, the debris was burned.  The fire consumed the root mat and organic matter on top of the soil, but left intact the trunks and branches of the downed trees.

Manioc, (Manihot esculenta) the principal crop grew well for one to two years when it was planted close to the decomposing logs. Only after the second year was there a big decline in crop growth due to a decrease in soluble phosphorus in the soil. We noticed that when the native farmers spread leaf litter from the forest, production was restored. Through a series of laboratory experiments, we found that the reason for resurgence of growth was chelation of iron oxide by organic acids leached from decomposing leaf litter on top of the soil, with the result that iron could no longer bind phosphorus.

Pictured at right: First year's growth of manioc

 It wasn’t the leaching of nutrients from the soil that inhibited growth.  Rather it was the decrease in phosphorus availability in the soil due to binding of phosphorus by the clay because there were no organic acids to keep it soluble. There was plenty of phosphorus in the soil. It was just not available to plants. We concluded that the key to sustainability was maintenance of organic matter in the topsoil                                                                                                                 

 Other studies showed that significant decreases in soil nitrogen and potassium began at about three years, when the residual trunks and branches on the soil surface had decomposed, If at that time the site is abandoned to forest, as is done by indigenous people, nutrient stocks begin to recover if the area cleared is small enough so that seeds and litter from the surrounding forest are easily transported in by birds and small mammals.  If the site continues to be used beyond about three years as usually happens with pasture, nutrient loss continues until all that can grow are unpalatable shrubs adapted to the sterile environment. [See Mesquita et al.(2015) for a 25-year study in Central Amazonas contrasting rain forest succession following clear cutting (similar to slash and burn agriculture) and succession following conversion to pasture.]

Soils of the Georgia Piedmont

Soils in the Georgia Piedmont are similar to many Amazonian soils:

  • Unlike glaciated regions of the U.S. where relatively new rock supplies nutrients to the soil, the rock from which the Piedmont soil derives is from the Appalachian mountains, more than 400 million years old.  Unlike the Andean region of South America where many soils are derived from recent volcanoes,  the lowland Amazon soils (except for river floodplains) are highly leached remnants of the ancient Guyana Shield, 1.7 billion years old.
  • The climates of the Amazon and of the Piedmont are conducive to rapid weathering of soil.  Both regions have hot, wet weather that results in intense biological activity for at least eight months of the year. As a result, nutrient –containing minerals are leached away, leaving only very stable iron and aluminum oxides.  The iron oxides cause the red color in both Amazon and Piedmont soils. 
  • As in the Amazonian soils, most of the nutrients in the original Piedmont soils are concentrated in the organic matter of the uppermost horizon.

There is almost no original soil left in the Georgia Piedmont. However, at Spring Valley Ecofarms, we have a stand of old growth forest that grows on soil that apparently has never been cultivated. There is a very distinct boundary between the upper soil rich in organic matter, and the underlying red clay.

Over 100 years of cotton farming destroyed the organic matter in the Georgia Piedmont through erosion and oxidation. All that was left was the red clay subsoil.The faint trace of brown  organic matter just below the surface results from decomposition of herbaceous roots. The clay is very dense, and must be mechanically loosened to permit water drainage and root growth.

Pictured at right: Typical Georgia red clay soil, the “Official Soil of Georgia”.

Management of Piedmont Soils

Early pioneers in the Piedmont were impressed by the rapid growth of cotton, the first few years after the forest was cleared. We believe that the later decline in growth was due to the same factor as in the Amazon – lack of soluble phosphorus available to the crops, following loss of the topsoil rich in organic matter. When this happened, the early plantation owners took one of two options. They abandoned the plantation and moved westward to clear forests in Alabama or Mississippi, or they spread guano, high in phosphorus, imported from Peru. As the availability of guano declined, cotton production began to decrease. The invasion of the boll weevil ended “King Cotton” in Georgia.

The key to developing sustainable agriculture in the Georgia Piedmont is the re-establishment of an “A” horizon, that is, an organic layer of topsoil. Techniques to build soil organic matter while managing for an economic crop are described in the “Application” page of this website.

Mesquita R.M. et al. (2015). Amazon rain forest succession: Stochasticity or land-use legacy?  BioScience 65: 849-861.

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