Why rainforest soils are generally poor for agriculture


Understanding the basic composition of forest soils helps explain the concept of nutrient cycling in the rainforest; why there are problems with clearing rainforest lands for agriculture; and how soils are an important factor influencing forest complexity.


Over two-thirds of the world's rainforests -- including much of those in Madagascar -- can be considered "wet-deserts" in that they grow on extremely poor soils which are acidic and low in minerals and nutrients. The key to the luxuriant vegetation of these forests lies in the rapid nutrient cycling of the rainforest.


In the rainforest, most of the carbon and essential nutrients are locked up in living vegetation, dead wood, and decaying leaves. As organic material decays, it is recycled so quickly that few nutrients ever reach the soil, leaving it nearly sterile.

Decaying matter (dead wood and leaf litter) is processed so efficiently because of the abundance of decomposers including bacteria, fungi, and termites. These organisms take up nutrients that are released as waste and when organisms die. Virtually all organic matter is rapidly processed, even fecal matter and perspiration. It is only a matter of minutes, in many rainforests, before dung is discovered and utilized by various insects. Excrement can be covered with brightly colored butterflies, beetles, and flies, while dung beetles feverishly roll portions of the waste into balls for use later as larval food.

As vegetation dies, the nutrients are rapidly broken down and almost immediately returned to the system as they are taken up by living plants. Uptake of nutrients by plant roots is facilitated by a unique relationship between the roots and a fungi, mycorrhizae. The mycorrhizae attach to plant roots and are specialized to increase the efficiency of nutrient uptake nutrient from the soil. In return, plants provide the fungi with sugars and shelter among their roots. Studies have also shown that mycorrhizae can help a tree resist drought and disease.

Tropical rainforest trees are well-adapted to the poor soils of their environment. Since the first 6-8" (15-20 cm) of soil is a compost of decaying leaves, wood, and other organic matter, it is richest pool of nutrients on the ground. Canopy trees are generally shallow rooted to better tap this resource.

Being shallow rooted, compounded by the wet soils, has serious disadvantages for tall rainforest trees, especially with the strong winds of the upper canopy that can accompany periodic cyclones. To counter this inherent instability, some rainforest tree species have extensive root systems that run for over 325 feet (100 m). Other trees, especially tall emergent species, have buttress roots, which are large, thin extensions of the trunk that begin some twenty feet from the ground. These structures are thought to also aid in water uptake and storage, increase surface area for gas exchange, and collect leaf litter for additional nutrition. Some trees, especially palms have developed stilt roots for further support.


Tavy, or slash-and-burn agriculture, is widely used in the moist forests of Madagascar. As currently practiced with fallow periods too brief for regrowth after land abandonment, tavy effectively destroys the nutrient cycle and resource base that enables forest to grow on deficient soils. While nutrients released from burning vegetation may sustain one or two years of crop production, in the longer term the loss of living trees and decaying plant matter means that there is no longer fuel to feed the cycle. Without the mycorrhizae and other soil organisms to fix nutrients, vital minerals are rapidly leached by the sun and eroded away with rain.

Tavy, as traditionally practiced with long (10 years or more) fallow periods, can probably be an effective strategy for subsistence agriculture in the forests of Madagascar. Increasing the length of the fallow period and leaving intact forest surrounding the cleared patch are key to making tavy a sustainable practice. Unfortunately population/economic pressures and the rapidly disappearing forest cover in eastern Madagascar have made it difficult to carry out tavy in such a manner. The problem is that it is typically in the best short-term interest for Malagasy to clear as much forest as quickly as possible to maximize rice production. For people where day to day subsistence is a question there is little concern for the long-term consequences of their actions. From their perspective, as long as there is more forest land freely availabe for clearing (forests are often considered common property in Madagascar), you might as well take advantage of it before a neighbor does.

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