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.
SOIL COMPOSITION
Over two-thirds of the world's rainforests, and three-fourths of the Amazonian rainforest can be considered "wet-deserts" in that they grow on red and yellow clay-like laterite soils which are acidic and low in nutrients. Many tropical forest soils are very old and impoverished, especially in regions—like the Amazon basin—where there has been no recent volcanic activity to bring up new nutrients. Amazonian soils are so weathered that they are largely devoid of minerals like phosphorus, potassium, calcium, and magnesium, which come from "rock" sources, but are rich with aluminum oxide and iron oxide, which give tropical soils their distinctive reddish or yellowish coloration and are toxic in high amounts. Under such conditions, one wonders how these poor soils can appear to support such vigorous growth.
Rainforests are tremendously vegetated. Early European settlers in the tropics were convinced (and even assured by scientists at the time) that the lushness of the "jungle" was due to the rich soils, so they cut down large patches of forest to create croplands. The cleared land supported vigorous agricultural growth, but only for one to four years, when mysteriously, plant growth declined to a point where copious amounts of fertilizer were required for any growth. Settlers wondered why their crops perished and how such poor soil could support the luxuriant growth of tropical rainforest. The answer lies in the rapid nutrient cycling in the rainforest.
NUTRIENT CYCLING
The colonial settlers did not realize that they were dealing with an entirely different ecosystem from their temperate forests where most of the nutrients exist in the soil. In the rainforest, most of the carbon and essential nutrients are locked up in the 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, which are released as wastes 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. Insects are not only attracted to dung for the energy value, but often for the presence of nutrients like calcium salts. Human sweat is a treasure for several species of butterflies, which gather on the necks and hats of tourists, and for annoying sweat bees, which can cover seemingly every inch of exposed skin in some forests.
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.
Buttress roots of giant strangler fig tree in Sulawesi, Indonesia. Click image for more pictures of buttress roots. (Photo by R. Butler).
Review questions:
Why does traditional agriculture often fail in rainforests?
How are nutrients recycled in the rainforest environment?
How are tropical forest soils different from soils in temperate forests?