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[Biomes - Living Worlds] :: Rainforest :: Soil
Soil in the tropical rainforests is very nutrient poor. The topsoil is only one to two
inches (2.5 to 5 centimeters) deep. The only reason plant life is so lush is because
the plants store the nutrients in themselves rather than getting them from the soil.
When plants decay, other growing plants tap the nutrients from the dead matter and
reuse nutrients left over from that plant. This is why farmers can only use the
rainforest soil for one or two years after they clear cut it, before all nutrients are
stripped from the soil.
The reason soils are so infertile is because they are more than 100 million years old,
and have taken a beating from the elements. After time, rain washes minerals out of
the soil, leaving them more acidic and nutrient poor. Soil exposed to the heat and
condensed sunlight turns soil into red clay. Other soils just cannot deal with minerals,
and turn them into compounds useless to plants. There are some fertile patches of
soil in the rainforest, but they are scattered throughout the thick vegetation.
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. Excretement 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. I
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, expecially palms have developed stilt roots for further support.