Soil processes
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4A. Soil profile
From Chapman et al.
(1985) |
Soil in the rainforest is very important because of the role that it plays in
recycling the organic matter
from the forest floor into a form that the trees can use for further
growth. Soils in the rainforest tend to have a profile that is not
very deep but which is very high in organic matter near the surface,
with not much organic matter past the top layers as it is highly
leached because of the high rainfall.
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| 4B. Exposed soil |
Just as we saw at earlier stops that the vegetation in the rainforest
is unique, because the factors that lead to it are slightly different
in each case, the same can be said about the soil. Firstly, just
as the soil affects the trees so the trees affect the soil. Trees
and other organisms can affect the chemical nature of the soil,
whether it is acid or alkaline. In a rainforest the soil tends to
be more acid because of the leaf litter on the ground. Secondly,
fauna can affect soil by burrowing into it, eating organic matter
and excreting it in other parts of the soil profile, where it might
be more useful in its more broken-down form. Examples include worms
which use organic matter for food, then burrow, moving some organic
matter into the top layer of soil as they go, and depositing digested
matter in this layer and further down. This helps to make the soil
healthier by allowing air into the deeper layers and improving drainage.
Organisms are one of five factors that affect soil formation; the
others are bedrock, the climate, topography, and time.
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| 4C. Geological cross-section
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As can be seen in the diagram of the geology of the Minnamurra
area, there are varying bedrocks throughout the Minnamurra
Rainforest, with sandstones and volcanic rocks being the dominant
types. These would produce podzolics from the sedimentary sandstone
materials and kraznozems from the volcanic materials. The type
of bedrock affects the soil type and this in turn affects the
type of forests which grow in that soil. Sandstone-based soils
tend to be less fertile than volcanic based soils and are therefore
normally associated with warm temperate rainforest in this area;
whilst volcanic soils are quite fertile and tend to be associated
with the more abundant subtropical forest.
Climate can affect soil because
the amount of rainfall influences how quickly organic matter,
nutrients and minerals are moved through the soil profile and
how quickly they break down. This is also affected by temperature.
If an area is likely to have rainforest it will tend to have a
warm, wet climate. This leads to soils that are highly leached
and quick at breaking down organic matter. In addition lime, or
calcium carbonate, which makes soils alkaline, is soluble in water,
so this is quickly dissolved and leached out of the soil, leaving
an acid soil.
Topography affects soils because the position of soil
on a slope or on flat ground affects how quickly water drains
either off or through it. If soil is located on a slope, some
of the water that falls as rainfall will not penetrate the soil
profile but will drain straight over the soil towards a lower
point. Flat land, in the same rainfall area, will have more of
the rain infiltrate into the soil profile. This means that the
effects of water as described above are more noticeable on flat
land than on adjoining slopes, which will have drier soil despite
the same rainfall. This affects the rainforest because it means
that volcanic soils, where we would expect to see subtropical
rainforest, there may be warm temperate rainforest instead, because
the soil will not be moist enough to support subtropical plants.
In addition, if a slope faces the direction of the strongest winds
it will tend to be dried out, making conditions less favourable
for subtropical rainforest, whilst slopes facing away from the
wind are often more protected, and it is more likely that subtropical
rainforest will grow there.
Time affects soils because the older the soil, the more
developed it is, in terms of where minerals, nutrients and organic
matter are found in the profile. Generally older soils will have
had time to move these things down the profile. However, once
soils attain a certain age, they reach an equilibrium where the
minerals etc. are moved through the profile and taken up by plants
at a constant rate. This continues as long as the organisms that
affect that soil continue to exist undisturbed. Rainforest soils
have generally reached this equilibrium, so it is hard to tell
how old they are apart from saying they are old enough to be well
developed.
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