Geography Reference
In-Depth Information
soil. In most soils, the majority of the solid material is mineral
matter which has been derived from rock weathering. Often only
2 to 6 per cent of the soil is organic matter but it is still very
important. Soil organic matter consists of decomposing plant and
animal debris known as
litter
.
It also consists of
organic matter
more resistant to decay known as
humus
, and living organisms
and plant roots known as the soil
biomass
. Indeed, soil usually
contains billions of bacteria in every handful. Litter is decomposed
by soil organisms to produce humus which is a stable end product,
resistant to further decomposition. Plant nutrients, especially nitro-
gen, phosphorus and sulphur are released as litter breaks down and
this process is known as
mineralisation
. Soil organic matter holds
mineral particles together, stabilising the soil; improves the water-
holding capacity; improves aeration; and it is a major source of
nutrients, important for soil fertility.
Air and water fill the gaps between solid soil particles. Soil air is
very important as soil animals, plant roots and most microorganisms
use oxygen and release carbon dioxide when they respire (breathe).
In order to allow soil organisms to survive, oxygen needs to move
into the soil and carbon dioxide must be able to move out of the
soil. Therefore, the aeration of the soil is an important component
influencing biological activity and litter decomposition.
Soil water contains dissolved substances which are important for
uptake by plant roots and moves dissolved chemicals through the
soil (both up and down) making them available for plants. Interest-
ingly, water is held in the soil despite gravity forces pulling it
down. Even in very hot, dry deserts, water is still found in the soil
showing that the forces holding water in the soil must be strong.
This water remains in the soil because the combined chemical
attraction of the water molecules to each other and the attraction
of water to the soil particles is greater than the gravitational force.
If you dip the end of a tissue into a bowl of water, then you can
watch the water being drawn up the tissue showing that water does
not always flow downhill. This is capillary action. Smaller pores
exert stronger attraction forces on the water than larger pores.
Therefore, capillary water will be driven to move from wetter parts
of the soil to drier parts because the drier parts have more small
pores that have little water thereby exerting a capillary attraction
force on the water. This capillary action is also how plant roots
