the greater the degree of alteration that will occur. Perhaps the first changes to bare rock
would be cracks or fractures due to mechanical weathering from ice wedging. Then plants
like lichens or grasses become established. As more and more layers of material weather, the
soil developssoil horizons,as each layer becomes progressively altered. The place where
the greatest degree of weathering occurs is the top layer. Each successive, lower layer is
altered just a little bit less. This is because the first place where water and air come in
contact with the soil is at the top. As water moves down through the layers, it is able to do
less work to change the soil. If you were able to dig a deep hole into the ground, you could
see each of the different layers of soil. All together, these are called asoil profile. Each
horizon has its own particular set of characteristics (Figure9.8).
Figure 9.8: Soil is an important resource. Each soil horizon is distinctly visible in this
photograph. ( 2 )
In the simplest soil profile, a soil is considered to have three horizons. The first horizon is
thetop soil,which is called the A horizon. The topsoil will usually be the darkest layer of
the soil, because this is the layer with the highest proportion of organic material. Remember
that humus forms from all the plant and animal debris that falls to the ground. This includes
branches and twigs, acorns and pine needles as well as waste from animals and fungi. The
top soil is the region of most intense biological activity. Many living organisms live within
this layer and plants stretch their roots down into this layer. In fact, plant roots are very
important to this layer because vegetation helps to hold this layer of soil in place. The top
soil layer is usually a layer where minerals that can dissolve and very small particles like
clay are absent. This is because clay sized particles get carried to lower layers as water seeps
down into the ground. Soluble minerals are missing because they readily dissolve in the fresh
water that moves through this layer and are carried down to lower layers of the soil.