- the balance between soil system inputs and outputs.
- the redistribution of and chemical changes in the soil constituents.
The three major soil horizons (can be further sub-divided) are referred to by specific letters to indicate their genetic origin.
1. The upper layer or A horizon, is where biological activity and humus content are at their maximum. It is also the zone that is most affected by the leaching of soluble materials and by the downward eluviation of clay particles.
2. Beneath this is the B horizon which is the zone of accumulation or illuviation, where clays or other materials moved down from the A horizons are redeposited. The A and B horizons together make up the true soil.
3. The C horizon consists mainly of weathered material (regolith) resting on the bedrock (unaltered undelying geology).
This 3 fold division is useful but is
over-simplified:
* Humus may be mixed throughout the soil or it may form a distinct
layer. Where humus is mixed into the soil it gives a crumbly, black,
nutrient rich layer known as mull (as in the diagram above under
deciduous woods). Where humus is slow to decompose - like in cold, wet
upland areas it produces a fibrous, acidic and nutrient-deficient
surface horizon known as mor (peat moorlands)
* The junctions of horizons are not always clear to see.
* All horizons need not always be present
* The depth of the soil and of each horizon vary at different sites.
Local conditions produce soils with characteristic horizons differing
from the classic A, B, C pattern - for example a waterlogged soil,
suffering from a shortage of oxygen, develops a gleyed (G)
horizon. (In waterlogged soils the Fe+++ ions (reddish) are reduced to
Fe++ (grey) so the soil is a 'grey gley'.