Factors of Pedogenesis - Major Soil Groups of the World - page 56

Fig. 2.19 Zonation of vegetation (and soils) in the Alps in relation to altitude and climate.
Photo : author.
￿ Drying out: this considerable when it is calculated that one kg of
dry plant matter requires for its formation 200-400 kg of water
in temperate climate.
￿ Absorption of basic cations and exchange with the H + -ions
excreted into the environment, which is thereby acidified at the
boundary with the root.
￿ Excretion of organic compounds and modification of the
rhizosphere (rhizodeposition).
￿ Consumption of oxygen.
￿ Physical roles: enlargement of fissures in the rock, stabilization
of the soil and reduction of erosion, but at times uprooting and
tree fall ( chablis ).
For further information the topic The Living Soil (Gobat et al . 2004)
may be referred to.

The principal roles of organic matter are the following:

Role of organic residues (Trichet 1999)

￿ acidification ; organic residues have the ability to exchange their
protons for the cations contained in minerals, thereby facilitating
their weathering;