Agriculture Reference
In-Depth Information
Salinolysis of feldspars leads to the following reaction:
Alkalinolysis may be written:
Hydration and oxidation
Hydration and oxidation may supplement the above processes. Hydration transforms
the original minerals through the addition of water molecules. A few specific hydration
reactions are relevant to soil formation e.g., formation of the iron hydroxide
goethite) from iron oxide which constitutes the cement of some sandstones and may lead
to their subsequent degradation. Oxidation is often mediated by oxygen dissolved in
water. When associated with hydrolysis, it particularly affects reduced iron which may
be retransformed into ferric oxide (Section I.1.2).
Neoformation
As weathering progresses, primary rock-forming minerals break down through hydrolysis
and other processes. Secondary minerals may form through the degradation of
the primary minerals or new (neoformed) minerals may crystallise from the ions released
into solution. These new minerals are more stable in the near-surface environment.
The types and amounts of neoformed minerals that develop depend on the weathering
environment and the age of the landscape. The distributions of the dominant chemical
weathering processes throughout the world and the typical products of neoformation are
discussed below in relation to soil-forming processes (Section II.4).
3.1.3
BIOLOGICALLY-MEDIATED PROCESSES
The four chemical weathering processes described above may involve purely abiotic
interactions and can occur in the absence of biological influences. However, they may
often be promoted directly by biological activity, or indirectly by the activities of simple
metabolites or organic compounds. Nutrient element uptake by plants is offset by
the release of equivalent quantities of protons to maintain a net electrochemical balance
(Grimaldi and Pédro, 1996). It has been well documented that simple organic and humic
acids from decomposing litter can destroy silicates through acidolysis (Lossaint, 1959;
Juste, 1965). Carbon dioxide produced by respiration and other metabolites may also
promote acid hydrolysis, alkalinisation, oxido-reduction or acido-complexolysis
( i.e., acidolysis by acid organic complexes) (Berthelin, 1976).
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