Geoscience Reference
In-Depth Information
lastly, they occupy the basins in perhumid tropical zones: these
are the lowland tropical Podzols, for example Podzols of Borneo
under an annual rainfall of 2600 mm and a mean temperature
of 26° C (§ 11.4.2, § 11.4.3).
In France, they are found in the mountains associated with a set
of conditions: humid climate or pedoclimate, acid rocks, exposure to
cold and also acidifying vegetation giving a humus of the 'mor' type.
When any of these factors is lacking, the Podzol disappears giving
place to an 'Entic Podzol' with less differentiated horizons. Podzols are,
therefore, rare in France and Entic Podzols are, on the contrary, common
(Duchaufour 1947; Legros 1975). This is true for Switzerland too (Egli
et
al
. 2009).
11.3
GENESIS OF PODZOLS
11.3.1 Features of Weathering in Podzols
Let us first look at the indisputable facts brought out by scientists.
Three principal causes determine acidification of the soil (Dambrine
2001):
Firstly, dissolution of CO
2
in water provides carbonic acid:
CO
2
+
H
2
O
´
HCO
3
-
+
H
+
and
HCO
3
-
´
CO
3
2-
+
H
+
Calculations show that pH = 3.90 - ½ log PCO
2
. As in a drop of
pure water in equilibrium with air (rain) the partial pressure of CO
2
is
about 10
-3.5
atm., the pH becomes 5.654 (not 7 as is often assumed!). But,
in the soil, the PCO
2
is much higher than in air because of biological
activity and the accompanying respiration. If we have PCO
2
= 10
-1
, the
pH becomes 4.4. Such production of protons is not negligible. Yet, the
equilibrium pertains to a weak acid; the dissociation diminishes when
the number of protons increases.
Secondly and above all, in humus there are organic acids that regulate
the pH between 3 and 5, even at low concentrations (10
-3
to 10
-5
moles
per litre), because of their COOH and phenolic OH functional groups.
These acids are secreted by plants or result from their decomposition
or are products of microbial metabolism.
Thirdly, in forests exploited by man, removal of trees eliminates basic
cations.
Production of protons
