Geoscience Reference
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doubly justified: on the one hand, they are abundant and play a major
role in pedogenesis; on the other hand, they are the most strongly held
by fulvic acids, if one disregards the particular case of mercury that is
fortunately rare in soils.
Table 11.1 gives a list of the principal acids, complexing or not, found
in podzolic soils.
Table 11.1
Principal types of soil acids with pH values corresponding to one
mole of acid per litre of water (H. Chamayou, pers. comm.). These acids are
arranged in order of decreasing efficiency in the decomposition of micas (Robert
et al
. 1979).
Acid
Type
pH
Oxalic
Low molecular weight (LMW) complexing acids,
2.8
Citric
representing from 0.5 to 5% of the dissolved organic matter
4.8
Tartaric
but 20 to 40% of that holding Al (Lundström
et al
. 2000a)
3.8
Formic
1.8
Malic
Low molecular weight, non-complexing
~ 1.7
Acetic
2.4
Fulvic
High molecular weight, complexing, the smallest are soluble
~ 2
in water
Humic
Very high molecular weight, complexing, insoluble in water
~ 2.3
at least in acid conditions
The list is far from exhaustive. The following acids are also found
in soils: hydroxybenzoic, vanillic, succinic, coumaric, shikimic, fumaric,
lactic, malonic, etc. But they play a very small role in podzolization
because they are not complexing agents, because they are present in
small amounts, or because the number of their acid groups per unit
mass is low (Lundström
et al
. 2000b).
In soil, the proportions of these various organic acids vary with
pedoclimatic conditions.
Fulvic and humic acids have the properties given in Table 11.2. But
we should keep in mind that the structure and molecular weight of
these substances are open to discussion (Chap. 2, § 2.5.2).
Even at suitable pH values, the solubility of humic acids and fulvic
acids in the soil solution is effective only if that solution is poor in
flocculating cations. In podzolic soils, that is, in absence of large
quantities of bases (Ca, K, etc.), it depends mostly on the acid/metal ratio
or, if one prefers, carbon/metal ratio or, still better, the C/(Al + Fe) ratio.
In these soils the last-mentioned ratio is generally between 30 and 60
(Dahlgren and Marrett 1991). If there is too much metal ion, the organic
