Andosols - Major Soil Groups of the World - page 304

Calculations give the following orders of magnitude (in %) of the
losses in Andosols: bases 90-100; silica 50; ferric iron 25; ferrous iron
90; aluminium 0-30.

In Andosols of the wettest climates, the most favourable for accumulation
of organic matter, mostly substances poor in silica are found: organic
complexes of aluminium as well as hydroxyaluminous 2/1 clay minerals
with wedges of aluminium in interlayer position (Ndayiragije and
Delvaux 2003; van Ranst et al . 2008). The 2/1 clay minerals and organic
matter thus trap the aluminium and prevent its precipitation in the form
of gibbsite. Some authors mention an anti-gibbsite effect (M.L. Jackson
in 1963). Similarly, many authors see an anti-allophane effect of carbon
compounds in the fact that allophane does not appear in Andosols rich
in aluminium complexed by organic matter.
We can summarize all this by saying that there is competition of
humus and silica for aluminium (FAO 2006) as shown in Figure 10.7.

Allophanic and non-allophanic Andosols

Allophanic Andosols
(young soils, pH > 5)

Non-allophanic Andosols
(old soils, very humid climate, pH < 5)

2/1 clay
minerals

Organic
matter

Silica

Al

Hydroxy-aluminous
clay minerals

Al-humus
complexes

Allophane

(Gibbsite)

Fig. 10.7 Diagrammatic representation of allophanic and non-allophanic Andosols and their
mineralogical composition.

The issue of iron-humus complexes is not evoked here. The oxides
and hydroxides of iron are more stable than its organic complexes. Thus
the latter are hardly found (Dahlgren et al . 2004).
When ( Al p + Fe p )/ C p is low, the organic matter is no longer protected;
it gets mineralized and equilibrium is re-established. When the ratio
is higher than 0.2, the organic matter is saturated with the metal and
precipitates. Thus, in andosolization there is no redistribution of Fe,
Al and organic matter in the profile. This is one of the features that
distinguish it from podzolization (Chap. 11). Studies have shown that
the contents of dissolved organic matter, Fe and Al in soil solutions are
very low in Andosols (Aran et al . 2001).