Geoscience Reference
In-Depth Information
It is surprising to find dark soils containing a sizeable quantity of
organic carbon (sometimes more than 1% in the surface horizon, and 50
to 100 t ha -1 of organic matter in the entire profile) in hot dry climates
favouring mineralization (Knowles and Singh 1998). This persistence
in Vertisols of organic carbon is because the organic matter is strongly
bound to the clay minerals. Up to 72 hours of repeated treatment with
hydrogen peroxide at 50° C is necessary to remove about 90 per cent
of the carbon originally present (Hyeong and Capuano 2000). The clay
then becomes light-coloured.
At the same time, the behaviour of smectites is peculiar in these soils
(Bornand et al. 1984):
￿ The clay minerals, when saturated with Mg, should normally
have a spacing of 15 Å; here the spacings are about 15.8 Å.
￿ After heating at 400° C, the layers do not collapse to 10 Å as
is usual for the smectites of other soils. The interlayer spacing
remains larger.
￿ Ethylene glycol that normally expands clay minerals is hardly
useful, except after the clay is treated with citrate.
All these are explained by the presence of organic matter, which
not only coats the clay particles but can also penetrate the interlayer
space. This penetration will be reserved for organic compounds with
low molecular weight. But other scientists think that the smectites retain
organic substances through cation bridges, through Ca ++ in particular
(Wattel-Koekkoek et al. 2001).
Organic matter in Vertisols
Table 6.1 presents the analytical characteristics of the horizons of a very
typical Vertisol of Cuba on alluvial deposits.
We can see the high clay contents, high cation exchange capacities
(meq/100 g) and the upper horizons that are decarbonated in contrast
to the ones below. The substratum is calcareous.
Example of analytical data
Table 6.1 Vertisol from Cuba (Paneque y Perez 1984).
Horizon Clay, % Silts (total),
Fine
Coarse
Texture
CaCO 3, %
(cm)
%
sand, %
sand, %
0-33
76.0
22.9
0.0
1.1
C
0
33-53
84.3
9.8
4.9
1.1
C
0
53-86
84.3
10.0
3.1
2.6
C
2.6
Table 6.1 Contd...
 
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