Fjölrit RALA - 15.06.2004, Page 134

Water properties of volcanie ash soils rich in high charge halloysite
Grégoire Pochet and Bruno Delvaux
Unité Sciences du Sol, Department of Environmental Sciences and Land Use Planning, Université catholique de
Louvain (UCL), Belgium
Highly weathered soils rich in low charge 1:1 clay minerals and iron oxides usually exhibit
high water infiltration rates. Their hydraulic conductivity at water saturation (A'sat) may range
between 10 and 50 cm h1. Since these clayey soils are also characterized by low available
water, they behave like sands at moisture tension (MT) below 1500 kPa (pF 2.5), and hke
clays above this MT value. This well-known behaviour is caused by a strong micro-
aggregation involving kaolinite and iron oxide.
In Tonga island (South Pacific, humid tropical climate), fine clayey soils (70-95% clay)
derived from basaltic ash are widespread. They are rich in organic carbon (1-5% in the top 50
cm), which significantly accumulated in the clay fraction. Their clay fraction is largely
dominated by halloysite and free iron (~ 9 %), with a large proportion of poorly crystallized
Fe oxide. The cation exchange capacity of the clay fraction (CECciay) ranges between 15 and
90 cmolc kg"1 clay. In some clay fractions, swelling 2:1 silicates were detected by X-ray
diffraction. The intensity of the XRD features of these 2:1 swelling clays was, however, not
correlated to the magnitude of CECciay. The soils exhibited low bulk density (0.9-1.2 g.cm"3),
and high hydraulic conductivity (ATsat) at water saturation in all horizons (10-90 cm h"1). Their
porosity and water properties, as measured through the water retention curve, showed low
available water despite a very high moisture content at wilting point (>30%). These hydric
and hydrodynamic properties suggest a strong micro-aggregation.
Micro-aggregation in halloysitic soils is poorly documented and may seems paradoxical
because of the CEC of the clay fraction which is typical for high activity clays (HAC), rather
than for low activity clays.
Our results suggest that the combination of a large proportion of poorly crystallized iron
oxide and the accumulation of humic substances in the clay fraction may provide a strong
basis for micro-aggregation in HAC soils. This hypothesis underlines current research
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