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Fe oxides must be taken into account to interpret these results (see Buurman et al., this
volume).
The Alp/Al0 ratio was <0.5 in most samples, indicating that the reactive A1 pool is
dominated by inorganic constituents. The content in allophane, estimated according to Mizota
& van Reeuwijk (1989), which is well-correlated with allophane contents estimated from Si0
(r2=0.93), ranged from negligible in the soils from Greece, Hungary and Italy (EUR01 and
02) to more than 20% in some horizons fforn France and Azores. In general, the higher
allophane contents occur in horizons with pHwater between 5.5-6.5. The average Al/Si molar
ratio ranged from 1.7 in the soils from Iceland to 2.7 in those from Azores, with intermediate
values in the other soils (1.9, in Italy; 2.1, in France; 2.6, in Tenerife). In spite of the
dominance of the inorganic Al-pool, Al-humus complexes are an important component in
many soils. Their proportion increases with carbon content (Alp/Al0 vs C content r2=0.76,
excluding the soils with very low Alp). In most horizons, CuCl2 extracted about of 35% of Alp
(r2=0.96), suggesting that it constitutes a specific fraction of organically-complexed A1
(García-Rodeja et al., 2004). In many mineral horizons with allophane, Alcu is highly
correlated to CEC (r2=0.76), which suggests that A1 dominates the adsorption complex.
The content in ferrihydrite (calculated from Fe0) is usually less than 2%, although it exceeds
4% in some horizons from Azores and France. As with allophane, the higher ferrihydrite
contents occur in the pH range 5.5-6.5. Calculating ferrihydrite directly from Fe0 suggests that
there is no extractable Fe in humus complexes. Because pyrophosphate extracts quantities
equal to 5-100% of Fe0, this assumption is irrealistic. Fep clearly increases with C content in
soils EUR03 and 04 from Italy (r2=0.99) and in those from Azores, Iceland and France (r2
0.73-0.75, excluding horizons with very high C contents). Extractable manganese is clearly
more abundant in the surface horizons of the soil EUR07. Citrate-dithionite extracted from
>80% (soil EUR20) to <10% (soil EUR09) of Mn,. Mn0 was close to Mnd in the soils from
Hungary or France, but clearly lower in EUR10 and 12. Good correlations of Mnt with Mn^
(r2=0.96) and of Mnd with Mn0 (r2=0.97) were found for the whole data set, but r2 decreases if
the 3 horizons richer in Mn are excluded from the regression analysis.
References
García-Rodeja, E., Nóvoa, J.C., Pontevedra, X., Martínez, and A., Buurman, P. 2004.
Aluminium fractionation of European volcanic soils by selective dissolution techniques.
Catena 56:155-183.
Borggaard, O.K. 1985. Organic matter and silicon in relation to the crystallinity of soil iron
oxides. Acta Agriculturae Scandinavica 35, 398- 406.
Buurman, P., van Lagen, B., and Velthorst, E.J. (Eds.) 1996. Manual for Soil and Water
Analysis. Backhuys Publishers, Leiden, The Netherlands. 314 pp.
Buurman, P., Meijer, E.L., Fraser, A., and García-Rodeja, E. 2004. Extractability and FTIR
characteristics of poorly-ordered minerals in a collection of volcanic ash soils. This volume.
Juo, A.S., and Kamprath, E.J. 1979. Copper chloride as an extractant for estimating the
potentially reactive aluminium pool in acid soils. Soil Sci. Soc. Amer. J. 43:35-38.
Holmgren, G.G.S. 1967. A rapid citrate-dithionite extractable iron procedure. Soil Sci. Soc.
Amer. Proc. 31:210-211.
Mizota, C., and van Reeuwijk, L.P. 1989. Clay mineralogy and chemistry of soils formed in
volcaic material in diverse climatic regions. Intemational Soil Reference and Information
Centre, Soil Monograph 2, Wageningen.
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