Aluminium and iron fractionation of European volcanic soils by selective dissolution techniques

Abstract

Soils developed from volcanic materials are characterized by mixture of short range order components such as allophane, imogolite, ferrihydrite, opaline silica, Al(Fe)-humus complexes and volcanic glass, in addition to the crystalline minerals halloysite, gibbsite, goethite, and aluminised 2:1 phyllosilicates. The accumulation of highly reactive components formed upon rapid weathering of volcanic rocks is the main cause for the unique physical and chemical properties of Andosols (Shoji and Fujiwara 1984, Dahlgren and Saigusa 1994). Depending on the dominant forms of Al, two kinds of Andosols are distinguished: 'allophanic' (Al in allophane and/or imogolite predominates) and 'non allophanic' (Al mainly in complexes with humus) (Shoji and Fujiwara 1984). When these components are less abundant, in the presence of volcanic glass, soils on volcanic materials are called 'vitric' (FAO 1998). Allophane and imogolite are formed preferentially at pH about 5. In more acid conditions, Al-humus complexes tend to be dominant (Shoji and Fujiwara 1984, Nanzyo et al. 1993). At pH<5, complexation of Al by organic matter exerts an anti-allophanic effect (Shoji et al. 1993), leaving no Al available to react with silica to form allophane or imogolite. The high organic carbon content of many of these soils is explained by protection exerted by non-crystalline alumino-silicates (Mizota and Van Reeuwijk 1989) or by a stabilizing effect of Al and to a lesser extent Fe on soil organic matter (Higashi 1983). Many properties of volcanic soils (phosphate retention, reactivity to NaF, water holding capacity, smeary consistence, etc.) are related to their active Al and Fe pools. Though mainly consisting of Al(Fe)-humus complexes, short-range ordered alumino-silicates (allophane or imogolite) and ferrihydrite, the pool may include other Al fractions, such as saltexchangeable Al, Al substituted in free iron oxides, hydroxy-Al polymers in interlayer positions of 2:1 layer silicates, and poorly crystallized gibbsite (Dahlgren and Walker 1993). In addition, Meijer and Buurman (1997) and Meijer et al. (this volume) suggested the presence of cation-depleted Alsilicate skeletons that may act as a source of active Al. Al-humus complexes play a major role in pH buffering and in the regulation of Al activity in the soil solution (Bloom et al. 1979ab, Cronan et al. 1986, Mulder and Stein 1994, Takahashi et al. 1995). Extractions with unbuffered salts, such as CuCl2 (Juo and Kamprath 1979) or LaCl3 (Bloom et al. 1979b, Hargrove and Thomas 1981), have been proposed as alternative methods for estimating the organic fraction of soil Al. Another fraction of interest is 1M KCl-exchangeable Al (Lin and Coleman 1960, Kamprath 1970). Although this is the standard method to measure 'exchangeable' Al, this interpretation was questioned for variable charge soils (Wada 1987ab, Dahlgren and Walker 1993, 1994), for organic matter rich soils (Ponnette et al. 1996), and for soils in which Al-humus complexes are abundant (Takahashi and Dahlgren 1998). In this chapter we evaluate several selective dissolution methods (SDM) for Al, Fe and Si, their use in the characterization of soil components in volcanic soils and their relation to some of the characteristic properties of these soils. Garcia-Rodeja et al. (2004) have previously studied the fractionation of Al in part of the COST622-Reference European volcanic soils, those from Italy, Azores, Iceland and Tenerife. © Springer-Verlag Berlin Heidelberg 2007.