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Soil Science Society of America Journal 67:198-207 (2003)
© 2003 Soil Science Society of America

DIVISION S-5—PEDOLOGY

Genesis of Tephra-derived Soils from the Roccamonfina Volcano, South Central Italy

A. Vacca*,a, P. Adamob, M. Pignab and P. Violanteb

a Dipartimento di Scienze della Terra, Università degli Studi di Cagliari, Via Trentino 51, 09127 Cagliari, Italy
b Dipartimento di Scienze del Suolo, della Pianta e dell'Ambiente, Università degli Studi di Napoli "Federico II", Via Università 100, 80055 Portici (Na), Italy

* Corresponding author (avacca{at}unica.it)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The properties of seven pedons, formed in three different tephra deposits inside the main caldera of the Roccamonfina volcano, were examined to better understand the genesis of these soils. Three of these soils derived from trachytic ash deposits are deep and porous and have andic properties, imogolite, and high allophane content. In these ash-derived soils, neogenesis of crystalline clays is limited but include halloysite and hydroxy-interlayered vermiculite. Hydrated halloysite is present only in the deepest horizons of two of these pedons. By contrast, the deep and clayey soil formed from alkali basaltic scoria and the shallow and slowly permeable soils derived from consolidated trachyandesitic tuffs have weak andic properties. In these soils, allophane content is negligible and pedogenesis induces the neogenesis of crystalline clays dominated by halloysite. Unlike the ash-derived soils, hydrated halloysite is present through most of the pedons. Climate, topography, and vegetation do not appear to be limiting factors for allophane formation in the area. The hydraulic properties of the parent material, possibly together with the time of pedogenesis, apparently affect the different pedogenetic trends of these soils. Allophanic soils developed in younger, porous, permeable ash deposits where rapid glass weathering is favored and Al and Si are readily available for allophane formation. By contrast, non-allophanic soils developed in older, less porous, and less permeable scoria and consolidated tuffs where pedogenesis induces the neogenesis of crystalline clays and hydrated halloysite is probably formed by in situ weathering of volcanic glass. Allophanic and non-allophanic soils are then coexisting in similar landscapes under similar climatic conditions.

Abbreviations: AAS, atomic absorption spectrometry • asl, above sea level • EA, exchangeable acidity • ECEC, effective cation-exchange capacity • D (subscript), sodium dithionite citrate extractable • O (subscript), ammonium oxalate extractable • OC, organic C • P(subscript), sodium pyrophosphate extractable • TEM, transmission electronic microscope • XRD, X-ray diffraction


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE GENESIS OF SOILS formed in tephra deposits has been extensively researched in many parts of the world (Mizota and Reeuwijk, 1989; Shoji et al., 1993). The fine particle size and the predominance of glass in tephra deposits favor preferential formation of short-range-order materials, most commonly allophane, imogolite, and ferrihydrite, and the crystalline clay mineral halloysite (Shoji et al., 1993). The weathering environment, which regulates Al and Si activities in the soil solution, determines which materials form (i.e., either noncrystalline components, or crystallized clay minerals, or both) (Ugolini and Dahlgren, 1991).

The Quaternary volcanic district of South Central Italy represents a Mediterranean environment with a temperate climate. This district is geographically divided into the Roman and Campanian petrographic provinces, and is characterized by extensive deposits with a wide range of compositions derived from potassium-rich magma (Sollevanti, 1983; Scandone et al., 1991; Chiesa et al., 1995). Soils derived from tephra belonging to the Roman province have been studied in detail to understand their pedogenesis (Bidini et al., 1984; Quantin et al., 1985, 1988; Lulli et al., 1988; Lorenzoni et al., 1995). Soils with andic properties occur at altitudes above 700 m, whereas soils typified by neogenesis and translocation of crystalline clay materials occur below 700 m. By contrast, few studies have investigated the genesis of soils derived from tephra belonging to the Campanian province. In particular, Lulli and Bidini (1978) demonstrated that five soils from the Roccamonfina volcano had andic characteristics with amorphous materials. Further studies on two of these soils identified large amounts of imogolite, proto-imogolite, allophane, and crystalline halloysite (Violante and Tait, 1979; Violante and Wilson, 1983). More intensive studies are needed to gain a better understanding of the soil properties in relation to soil-forming factors in this environment. Information on this topic could clarify pedogenic processes in similar environments in other volcanic districts.

The purpose of this study was to investigate the genesis, properties, and classification of seven pedons, formed in three different tephra-derived parent materials inside the main caldera of the Roccamonfina volcano. This setting provides a unique opportunity to elucidate the influence of parent material on pedogenic processes under a Mediterranean temperate climate. Although chemical composition is similar, we hypothesize that porous and highly permeable ash deposits favor allophane formation, whereas less permeable scoria and tuffs favor neogenesis of crystalline clay minerals.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Description of the Study Area
The Roccamonfina volcano lies in Northwestern Campania (South Italy) (Fig. 1) , between 41°15' to 41°20' N lat. and 13°56' to 14°02' E long., and belongs to the Campanian petrographic province of the Quaternary south central Italian volcanism (Chiesa et al., 1995). Volcanic activity in this region started 583 to 570 ± 11 kyr BP and ended 160 kyr BP, with the main caldera formed between 360 and 300 kyr BP (Chiesa et al., 1995). The postcollapse caldera phreatomagmatic activity, described in detail by Cole et al. (1992), led to the accumulation of pyroclastic surge deposits and associated airfall material, intercalated with lacustrine beds. The final stage in the main caldera was the formation of lava domes. The youngest volcanic product in the main caldera is a distal part of the Campanian Ignimbrite, which erupted about 30 kyr BP in the Campi Flegrei region (Fisher et al., 1993). In the survey carried out for the present work, we found previously unreported multiple trachytic ash deposits covering the inner slope of the caldera, the lava domes, and the Campanian Ignimbrite.



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  		Fig. 1. Map of the study area in Italy and within the Campanian petrographic province. Numbers 1 through 7 indicate the locations of the pedons examined in this study.

 
Our study focused on seven pedons (Fig. 1, Table 1) developed from the most common tephra outcroppings in the main caldera. The pedons were selected to represent soil development in the ashes, scoria, and consolidated tuffs deposited after the caldera collapse (Table 1). The dominant natural vegetation is chestnut (Castanea sativa P. Mill.) forest, but in the past the flat and gently sloping areas inside the caldera were used for agriculture. In these areas, in the 18th and 19th century, the most important crops were wheat (Triticum aestivum), oat (Avena sativa L.), and flax (Linum usitatissimum L.) (Panarello et al., 2000).


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  		Table 1. Selected site characteristics, locations, and time of deposition of parent materials for all pedons studied.

 
Elevations range from 500 to 1005 m above sea level (asl). There is one weather station located at 545 m asl. The climate is cold temperate with mean annual precipitation of 1479 mm and with an average seasonal precipitation of 633 mm in autumn, 420 mm in winter, 249 mm in spring, and 177 mm in summer (Ufficio Idrografico e Mareografico di Napoli, 1960–1995). The average annual air temperature is 13.4°C, with an average seasonal temperature of 10.5°C in autumn, 6.9°C in winter, 15.4°C in spring and 20.8°C in summer (Ufficio Idrografico e Mareografico di Napoli, 1960–1995). Average annual potential evapotranspiration (Thornthwaite and Mather, 1957) is 735 mm. The soil temperature and the soil moisture regimes are estimated to be mesic and udic.

Field Methods
The pedons were described by standard soil survey methodology (Soil Survey Division Staff, 1993). Bulk soil samples were collected from each genetic horizon of each pedon for laboratory analyses. From these horizons undisturbed samples were collected with metal cylinders (Soil Survey Laboratory Staff, 1996) for bulk density determination.

Laboratory Methods
Bulk samples were air-dried and crushed to pass a 2-mm sieve. All analyses were performed on air-dried <2-mm soil according to the procedures published by Ministero delle Politiche Agricole e Forestali (2000) unless otherwise specified. Coarse (2.0–0.2 mm), fine-sand (0.20–0.02 mm), silt (0.020–0.002 mm), and clay (<0.002 mm) fractions were separated by pipette and sieving following pretreatment with H2O2 to oxidize organic matter and dispersion aided by sodium hexametaphosphate. Bulk density of field moist core samples was determined according to Soil Survey Laboratory Staff (1996). Water content at 1.5 MPa was determined on air-dried and field-moist <2-mm soil (Soil Survey Laboratory Staff, 1996).

Soil pH was measured by potentiometry in soil/solution suspensions of 1:2.5 H2O, 1:2.5 1 M KCl, and 1:50 1 M NaF. Organic C (OC) was estimated by wet digestion with a modified Walkley-Black procedure. Exchangeable-base cations (Ca, Mg, Na, and K) were extracted with 1 M NH4OAc at pH 7.0 and quantified by atomic absorption spectrometry (AAS). Exchangeable acid cations (Al+3 and H+) were extracted with 1 M KCl and the extract was titrated to measure the total exchangeable acidity (EA). Effective-cation exchange capacity (ECEC) was calculated as the sum of exchangeable-base cations plus KCl-exchangeable acidity. Phosphate retention was quantified (Blakemore et al., 1987). Iron, Al, and Si were extracted by sodium dithionite citrate (Fed, Ald, Sid) and by ammonium oxalate (Feo, Alo, Sio); Fe and Al were also extracted by sodium pyrophosphate (Fep, Alp). Iron, Al, and Si in solution were determined by AAS. The allophane content was estimated from selective dissolution extracts (Parfitt, 1990). Allophane Al/Si molar ratio was estimated from (Alo - Alp)/Sio. Allophane was calculated by multiplying Sio by a factor depending on the Al/Si molar ratio, assuming allophanes with Al/Si = 1 contain 20% Sio and allophanes with Al/Si = 3.5 contain 6.25% Sio. Intermediate Sio contents of allophanes with intermediate Al/Si molar ratios were determined assuming a linear relationship between Sio and Al/Si molar ratios of 1 and 3.5.

Volcanic glass was identified and quantified in the sand fraction (2.0–0.02 mm) using a petrographic microscope (Soil Survey Laboratory Staff, 1996).

The clay fraction (<0.002 mm) was separated by sedimentation in water from the H2O2–treated <2-mm fraction following 15-min ultrasonic pretreatment (20 kHz and 200 W). The clay samples were split and saturated with 0.5 M MgCl2 or 1 M KCl solution (Jackson, 1974). The Mg- or K-saturated clay suspension was prepared by successive washings with water, 80% methanol, and acetone, then air-dried, gently ground in an agate mortar, and stored in a plastic container. The Mg- and K-saturated clay samples were oriented on glass slides from a water suspension. The Mg-saturated samples were glycerol-saturated by adding directly onto the dried clay film drops of the organic liquid solution (6% in ethanol). The mineralogy of the clay fraction was determined using a Rigaku Geigerflex D/Max IIIC X-ray diffractometer (Rigaku/MSC, Kent, England) equipped with iron-filtered Co-K{alpha} radiation generated at 40 kV, 30 mA, with divergent, receiving, and antiscatter slits at 1°, 0.15 mm, and 1°, respectively. Scan speed was of 1° 2{theta} min-1. The following treated slides were scanned for each soil sample: Mg-saturated, air-dry; Mg-saturated, glycerol-saturated; K-saturated, air-dry; and K-saturated, heated to 110, 350, and 550°C.

Transmission electron microscopic (TEM) observations of the clay fractions (<2-µm), separated with the same procedure used for X-ray diffraction (XRD) analyses, were performed with a Philips CM12 electron microscope (Philips Industrie S.A., Wavre, Belgium). Samples were prepared by applying and drying one drop of suspended material directly onto carbon-coated Formvar film Cu grids.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Morphological Properties
Pedons 1, 2, and 3 are deep soils reflective of repeated ash depositions to the soil surface (Table 2). Pedons 1 and 2 are characterized by five and three, respectively, distinct ash deposits. It is not known at what depth the lava occurs in Pedon 2. Pedon 3 developed from three distinct ash deposits overlying the Campanian Ignimbrite (4R horizon). These three pedons have thick black to dark brown granular A horizons and well-expressed Bw horizons with subangular blocky structure (Table 2). Both A and Bw horizons are very friable and highly permeable. Stratification, dark granular A horizons, Bw horizons with subangular blocky structure, very friable moist consistence, and high permeability are common to volcanic ash soils (Shoji et al., 1993). Although Pedons 1 and 2 are located on steep and moderately steep slopes, respectively, there were no signs of significant erosion observed in the field, probably because of the dense vegetation covering the areas and highly permeable soils.


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  		Table 2. Selected morphological properties of all pedons.

 
Pedon 4 is characterized by three distinct tephra layers (Table 2). The upper 110 cm of the soil originated from alkali basaltic scoria and is clayey in both the A and Bw horizons. Many thin clay coatings were observed on the surfaces of the weathered scoria in the Bw horizon, but previous micromorphological studies on similar soils in the area did not show any oriented clay (Lulli and Bidini, 1978). We interpret these clay coatings as the result of in situ weathering of the scoria without translocation. The underlying 2C horizon, occurring in an ash bed, is massive, but not cemented, and overlies a buried A horizon, darker in color with clay texture and angular blocky structure.

Pedons 5, 6, and 7 are shallow, developed from a thin ash layer (A1 horizons) overlying consolidated tuffs (Table 2). They have very dark to dark A horizons with granular structure in the upper part (A1) and subangular blocky structure in the lower part (2A2), over thin 2Bw horizons with subangular blocky structure. The 2Cr horizons are massive and are restrictive to roots and water. In Pedon 6, water permeability in the 2Cr horizon is restricted to the extent that the thin overlying 2Bw horizon was wet in the field and had a dry color of 2.5Y6.5/2, indicating reduction (Soil Survey Division Staff, 1993). The 2Cr horizon directly overlies an R horizon in Pedon 6, whereas in Pedons 5 and 7 the 2Cr horizons overlie a sequence of Cr horizons, which were not sampled, derived from different layers of the pyroclastic surge deposit.

Physical Properties and Volcanic Glass Content
The differences in texture among the pedons reflected their origin from different volcanic materials (Table 3). Generally for Pedons 1, 2, and 3, the texture determined in the laboratory by the pipette method is lower in clay and silt than textures estimated in the field. This is probably caused by the presence of noncrystalline materials, which contribute to the formation of stable aggregates that strongly resist dispersion, together with the possible cementing action of Fe oxides and hydroxides (Shoji et al., 1993). Pedon 4 is clayey throughout, except for the loamy 2C horizon, which is characterized by 398 g kg-1 of silt and 121 g kg-1 of clay. Texture is generally sandy clay loam in Pedons 5, 6, and 7. Textural changes within each soil mainly reflect the presence of discontinuities.


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  		Table 3. Selected physical properties and volcanic glass content of all pedons.

 
All horizons of Pedons 1, 2, and 3 have oven-dry bulk densities of <0.80 Mg m-3 (Table 3). This is typical for volcanic ash soils at the weathering stage where a porous soil structure has developed, strongly influenced by noncrystalline materials and high soil organic matter concentrations (Wada, 1985; Shoji et al., 1993). In Pedon 4, oven-dry bulk densities from the surface to the 110-cm depth are very similar, ranging from 0.98 to 1.06 Mg m-3, and are influenced by the clayey texture. Pedons 5, 6, and 7 have bulk densities of <0.9 Mg m-3 only in the thin surface horizon, which is richer in organic matter. Subsurface horizons of Pedons 5, 6, and 7 for which data are available have bulk densities >=0.9 Mg m-3.

Water retention at 1.5 MPa on both air-dry and field-moist samples is relatively high for Pedons 1 and 2 (Table 3), and is attributed to the presence of allophane and allophanelike materials with fine particle-size and hollow spherical structure (Wada, 1989; Shoji et al., 1993). The large reduction of 1.5 MPa water content with drying (>=5% absolute) suggests the presence of allophane (Shoji et al., 1993). For the other pedons, water retention at 1.5 MPa is generally lower, as is the reduction with drying. One exception is the Bw horizon of Pedon 4, which has a trend similar to that of Pedons 1 and 2.

Volcanic glass ranges from 17 to 48% of the 0.02- to 2.0-mm fraction of all pedons (Table 3). The absence of significant differences in glass content between pedons formed on tephra deposits of different age was observed. Assuming a very glassy nature of all parent materials, this result could be explained by the different weathering rate of glass in volcanic ash, scoria, and consolidated tuffs. Rapid weathering of glass in the porous, permeable, and younger ash deposits would have rapidly reduced the glass concentration. By contrast, the weathering of the older consolidated volcanic tuffs and scoria would have been much slower, resulting in a much slower decrease in total glass content.

Chemical Properties and Selective Dissolution Analysis
Water pH values, ranging from 5.0 to 6.9, are generally higher than those reported by Shoji et al. (1993) for volcanic soils, and the values are higher in surface than in subsurface horizons (Table 4). This is attributed to the deposition of volcanic ash layers on the soil surface. Sodium fluoride pH values are >10.5 for all horizons of Pedons 1, 2, and 3, and in the 2C horizon of Pedon 4, which can be indicative of amorphous material (Shoji and Ono, 1978).


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  		Table 4. Selected chemical and mineralogical properties of all pedons.

 
Organic C concentrations are highest in the surface horizons and decrease with depth in all soils (Table 4). In Pedons 1, 2, and 3, appreciable amounts of OC are also present in the subsurface horizons, suggesting intermittent ash deposition. Organic C >100 g kg-1 occur only in the surface horizons of Pedons 1 and 2. These two pedons are located at elevations >=800 m, where low temperatures may facilitate OC accumulation.

Exchangeable-base cation concentrations are generally higher for Pedons 5, 6, and 7, except in the surface horizons of Pedons 1 and 2, and generally decrease with depth (Table 4). These differences are probably related to differences in the composition of parent materials and to the presence or absence of an ash layer at the soil surface. Pedons 5, 6, and 7 are formed from parent materials which are higher in base-cation composition compared with Pedons 1, 2, and 3. However, all these pedons have an ash layer at the soil surface which could supply the bases necessary for maintaining higher exchangeable-base cation concentrations. In Pedon 4, formed from alkali basaltic scoria, the lack of surface fresher glassy material could explain the lower exchangeable-base cation concentrations and pH(H2O) values <=5.5. The concentration of exchangeable cations in surface horizons reflects vegetation nutrient cycling. As expected from pH values, exchange acidity is generally very low (<1 cmolc kg-1) for all pedons. Only the A2, Bw, and 3Ab horizons of Pedon 4 have exchange acidity values >1 cmolc kg-1. Effective-cation exchange capacity generally decreases with depth and is higher for Pedons 5, 6, and 7, except in the surface horizons of Pedons 1 and 2. Base saturation is >=96% for Pedons 1, 2, 5, 6, and 7. In Pedon 3, it ranges from 84 to 93% and in Pedon 4 from 60 to 90%, reflecting the absence of resupply of base cations from the surface ash layer as in other pedons.

Phosphate retention is >=85% for Pedons 1, 2, and 3, suggesting the presence of amorphous materials. Only 14% phosphate retention was determined for the 2C horizon of Pedon 6, but all the other horizons in Pedons 4 through 7 have phosphate retention >25%. In these pedons, phosphate retention is probably related to the presence of Fe-oxides (Tables 4 and 5).


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  		Table 5. Results of selective dissolution analysis of <2-mm soil of all pedons.

 
Selective dissolution results (Table 5) show that the concentrations of the various Fe forms are different among pedons, but their relative abundance is similar. In all pedons, concentrations of Fed are higher than Feo and Fep, indicating that considerable amounts of Fe released from the weathering of Fe-bearing minerals are transformed into more or less crystalline Fe-oxides. Concentrations of Feo are always higher than Fep, indicated by Fep/Feo ratios <=0.35, suggesting that the non or poorly crystalline form of Fe is largely ferrihydrite as opposed to organically bound Fe. The Feo/Fed ratio is related to the degree of crystallinity of the Fe oxides (Schwertmann, 1985) and has been found to reflect the degree of soil development in volcanic soils (Malucelli et al., 1999). The A and B horizons of the studied pedons have mean Feo/Fed ratios ranging from 0.36 (Pedon 1) to 0.63 (Pedon 4), which reflect the occurrence of a consistent poorly crystalline fraction of Fe oxides (Cornell and Schwertmann, 1996). According to the different ages of the parent materials, it was expected that weathering would have led to a decrease in Feo/Fed ratio with age. However, no trend was observed suggesting that in the studied soils the solubility of the Fe oxides, like their concentration, is independent of pedogenic time. It seems likely that the controls on the types of secondary Fe oxides formed are similar in the studied soils, perhaps since parent material chemical composition, climate, and vegetation are similar for all pedons.

In Pedons 1, 2, and 3 the Alp/Alo ratio ranges from 0.07 to 0.23 (Table 5), suggesting that active Al is incorporated mainly into short-range-order aluminosilicates (e.g., allophane and imogolite) instead of Al-humus complexes. In Pedons 4 through 7, the Alp/Alo ratio is generally higher, because of Alo contents generally <10 g kg-1. In Pedon 4, Alp values are very high (>20 g kg-1) for the Bw and 3Ab horizons. The relative amounts of Al extracted by sodium pyrophosphate are highly dependent on the centrifugation and filtration methods employed since colloidal Al phases are important components dispersed with this extractant (McKeague and Schuppli, 1982; Loveland and Digby, 1984). The Bw and 3Ab horizons of Pedon 4 are rich in clay (511 and 545 g kg-1, respectively) and the very high values of Alp are likely because of a significant fraction of the colloidal Al passing the filter.

Acid-oxalate extractable Al and Sio, together with Alp, are used to estimate the Al/Si molar ratio of the short-range-order Al-Si material and the contents of allophane and imogolite (Dahlgren, 1994). The Al/Si molar ratio is estimated using the formula (Alo - Alp/Sio), which corrects for the Al dissolved from humic complexes by the acid-oxalate treatment. Aluminumrich allophanes and imogolite typically have an Al/Si molar ratio close to 2:1. The siliconrich allophanes have an Al/Si molar ratio close to 1:1. Short-range-order Al-Si materials with Al/Si molar ratios between 1:1 and 2:1 consist of mixtures containing various proportions of imogolite and Al- and siliconrich allophanes rather than a single phase (Parfitt, 1990). Relatively high concentrations of Alo and Sio in Pedons 1, 2, and 3 suggest that these soils are rich in allophane and imogolite. Multiplying Sio values by appropriate conversion factors provided by Parfitt (1990), the concentration of allophane/imogolite in these pedons was estimated to range from 9 to 27%, and generally increased with depth (Table 5). Imogolite is commonly found in association with allophane in many Andisols. Imogolite may form either by transformation from allophane through desilication, or by precipitation from weathering solutions (Wada, 1989). Allophane and imogolite have distinct morphologies and may be easily detected by electron microscopy. In Pedons 1, 2, and 3 imogolite seems to coexist with allophane, both forms being identified in the clay fraction using the transmission electron microscopy (Fig. 2a) . In contrast, Pedons 4 through 7 have estimated allophane concentrations <=3%, with the exception of the 2C horizon of Pedon 4 (42%).



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  		Fig. 2. Transmission electron micrographs of the clay fraction of selected soil horizons: (a) smooth and curved fibres of imogolite in the 3Bw1 horizon of Pedon 2; (b) aggregates of spherical, ring-shaped particles of allophane in the 2C horizon of Pedon 4; (c) spheroids and tubes of halloysite in the 4Bw2 horizon of Pedon 1; and (d) tubes of halloysite in the Bw horizon of Pedon 4.

 
Pedons 1 and 2 occur on slopes >=18% at elevations >=800 m, where the environment is probably moister and cooler. However, Pedon 3 occurs 200 m lower in elevation on 2% slope in a landscape similar to where Pedons 4 through 7 occur. Because present vegetation and past land use are similar in this landscape, climate, topography, and vegetation may not be considered as limiting factors for allophane formation in Pedons 4 through 7. Parent material, possibly together with the time of pedogenesis, seems to have played a significant role in determining the different trends of allophanic materials formation in the studied soils. Pedons 1, 2, and 3 developed in younger, porous, permeable ash deposits, which would favor rapid weathering of glass, making Al and Si easily available. The abundance of Al and Si in a pedoenvironment with pH(H2O) values between 5.7 and 6.7 favors coprecipitation and allophane/imogolite formation. The presumed moister and cooler environment that characterizes Pedons 1 and 2 relative to Pedon 3 may facilitate this process. In contrast, Pedons 4 through 7 formed in scoria and consolidated tuffs that are older, less porous, and less permeable than ash. Although volcanic glass content is high in these soils, the hydraulic properties of these parent materials, possibly together with a longer period of pedogenesis, favor the neogenesis of crystallized over short-range-order aluminosilicate minerals. Restriction of vertical water movement is likely to increase Si in soil solution and volcanic materials may directly weather to halloysite (Parfitt et al., 1984; Parfitt and Wilson, 1985).

X-Ray Diffraction of the Clay Fraction
Crystalline clay mineralogy is similar for all the studied pedons (Table 4), except for the 2C horizon of Pedon 4, which did not show crystalline minerals. This suggests that the chemical composition of the parent materials does not greatly affect crystalline weathering products. De Gennaro et al. (1973) also found the same pool of clay minerals in soils formed from volcanic materials with different chemical compositions in other areas of the Roman and Campanian volcanic provinces. However, the XRD patterns for Pedons 1, 2, and 3 were generally poorly resolved because of the abundance of allophane. For example, for the same clay mineral, peak intensity was much less in Pedons 1, 2, and 3 than in Pedons 4 through 7. This supports the hypothesis that neogenesis of crystallized clays is more important in Pedons 4 through 7. Halloysite was detected in all horizons within the studied pedons. However, hydrated halloysite occurs only deeper than 110 cm in Pedons 1 and 3, while it occurs in shallow depths in Pedons 4 through 7. In Pedons 1, 2, and 3, halloysite could have been formed during dry periods, which caused increased Si concentration in the soil solution (Parfitt et al., 1984; Parfitt and Wilson, 1985; Wada, 1985). In the slowly permeable Pedons 4 through 7, halloysite could be formed by in situ weathering in an environment where Si in the soil solution is likely to be high (Parfitt et al., 1984; Parfitt and Wilson, 1985). Quantin et al. (1988) described in detail this type of weathering and clay neogenesis on the Vico volcano.

The group of minerals represented by a peak at 1.4-nm is presumably chlorite or vermiculite, and hydroxy-interlayered vermiculite. Mica and plagioclase are also present in all horizons. The highly ordered nature of mica may suggest its inheritance from the parent material, while the poor crystallinity of the 1.4-nm intergrade may suggest a pedogenic origin, in accordance with the findings of Violante and Wilson (1983) in similar soils in the area. Amphibole, inherited from parent material, is revealed by a small peak at 0.8 nm present only on clay samples from the A1, A2, and Bw horizons of Pedon 4, the A1 horizon of Pedon 5, and the A1 and 2Bw horizons of Pedon 7. The presence of easily weatherable clay minerals, such as amphiboles, indicates that these soils have developed in a fairly short time.

Transmission Electron Microscopy
Transmission electron microscopy confirmed the results obtained by selective dissolution and XRD analysis by revealing several distinct clay crystal morphologies commonly observed in volcanic soils. These morphologies are smooth and curved fibers of imogolite varying in diameter from 5 to 15 nm and extending up to several micrometers in length (Fig. 2a); aggregates of spherical, ring-shaped particles of allophane with diameters of 10 to 20 nm (Fig. 2b); spheroids (about 0.2 µm wide) (Fig. 2c) and tubes (from about 0.1 to 0.2 µm long) of halloysite (Fig. 2c,d).

Classification
Criteria for andic soil properties (Soil Survey Staff, 1998) were examined for horizons composing the upper 60 cm or the whole pedon if less than 60 cm thick. Pedons 1, 2, and 3 meet the first requirement for andic soil properties (Alo + 0.5 Feo >=20 g kg-1; 33 kPa bulk density <=0.9 Mg m-3; and phosphate retention >=85%). Pedons 4, 5, and 7 have a phosphate retention >=36%, 303 g kg-1 or more particles 0.02 to 2.0 mm in size, Alo + 0.5 Feo totaling 4 g kg-1 or more, and volcanic glass content >=17%. The volcanic glass content is always enough so that the glass percentage, when plotted against the Alo + 0.5 Feo value, falls within the andic soil properties area (second requirement for andic soil properties). All these pedons are classified as Andisols (Table 6). Difference in these Andisols is mainly reflected at the family level by their mineralogy classes. Pedons 1, 2, and 3, where the concentration of allophane/imogolite was estimated to range from 9 to 27%, belong to the amorphic mineralogy class. Pedons 4 and 5, where the concentration of allophane/imogolite was estimated to be <=3% and Feo contents are considerably higher than Sio contents, belong to the ferrihydritic mineralogy class. Pedon 7 contains less Feo than Pedons 4 and 5 and belongs to the glassy mineralogy class because of its volcanic glass content >=36% in the 0.02- to 2.0-mm fraction. The shallow family of Pedons 5 and 7 is because of the presence of a paralithic contact <50 cm deep (the 2Cr horizons).


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  		Table 6. Family classification of all pedons.

 
Pedon 6 does not meet the requirements for andic soil properties in 60% or more of its thickness because Alo + 0.5 Feo and volcanic glass content are to low in the 2A2 horizon. This pedon has a mollic epipedon, a lithic contact at 50 cm, and classifies as a Mollisol (Table 6).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Pedogenesis in the studied soils appears to be greatly affected by the different hydraulic properties of the parent materials, possibly together with the time of pedogenesis. In the deep and porous soils formed from younger volcanic ash the most important pedogenic process is the formation of short-range-order alluminosilicates. Neogenesis of crystallized clay minerals in these soils is limited. In contrast, pedogenesis in the deep and clayey soils formed from older scoria and in the shallow and slowly permeable soils derived from older consolidated tuffs is characterized by the neogenesis of crystallized clay minerals. Ferrihydrite and metal-humus complexes are also present in these soils. Therefore, allophanic and nonallophanic soils are capable of coexisting in similar landscape and climatic conditions. Allophanic soils classify into the amorphic family of Andisols, while the nonallophanic soils belong to the ferrihydritic or glassy family of Andisols or are Mollisols (relative lack of short-range-order alluminosilicates).


   ACKNOWLEDGMENTS
 
We are very grateful to J.L. Boettinger and anonymous reviewers for their many helpful comments and suggestions.

Received for publication July 19, 2000.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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A. A. Jara, A. Violante, M. Pigna, and M. de la Luz Mora
Mutual Interactions of Sulfate, Oxalate, Citrate, and Phosphate on Synthetic and Natural Allophanes
Soil Sci. Soc. Am. J., February 2, 2006; 70(2): 337 - 346.
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