SSSAJ Journal of Natural Resources and Life Sciences Education
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via ISI Web of Science (1)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Jongmans, A.G.
Right arrow Articles by Nieuwenhuyse, A.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Jongmans, A.G.
Right arrow Articles by Nieuwenhuyse, A.
GeoRef
Right arrow GeoRef Citation
Agricola
Right arrow Articles by Jongmans, A.G.
Right arrow Articles by Nieuwenhuyse, A.
Soil Science Society of America Journal 64:254-262 (2000)
© 2000 Soil Science Society of America

DIVISION S-5-PEDOLOGY

Induration of C Horizons by Allophane and Imogolite in Costa Rican Volcanic Soils

A.G. Jongmansa, L. Denaixb, F. van Oortb and A. Nieuwenhuysec

a Dep. of Soil Science and Geology, Agricultural Univ., Wageningen, The Netherlands
b Unité de Science du Sol, INRA, Versailles, France
c REPOSA, Guapiles, Costa Rica

toine.jongmans{at}bodeco.beng.wau.nl


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Little is known about the contribution of allophane and imogolite to induration of soil horizons in the perudic tropics. In this study we observed, to a depth of 10 m, indurated sandy C horizons that contained isotropic translucent fine-textured coatings in Holocene and Pleistocene alluvial andesitic deposits situated in the perudic moisture regime in Costa Rica. The horizons have a firm to extremely firm consistence and do not slake in water. Optical microscopy showed that the coatings cover and bridge primary minerals, forming a three-dimensional framework in the horizons, which indicates a post-depositional origin. Bulk chemical analyses show the presence of allophane in the indurated horizons. Scanning electron microscopy (SEM) of undisturbed mineral surfaces confirmed the existence of the coating framework. Chemical analyses, obtained by SEM energy-dispersive x-ray, indicate an alumino-silicate coating with Al/Si ratios ranging from 1.6 to 2.1. Coatings were studied by TEM, carried out on an ultra-thin section (50 nm) cut from a coating that was in situ sampled by microdrilling in an uncovered thin section. All clay material in the horizons consists of coatings of allophane and imogolite. Rapid weathering of the andesitic parent material released large amounts of Al and Si, which accumulated as amorphous gels due to oversaturation of the soil solution near the wetting front or in deeper water-conducting layers. We conclude that the mechanisms for soil horizon induration include changes in moisture conditions due to short seasonal drying, climatic changes, tectonic uplifting, and erosion, which lead to irreversible drying of allophane and imogolite.

Abbreviations: SEM, scanning electron microscopy • TEM, transmission electron microscopy


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
INDURATED HORIZONS are common features of volcanic soils in regions with pronounced dry periods (Quantin, 1994). At shallow depths they may cause serious problems with respect to agricultural management and they may enhance soil erosion (De Noni et al., 1994). Two main opinions exist with respect to the genesis of such horizons: (i) inheritance as a result of previous geological processes (Prat, 1992; Prat and Quantin, 1994); (ii) formation and accumulation of various secondary materials resulting from pedogenic processes under different environmental conditions.

A great deal of information is available about cementation of soil horizons by secondary minerals. Various cementing agents have been identified: silica (Campos et al., 1991; Campos, 1992; Rossignol et al., 1992; Hildago et al., 1993; Yamada et al., 1994, Dubroeucq and Thiry, 1994), carbonates (Fedoroff et al., 1994), gypsum or other soluble salts (Harden et al., 1991; Nimlos and Zamora, 1992), iron compounds (Luzio and Palma, 1994), illuviated or in situ neoformed clay minerals (Oleschko et al., 1992; Hessman, 1992).

On the other hand, papers dealing with induration of soil horizons by allophane and imogolite are scarce and mainly restricted to podzols: McKeague and Kodama (1981) suggested induration of BC and C horizons in podzols from British Columbia by allophane and imogolite. Also, Farmer et al. (1985) reported the presence of allophane and imogolite as likely cementing agents in Scottish and Australian podzols. Vogel et al. (1994) speculated that allophane may indurate soil horizons in volcanic soils in ustic moisture regimes, but the data provided did not support their speculations. Leamy et al. (1990) suggested that indurated layers may not be cemented only by Si, but that allophane and imogolite may be involved as well.

Studies of indurated horizons in volcanic soils are most often restricted to regions with ustic or (semi) aridic moisture regimes (Soil Survey Staff, 1998), e.g., the Tepetate formation in Mexico and Ecudador (Hildago et al., 1992). The pronounced dry period favors hardening of the accumulated secondary materials present in soil horizons (Miehlich, 1992; Quantin, 1994; Zebrowski, 1992). Under perudic climatic conditions this phenomenon has not yet been described in volcanic soils.

We observed induration of sandy CB and C horizons in volcanic soils (Andisols–Inceptisols) from 2000 years and older, developed on alluvial, Holocene and Pleistocene volcanic parent materials in perudic, tropical Costa Rica. Induration is absent in younger Entisols. Within the indurated horizons isotropic coatings were observed in intergranular microvoids, forming a three-dimensional framework of various thickness.

The goal of our study is to assess the morphology, chemistry, and mineralogy of isotropic coatings in indurated sandy horizons. In addition their genesis and effects on soil horizon induration will be discussed in relation to environmental conditions.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Site Conditions
All study sites are in the lowland of the Atlantic zone in Costa Rica (Fig. 1) . Fine to coarse textured sediments, which originate from the inland volcanic cordillera, are deposited throughout the lowlands. Profiles H1 through H7 are formed in Holocene, unconsolidated, stratified sediments. H1 through H3 are developed in fine sandy, marine beach-ridges, along the Caribbean sea. Profile H4 and H5 are developed in sandy (H4) and fine sandy (H5) fluvial deposits, whereas profiles H6 and H7 occur in coarse sandy-to-gravelly textured alluvial fan sediments. All Holocene sediments have a flat topography with no evidence of erosion. Profiles P1 through P3 are on Pleistocene stratified alluvial deposits, which are scattered throughout the lowland as residual erosion hills with flat tops, 10 to 25 m above the surrounding Holocene alluvial sediments (Nieuwenhuyse, 1996b). Parent materials of all study locations have an andesitic composition.



View larger version (55K):
[in this window]
[in a new window]
  		Fig. 1 Location of the sample area

 
All study sites have a perudic moisture regime. Mean annual rainfall ranges from 3500 to 5700 mm (Nieuwenhuyse, 1996b); relative humidity is 85 to 90%. Potential evapotranspiration ranges from 3.5 to 4.5 mm d-1. From December to April, dry spells of several weeks may occur at all sites causing the soils to dry out (Nieuwenhuyse et al., 1993, 1996b). Mean annual air temperature is 26°C (iso-hyperthermic temperature regime, Soil Survey Staff, 1998). Vegetation at the H1 through H4 profiles is tropical lowland forest, while H5 through H7, and P1 through P3 are under extensive pasture.

Drainage classes range from somewhat excessively to moderately well drained (Table 1) . Soil classification (Soil Survey Staff, 1998), estimated age, altitude, and average depths of the groundwater table are shown in Table 1.


View this table:
[in this window]
[in a new window]
  		Table 1 Soil classification, estimated age, average depth of groundwater, drainage and altitude of the Costa Rican lowland soils studied

 
Analytical Methods
A soil pit was dug in all Holocene study locations except H6 and H7. Profile P1 was situated in a 15m deep drainage gulley of a banana plantation. Sites H6, H7, P2, and P3 were surveyed in road cuts. At sites were sampled after refreshing the exposed wall for a 1-m width. Undisturbed samples of the indurated horizons were taken for preparation of thin sections (10 x 10 cm) as well as for scanning electron microscopy (SEM). Thin sections were prepared according to Fitzpatrick (1970), studied with a Zeiss petrographic polarizing microscope, and described according to Bullock et al. (1985). Stereo-microscope observations were carried out on undisturbed mineral surfaces in order to characterize the indurated horizons at the millimeter scale of observation, and to select samples for SEM and TEM studies.

Undisturbed mineral surfaces were sputter-coated with Au Pd in a vacuum evaporator and studied with a Philips 525 SEM-EDXRA operated at 20 kV, providing micrographs and qualitative chemical data.

Undisturbed parts of isotropic coatings present in the indurated horizons of H7 and P2 were isolated by microdrilling in thin sections (Verschure, 1978). Such coating parts were reimpregnated and ultra-thin sections ({approx}50 nm) were cut using an ultramicrotome, and prepared according to van Oort et al. (1994). Simultaneously, clay suspensions of coatings from H7 and P2 were made, and a drop of the suspensions was deposed on Cu microgrids covered with holey-C films. Both coating fragments and suspensions were studied by transmission electron microscopy (TEM; Philips EM 420) operated at 120 kV. Sites H7 and P2 were selected for TEM research, based on previous optical examination of indurated horizons.

Bulk samples of the indurated horizons were taken for chemical analyses. Organic C was determined using the Walkley-Black method (van Lagen, 1993). The pH H2O was measured potentiometrically in a 1:5 soil to solution suspension after 2 h shaking (van Lagen, 1993). Sodium dithionite extractable Fe and Al (Fed and Ald) , ammonium oxalate extractable Fe, Al, and Si (Feo, Alo, Sio) and sodium pyrophosphate extractable Al and Fe (Alp and Fep) were determined according to analytical procedures described by van Lagen (1993). Allophane contents were calculated using the formula proposed by Parfitt and Wilson (1985), and later simplified by Mizota and van Reeuwijk (1989): .


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
Field Observations
Major macromorphological characteristics of indurated horizons are shown in Table 2 . These horizons occur exclusively below the Bw horizons: at 50 to 70 cm depth in the younger Holocene sediments, and from 80 cm to 10 m depth in the Pleistocene sediments. Their thickness ranges from 15 to 60 cm and their textures, from coarse to fine sand. The sand grains in indurated horizons of the Pleistocene sediments show pellicular, linear, and dotted alteration, indicating that they were affected by weathering.


View this table:
[in this window]
[in a new window]
  		Table 2 Major macromorphological characteristics of the indurated horizons of the Costa Rican lowland soils

 
The young Holocene Entisol (Udipsamment, H1) consists of loosely packed fine sand. None of the features due to mineral alteration or induration were observed. Textures of A and B horizons in the older Holocene Andisols (Durudands H2 H7) range from loam (A horizon) to sandy loam (B horizon). Textures of the A and B horizons in the Pleistocene Inceptisols (Distropepts P1 P3) range from clay loam to clay as a result of strong weathering of the primary minerals, such as volcanic glass, feldspars, and pyroxenes. Consistence of the A and B horizons of both Andisols and Inceptisols is (very) friable. By contrast, the consistence of the underlying, dominantly coarse textured, indurated parts of the C horizons varies from firm to extremely firm. The indurated horizons had an extremely firm consistence that was difficult to dig with a spade. None of the horizons will slake in water.

During sampling (May 1995), the groundwater level in H2 through H7 occurred 10 to 20 cm below the indurated layers. In the Pleistocene study sites groundwater was always deeper than 8 m.

Optical Microscope Data
The parent material in the sandy C horizons of all soils consists of andesitic rock fragments in which the fine textured groundmass is dominated by microcrystals (microlites, MacKenzie et al., 1982) and some volcanic glass. The microlites and the phenocrysts within the rock fragments consist predominantly of plagioclase and pyroxene. In addition, plagioclase, pyroxene, and opaque iron occur as single mineral grains. The content of volcanic glass in H2 and H3 range from 4 to 11% (Nieuwenhuyse et al., 1996a). The amount of volcanic glass in H1, and H4 through H6 is in the same range. In H7 and in P1 through P3, volcanic glass (<2%) occurs only in the fine-textured groundmass of some andesitic rock fragments. Yellowish green, sand-sized clay bodies consisting of 2:1 phyllosilicates, are observed (Jongmans et al., 1994c). All fine material (<5 µm) occurs as coatings, which partially or completely fill inter-mineral microvoids.

In the C horizons of the young Holocene profile H1, no alteration of primary mineral grains is observed, and fine textured, translucent, isotropic coatings were absent.

In the (very) firm parts of the C horizons of the older Holocene profiles H2 through H6, few plagioclase and andesitic rock fragments show weak pellicular alteration, indicating that little weathering of primary minerals occurred in these horizons. Many, thin (10–20 µm) pale yellow, translucent, isotropic coatings partially cover sand grains and form bridges between them (Fig. 2A) .



View larger version (155K):
[in this window]
[in a new window]
  		Fig. 2 Selected micromorphological features. (A) Isotropic, fine textured coatings in the indurated horizon (80 cm) of soil H5. (B) Isotropic, fine textured coatings in the indurated horizon (1.8 m) of soil P2. Left images in crossed polarized light, right images in plane polarized light.

 
In the indurated part of the C horizon of the oldest Holocene profile H7 some plagioclase minerals show pellicular and dotted alteration, whereas the color of volcanic glass inside some rock fragments shifts to orange-brown, indicating weathering of volcanic glass and primary minerals. Abundant translucent, isotropic coatings occur with sizes up to 60 µm, and sometimes the outerparts are partially impregnated with iron compounds.

In the indurated part of the C horizons in the Pleistocene profiles P1 through P3, primary minerals frequently displayed dotted and irregular linear alteration, indicating partial weathering of primary minerals. The volcanic glass in andesitic rock fragments showed similar weathering features as observed in H7. In addition, abundant translucent, fine textured, isotropic, (pale) yellow to almost colorless coatings up to 60 µm thick cover sand grains and form bridges between them (Fig. 2B).

Stereo microscope observation show that the coatings in all profiles form a three-dimensional framework between the sand grains (bridged-grain microstructure).

All optical observations reveal that the coatings in all study sites have similar morphology except with respect to their thickness.

Chemical Analyses
Data for pH, extractable Al, Fe, and Si, calculated allophane and Al:Si molar ratios are shown in Table 3 . The pH in the young Udipsamment H1 is above 6.5, the older soils are slightly acid. Low values for Alp and Fep in all indurated horizons indicate an absence of organically bounded iron and aluminium. The Alo and Sio contents show that Al and Si are dominantly present as inorganic amorphous constituents. In the Udipsamment H1, allophane is absent. In the indurated layers of the Durudands H2 through H6 the allophane content increases with soil age and ranges from 38 kg-1 in H2 to 240 g kg-1 in the H7 soil. In the indurated layers of P1 through P3, allophane contents vary from 130 to 152 g kg-1. These differences between the sites suggest different stages of indurated horizon development. The Al:Si atomic ratios are close to 2 indicating the presence of Al-rich allophane and/or imogolite. The presence of noncrystalline material like allophane and imogolite in the indurated horizons coincides with the occurrence of isotropic fine coatings in these horizons.


View this table:
[in this window]
[in a new window]
  		Table 3 Chemical data, calculated allophane contents, and atomic Al/Si ratio of the indurated horizons of lowland Costa Rican soils

 
Scanning Electron Microscopy-EDXRA
Our SEM micrographs of natural mineral surfaces from indurated horizons in the Holocene soils H2 through H7 show the presence of thin, very fine textured, homogeneous coatings (Fig. 3A, B) , present in intergranular voids. They cover mineral grains and are connected with each other, forming a continuous three-dimensional framework. Qualitative chemical spectra show that the coatings consist of Al and Si with a Al:Si ratio of approximately 2 suggesting the presence of an aluminum rich alumino-silicate (Fig. 3C). In the H1 soil, such coatings are absent.



View larger version (89K):
[in this window]
[in a new window]
  		Fig. 3 Scanning electron microscopy of rough surfaces from indurated horizons in the Holocene profiles (H4). (A) Coatings forming a three-dimensional framework between the sand grains. (B) Detailed view of A. (C) Qualitative chemical spectrum of the coatings

 
Our SEM micrographs of samples from profiles P1 through P3 reveal the occurrence of similar, but thicker coatings (up to 60 µm) with the same distribution pattern as found in the Holocene soils (Fig. 4A, B) . EDXRA spectra show that they consist of Al and Si, with an Al:Si ratio of 1.3 (Fig. 4C). Traces of iron are measured.



View larger version (87K):
[in this window]
[in a new window]
  		Fig. 4 Scanning electron microscopy of undisturbed mineral surfaces from indurated horizons in the Pleistocene profiles (P2). (A) Coatings forming a three-dimensional framework between the sand grains. (B) Detailed view of A. (C) Qualitative chemical spectrum of the coatings

 
Transmission Electron Microscopy-EDS
The TEM micrographs from ultrathin (50 nm) undisturbed parts of isotropic coatings of H7 and P2, isolated by microdrilling show the presence of a gel, consisting of allophane units and imogolite threads. This gel covers primary minerals but no weathering relationship is evidenced between them (Fig. 5A, 6A and B) . Our EDS analyses reveal alumino-silicate compositions with Al:Si ratio 1.6 to 2 for H7, and 1.45 to 1.7 for P2. Differences in Al:Si ratios, compared with SEM-EDXRA data, are ascribed to differences in sample surface roughness and thickness. Although the allophane and imogolite composition of the coatings is confirmed by the TEM analyses of undisturbed ultrathin sections, the characteristic morphology of the noncrystalline compounds can better be visualized by performing TEM on extracted clay suspensions. Since optical microscopy and SEM results show that all clay-sized material present in the indurated horizons consists of isotropic coatings, extracted clay from these layers corresponds to these coatings. Our TEM images obtained from such clay suspensions show the occurrence of both allophane and imogolite (Fig. 5B, 6C). Detailed images confirm the characteristic morphology of imogolite threads (Fig. 5C) and allophane spheres (Fig. 5D). No gibbsite was found in the coatings.



View larger version (113K):
[in this window]
[in a new window]
  		Fig. 5 Transmission electron microscopy of coatings in the Holocene profiles (H7). (A) Micrograph made in the ultra-thin section of a coating, showing the presence of allophane and imogolite. (B) Micrograph made of a suspension. Overview of allophane and imogolite. (C) Detail of B showing the characteristic threads of imogolite. (D) Detail of B showing the characteristic spheres of allophane

 


View larger version (103K):
[in this window]
[in a new window]
  		Fig. 6 Transmission electron microscopy of coatings in the Pleistocene profiles (P2). (A) Micrograph made in the ultra-thin section of a coating showing the presence of allophane, imogolite and pieces of volcanic glass (v). (B) Detail of A showing imogolite threads (i). (C) Micrograph made of a suspension showing allophane spheres and imogolite threads

 
Mechanisms of Horizon Induration
Indurated soils are common in Central and South America (Quantin, 1994; Zebrowski, 1992). They occur in regions with a pronounced warm and dry season (Shoji et al., 1993; Quantin, 1994) and frequently have a duripan within 1 m of the soil surface. Duripans are thought to be formed as a result of silica release caused by rapid weathering of primary minerals, percolation of silica-rich soil solutions into deeper soil horizons, and precipitation of silica either at lithological discontinuities or near the depth of the wetting front.

In our study the optical and in situ submicroscopical data, ranging from the millimeter to the nanometre scale of observation, all point to the occurrence of noncrystalline alumino-silicate coatings. Bulk chemical analyses and deduced calculations reveal the presence of appreciable amounts of allophane and imogolite with Al:Si ratios close to 2. Our TEM analyses performed both on undisturbed ultrathin sections of in situ coating fabrics as well as on extracted clay suspensions confirm and visualize the allophane and imogolite composition of the coatings. Noncrystalline coatings in the indurated horizons probably result from translocation of aluminium and silica, liberated by rapid and intensive weathering of volcanic glass and Ca-plagioclases in the overlying andesitic deposits. An increase of the soil pH, near the wetting front present in the C horizons, may induce supersaturation and precipitation of allophane and imogolite, as reported by Dahlgren and Ugolini (1989) in tephretic Spodosols. Formation of noncrystalline coatings begins because of the abundance of Si and Al in the soil solution due to rapid weathering under the prevailing perudic conditions. Noncrystalline materials form due to faster precipitation kinetics compared with crystalline clay minerals (Shoji et al., 1993). The resulting three-dimensional framework of allophane imogolite coatings leads to consolidation of originally loose, sandy soil horizons. It reflects the first step in the induration process of sandy soil horizons. Consolidation effects in sandy sediments, caused by coatings of illuviated clay, was reported by Hessman (1992).

Leaching processes dominate in the perudic tropics due to high precipitation and temperature and the related intensive weathering. The downward solute flow moves out of the profile, and allophane and imogolite accumulations occur in C horizons. Jongmans et al. (1994a and 1994b) reported the presence of allophane coatings in perudic Andisols to a depth of 6 m. In our study, allophane and imogolite were observed to a depth of 11 m below the soil surface. No unequivocal explanation can be provided about the depth at which the indurated layer forms. An increase of pH may favor oversaturation of Si and Al in the soil solution and precipitation of allophane and imogolite (Dahlgren and Ugolini, 1989). The wetting front hypothesis and the occurrence of textural discontinuities are often advocated as mechanisms responsible for determining the depth at which the indurated horizon develops. In our Holocene sites, the indurated horizon is situated above the groundwater table, suggesting a mutual relationship. The soil solution containing Al and Si percolates downwards, and will meet groundwater, transported upwards by capillary rise. We speculate that differences in pH between both solutions may induce oversaturation and precipitation of allophane and imogolite just above the groundwater table. The same mechanism might be valid in the Pleistocene sites. The depth at which the indurated horizons currently occur may reflect historic zones of capillary rise and groundwater levels in a sedimentary landscape that is subject to uplifting due to tectonic movements, and sea level lowering in the Pleistocene. Milnes et al. (1991) reported that silicified horizons in sediments of different ages in South Australia might reflect former positions of the phreatic groundwater tables.

It is well known that allophane and imogolite have high water-holding capacities (Wada, 1989), as demonstrated by thixotropic properties. In Andisols, soil consistence is closely related to soil water conditions: moist and wet Andisols display friable to sticky consistence (Shoji et al., 1993). As long as horizons with allophane coatings remain wet, thixotropy and sticky consistence will be maintained.

Kubota (1976), Warkentin and Maeda (1980) and Wada (1989) described that upon drying, noncrystalline alumino-silicate gels form stable aggregates, due to formation of high energy chemical bondings between allophane and imogolite units. Therefore it is likely that induration of soil horizons by allophane and imogolite is initiated by irreversible drying from the three dimensional coating framework due to changes in soil moisture conditions. Seasonal drying predominantly occurs in areas with an ustic or (semi)aridic moisture regime. As a result, indurated soil horizons in volcanic soils are most often described and characterized in such regions (Quantin, 1994).

It appears contradictory that induration by allophane–imogolite is possible in a region with a perudic moisture regime (Soil Survey Staff, 1998). However, Nieuwenhuyse et al. (1993) and Nieuwenhuyse (1996b) reported that from December to April dry spells of some weeks occur in the study area, causing water shortages for pasture and crops. During such periods the evaporation of the tropical rainforest may contribute to the drying out of the profiles as well. We speculate that during such short dryer periods within the average perudic climate, today or in the past, the allophane coatings in the younger and older Holocene sites H2 through H6 start to dry out and initiate induration of the C horizons. With time, induration will increase, finally resulting in a (very) firm consistence of such layers. The indurated three-dimensional coating framework persists due to low faunal activity in the C horizons. The hardening of such layers in the oldest Holocene H7 and the Pleistocene P1 through P3 profiles is probably dependent on more than one factor. Uplifting of the landscape by tectonic movements (Nieuwenhuyse, 1996b), as well as lowering of the mean sea level during the Pleistocene period and subsequent landscape incision has contributed to a permanent lowering of the groundwater level. In turn, this may have lead to irreversible drying of the allophane–imogolite coatings and soil horizon induration.


   Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
Formation of a three-dimensional framework of allophane and imogolite coatings at different depths in unconsolidated stratified sandy volcanic deposits may occur in the perudic tropics. This framework hardens upon irreversible drying, due to long and short-term changes in moisture conditions at depths where it is protected against disturbance by biological activity. It eventually leads to formation of hard, cemented sandy horizons. In older Holocene and Pleistocene soils, indurated horizons that occur close to the rooting zone, may cause problems to agricultural management and erosion hazards.Food and Agricultural Organization of the United Nations 1990; Hessmann 1992; Nieuwenhuyse Jongmans van Breemen 1992; Nieuwenhuyse 1996


   ACKNOWLEDGMENTS
 
The authors are indebted to P. Verburg for his constructive critical comments on an earlier version of the manuscript.

Received for publication April 27, 1998.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 





This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via ISI Web of Science (1)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Jongmans, A.G.
Right arrow Articles by Nieuwenhuyse, A.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Jongmans, A.G.
Right arrow Articles by Nieuwenhuyse, A.
GeoRef
Right arrow GeoRef Citation
Agricola
Right arrow Articles by Jongmans, A.G.
Right arrow Articles by Nieuwenhuyse, A.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome