Published online 1 January 2007
Published in Soil Sci Soc Am J 71:181-188 (2007)
DOI: 10.2136/sssaj2004.0382
© 2007 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
PEDOLOGY
Pre- and Post-Mined Bauxite Soils of Jamaica: Physical and Chemical Properties
W. A. Greenberg*
Biology Dep., Bemidji State Univ., Bemidji, MN 56601
L. P. Wilding
Soil & Crop Sciences Dep., Texas A&M Univ., College Station, TX 77843
* Corresponding author (wgreenberg{at}bemidjistate.edu).
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ABSTRACT
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Earthy bauxite (aluminum ore) deposits over limestone bedrock cover over 20% of Jamaica's surface area. With the shortage of arable land in Jamaica, there is great interest in using post-mined bauxite lands for agriculture. The purpose of this study was to compare physical and chemical properties of pre- and post-mined bauxite soils that are relevant in assessing suitability of these lands for small-scale agriculture. Pedons were described and sampled for characterization at two pre-mined sites and four post-mined sites near Mandeville, Jamaica. Post-mined soils were generally steeper, shallower, and higher in limestone rock fragments than pre-mined soils. Pre-mined soils consisted of Ap and Bo horizons making up clayey Oxisols with low charge and low bulk density. The new caret (^) symbol, which indicates horizons formed from human transported material, was helpful in describing pedons of post-mined soils. Post-mined soils consisted of ^Ap horizons (replaced topsoil) and 2^C horizons (fill material) over limestone. Some post-mined pedons also had 2Bob or 3Bob horizons (Bo in situ buried by replaced topsoil and/or fill material). Post-mined soils had higher bulk density in the ^Ap horizons than pre-mined Ap horizons and higher pH throughout due to the incorporation of limestone rock fragments. In ^Ap horizons, organic matter and granular structure increased with time after reclamation and establishment of grass cover.
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INTRODUCTION
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In 1938, Sir Alfred D'Costa began having soil tests done with a goal of improving soil fertility on his farm in St. Ann Parish, Jamaica. Upon analysis, his soil proved to be highly aluminous, and by 1942 Jamaica's vast bauxite (aluminum ore) deposits were recognized (Jamaica Bauxite Institute, 1992; Zans, 1953). Earthy bauxite deposits cover over 20% of Jamaica's land surface (Lyew-Ayee and Stewart, 1982) (Fig. 1
). Bauxitic soils (mostly clayey Oxisols) cover over half of the island country (Ahmad et al., 1966). Bauxite deposits in Jamaica occur primarily as surface infillings of irregularly shaped karst depressions, creating an unusual juxtaposition of oxic material and carbonate rock. After open pit mining, bauxite lands are reshaped, covered with stockpiled topsoil, and revegetated with grass (Morgan and Stevens, 1979; Zans, 1953; Jamaica Bauxite Institute, 1992). The resulting post-mined soils are generally steeper, shallower, and higher in limestone rock fragments than pre-mined soils.
"The relationship between bauxite and agriculture was inevitably a competitive one" (Stone, 1975) because the bauxite deposits are also locally the best agricultural soils. With the shortage of arable land in Jamaica, there is great interest in using post-mined bauxite lands for agriculture (Morgan, 1971; Thomas, 1973), particularly by small (<5 ha) farmers. The Mining Act of 1947 requires bauxite mining companies in Jamaica to restore disturbed land to the level of agricultural productivity that existed before mining (Jamaican Government, 1947; Coke et al., 1987; Jamaica Bauxite Institute, 1992). The purpose of this study was to evaluate and compare properties of pre- and post-mined bauxite soils in Jamaica that are relevant to assessing suitability of these lands for small-scale agriculture. The focus was on physical and chemical properties that influence water and nutrient movement and retention.
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MATERIALS AND METHODS
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Study Area
Study sites were selected in Manchester Parish near the town of Mandeville (18°2' N, 77°30' W) (Fig. 1). Mean annual precipitation in Mandeville is just over 2000 mm, and precipitation exceeds potential evapotranspiration (PET) for most of the year (Meteorological Service, Kingston, Jamaica, as reported in Scholten and Andriesse, 1986). Distribution of precipitation is bimodal, with wetter periods in spring and fall and the driest period in the winter, when PET exceeds precipitation for 3 mo. Mean annual air temperature for Mandeville is about 23°C, with <5°C difference between winter and summer (Morrissey, 1990; Ahmed et al., 1990).
In Jamaica, mined lands that have been reshaped and topsoiled are called "reclaimed", and mined lands that have been subsequently revegetated over several years are called "restored". For this study all mined sites were referred to as post-mined. Age of post-mined soils was considered to be time since reclamation, that is, reshaping and topsoiling. Two pre-mined sites and five post-mined sites with varying ages of reclamation were selected north and east of Mandeville (Table 1). Mandeville itself is hilly, with an average elevation of about 620 m. All of the study sites were at lower elevations, so they had somewhat lower precipitation and higher temperatures than Mandeville. Pre-mined sites and post-mined sites reclaimed at least 5 yr were vegetated before the study, primarily with African Star grass (Cynodon nlemfuensis Vandercyst.), with small areas of Guinea grass (Panicum maximum Jacq.) and Brachiaria species.
Pedon Description and Sampling
Pits were excavated by backhoe at the study sites to a depth of 2 m or to the depth of limestone bedrock. Detailed morphological descriptions were made, and samples were taken from each horizon. Pit samples included 2 to 4 kg of bulk samples and three clods approximately 200 cm3. Additional bulk samples were taken at some sites by horizon using a bucket auger down 30 to 60 cm from the surface. Additional clods were taken from other backhoe pits to supplement bulk density and water retention data. Morphological descriptions were also done for pedons sampled by bucket auger and for backhoe pits from which only clods were sampled, as well as for additional pedons from backhoe pits for which no samples were taken (Table 1). All clods were coated with Saran in the field, and bulk samples were air dried in Jamaica.
Physical and Chemical Characterization
Particle-size distribution for the fine-earth fraction (<2 mm) was determined using the pipette method of Kilmer and Alexander (1949). Duplicate 10-g samples were dispersed by shaking overnight in a sodium hexametaphosphate solution. Sands were separated by sieving. Water dispersible clay (WDC) was determined for selected samples using the same method but without dispersing with sodium hexametaphosphate.
Soil pH was determined in duplicate for 1:1 soil/water mixtures. For selected samples pH was also determined in duplicate for 1:1 mixtures with KCl. Extractable Ca, Mg, Na, and K cations were determined by extracting with ammonium acetate (pH 7) (Holmgren et al., 1977). Cations were measured by flame emission (Na and K) and atomic absorption (Ca and Mg). Cation exchange capacity using 1 M sodium acetate (pH 8.2) was determined by the procedure of the USDA Handbook 60 (U.S. Salinity Laboratory staff, 1969), but using a variable rate mechanical extractor to obtain leachate. Citrate-dithionate Fe was extracted according to the procedure of Holmgren (1967). Iron in the extract was analyzed by atomic absorption with an air-acetylene flame to determine percentage of Fe. Percentages of calcite and dolomite were determined using the gasometric procedure of Dreimanis (1962). The CaCO3 equivalent was calculated from the calcite and dolomite percentages. Total C was determined by dry combustion (Nelson and Sommers, 1982). Organic C (OC) was calculated as a difference between total C and inorganic C as quantified in the CaCO3 equivalent analyses.
Air dry and 33-kPa bulk densities were measured using the coated clod method of Brasher et al. (1966). Saran-coated clods were prewet, desorbed to 33 kPa, weighed in air, weighed in water, air dried, and weighed in air and water again. These measurements provided volumes at 33 kPa and air dry conditions and air dry weights to calculate 33 kPa and air dry bulk densities. The coefficient of linear extensibility was calculated from bulk density measurements (Grossman et al., 1968). Gravimetric water content was determined for clods at 33 kPa and for ground bulk samples at 1500 kPa using porous ceramic plates in a pressure-plate extractor (Klute, 1986).
Much of the statistical analysis for this study was based on grouping together similar horizon types from different pedons to get a better understanding of the mean, range, and variability of values. Significant differences between means were assessed using Fisher's least significant difference, which entailed a preliminary analysis of variance to determine if any means are different. Then differences in means were compared with a calculated LSD value, which accounted for differing sample sizes (Ott, 1993).
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RESULTS AND DISCUSSION
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Macromorphology
Post-mined soils were generally shallower, higher in limestone rock fragments, and on steeper gradients than pre-mined soils. Table 2 provides a summary of salient macromorphological features of post- and pre-mined soils. Three types of horizons were described and sampled in post-mined soils over limestone bedrock: ^Ap, 2^C, and 2Bob or 3Bob horizons. The caret (^) symbol, which indicates horizons formed from human transported material (HTM), was introduced in the 10th edition of Keys to Soil Taxonomy (Soil Survey Staff, 2006). The ^Ap horizons consisted of replaced stockpiled topsoil, often mixed with limestone rock fragments and clods of dense residual bauxite. The 2^C horizons consisted of fill material composed of leftover bauxite mixed with limestone fragments. Some post-mined pedons had Bob horizons, which consisted of residual bauxite in situ over limestone bedrock that had been buried by replaced topsoil and sometimes also by fill material. These horizons were designated 2Bob if they were just buried by replaced topsoil and 3Bob if they were buried by replaced topsoil and fill material. Boundaries between post-mined horizons were clear or abrupt. The bedrock was soft, crumbly, or hard fissured limestone, often changing form and depth within a single pit. Depth to limestone ranged from 15 cm to over 2 m, but was commonly between 30 and 150 cm.
The extremely low macroporosity of most Bob horizons and their location in the lower part of landscape suggest that they were originally formed many meters below the surface (Bardossy; 1982). Slickensides at high angles were found in many Bob horizons, which was surprising in soil material with such low shrink-swell potential. Dense root mats were found just above Bob horizons and along slickensides planes. Bardossy (1982) indicated that in some earthy bauxite deposits over limestone, bedrock karstification after bauxite formation has triggered slumps and slides in the bauxite, resulting in slickensides. Therefore, these slickensides are believed to be inactive and not related to current shrinking and swelling processes. Some Bob horizons were quite different, exhibiting friable consistence and other properties similar to upper Bo horizons in pre-mined soils. These Bob horizons were probably formed near the surface.
Most pre-mined (natural) soils were very deep, but there were some areas where limestone bedrock was within 2 m of the surface. Two types of horizons were described and sampled in pre-mined soils (Table 2). The Ap horizons formed when these soils were used for pasture and sometimes for cropping. Although clayey, the Ap horizons were friable and well aggregated. In the Bo horizons, there were some shiny ped faces, increasing with depth, while number and size of tubular pores decreased with depth. Boundaries between pre-mined horizons were gradual. The primary difference between red and brown bauxite was the dominant type of iron oxide present. Red bauxite had more hematite, and brown bauxite had more goethite. All the bauxite soils were high chroma (mostly 4 to 6). Both pre- and post-mined soils were well, or in some cases extremely well, drained, so few redoximorphic features were evident.
Chemical Characterization
A comparison was made of mean chemical properties of pre- and post-mined soils by horizon (Table 3). Shallow limestone bedrock and the incorporation of limestone rock fragments in post-mined soils affected many properties, including pH, extractable Ca, and CaCO3 equivalent. All post-mined horizons had significantly higher pH than pre-mined horizons. Coke et al. (1987) also found a higher pH for post-mined Ap horizons (7.3–7.9) than for pre-mined Ap horizons (5.4–6.5). The pH of pre-mined soils increased with depth, as is common for these soils (Ahmad et al., 1966, Ahmed et al.,1990). Extractable Ca was highest in post-mined ^Ap and 2^C horizons due to the high limestone rock fragment content. Extractable Ca was also at least double the CEC of ^Ap and 2^C horizons. Since soluble salt content was negligible, this indicated that extractable Ca considerably overestimated exchangeable Ca due to partial dissolution of fine-earth carbonates by the ammonium acetate extractant.
Extractable Mg was not correlated with limestone rock fragments, but was significantly higher in Ap horizons of pre- and post-mined soils than in all other horizons. This suggested that plants have accumulated Mg near the surface and/or the primary source of Mg was the manure added to facilitate revegetation. In either case, the limestone was a poor source of Mg. Analysis of limestone rock fragments confirmed that they were overwhelmingly dominated by calcite and contained little Mg-rich dolomite or Mg-calcite. Coke et al. (1987) found noticeably lower Mg in post-mined Ap horizons than in pre-mined Ap horizons, probably because their post-mined samples were taken before revegetation or addition of manure.
Cation exchange capacity (CEC) for all samples was strongly positively correlated with OC (R2 of 0.92) and negatively correlated with the percent clay, probably due to the fact that horizons with higher clay content were usually deeper and therefore had lower OC. For both pre- and post-mined soils, Ap horizons had significantly higher mean %OC and CEC than all other horizons (Table 3). Mean CEC was 18.8 cmolc kg–1 for pre-mined sites and 16.5 cmolc kg–1 for post-mined sites. Previously reported values for CEC of Ap horizons ranged from 8.8 to 16.4 cmolc kg–1 for pre-mined soils (Ahmad et al., 1966; Scholten and Andriesse, 1986; Ahmed et al., 1990) and from 2.2 to 5.9 cmolc kg–1 for recently reclaimed post-mined soils without organic amendments (Coke et al., 1987).
Methods developed to determine CEC for temperate soils, with mostly permanent charge, dramatically overestimate in situ charge for weakly buffered, variable charge soils common in the humid tropics (Gillman, 1979; Juo et al., 1976; Sanchez, 1976; Van Raij and Peech, 1972). Determination of effective cation exchange capacity (ECEC) is more appropriate for variable charge soils such as in this study. The ECEC is considered to be the CEC at the soil pH, but it is often determined by adding the sum of bases and extractable Al or salt extractable acidity (which includes exchangeable Al and H ions) (Soil Survey Staff, 2003; Soil Science Society of America, 1997). Ahmed et al. (1990) used the sum of bases as an approximation of ECEC for some soils similar to the pre-mined soils in this study. However, ECEC cannot be estimated using the sum of bases for post-mined bauxite soils because the fine-earth CaCO3 causes extractable Ca values to considerably overestimate exchangeable Ca.
With soils dominated by variable charge constituents, comparing pH in KCl to pH in H2O is useful to evaluate whether colloids have a net negative or positive charge at the soil pH. In a study of two Oxisols and one Alfisol from Brazil, Van Raij and Peech (1972) determined that if pH in H2O is lower than pH in KCl, then the soil pH is lower than the point of zero charge (PZC) and colloids have a net positive charge at the soil pH. This appears to be the case for many of the oxic horizons of pre- and post-mined pedons for which pH in KCl was determined (Table 4). Even some of the fill material (2^C horizons), which had pH in water of 7.6, had higher pH in KCl. This indicates that these are very low charge systems, with organic matter being the primary source of CEC at any soil pH. This is not surprising since the primary constituents of bauxite soils are Al and Fe oxides, which generally have a very high PZC of pH 8 to 10 (McBride, 1989). The Ap horizon of pre-mined Pedon 1994–4 (Table 4), which had 2.3% OC and a pH in H2O of 5.7, was found to have a PZC between 3.5 and 4.0 (Greenberg, 2001). Because the PZC of OC is about pH 3 (Senesi and Loffredo, 1999; Tate and Theng, 1980), organic matter has the effect of lowering the soil PZC, and thus increasing CEC, in Ap horizons relative to subsoil horizons (Van Raij and Peech, 1972; Gillman and Uehara, 1980).
A comparison was also made of chemical properties of post-mined ^Ap horizons for sites with different ages of reclamation (Table 5). Organic C, Mg, and CEC were all significantly higher at Battersea, the oldest site, than at Comfort, the youngest site, with the means for the other sites falling in between. These differences were likely due to Battersea having the longest established grass cover, which would increase OC, and thus CEC, as well as accumulate Mg. Although the Comfort site was just reclaimed when sampled, it had a mean of over 1% OC because the replaced topsoil contained some organic matter.
Physical Characterization
Mean physical properties of pre- and post-mined soils were compared by horizon (Table 6). The texture of most pre- and post-mined horizons was clay, with some silty clay pre-mined Bo horizons and some gravelly to very gravelly clay loam or loam 2^C horizons. As expected, the mean percentage of clay (determined by shaking with sodium hexametaphosphate) was significantly higher in pre-mined Bo horizons (73%) and post-mined Bob horizons (69%) than in other horizons. Pre-mined soils also had a significant increase in proportion of fine clay with depth.
Determination of total clay can vary considerably with treatment in clayey Oxisols because clay particles are often tightly bonded into sand and silt-sized microaggregates by Fe oxides (El-Swaify, 1980; Bartoli et al., 1992; Ahmad et al., 1966). Reported amounts of clay in Jamaican soils vary with type and degree of dispersion. Scholten and Andriesse (1986) measured 20% clay in Ap horizons and 8% clay in Bo horizons dispersing just with H2O, 47% clay in Ap horizons and 40 to 50% clay in Bo horizons dispersing with sodium hexametaphosphate, and 68 to 76% clay in Ap horizons and 67 to 87% clay in Bo horizons with sodium hexametaphosphate and removal of Fe oxides. With sodium hexametaphosphate and repeated use of an ultrasonic vibrator, Ahmad et al. (1966) reported 75% clay in Ap horizons and over 90% clay in lower Bo horizons. They also found that additional ultrasonic treatment resulted in the complete dispersion of the whole fine-earth fraction to clay-size particles, suggesting that, in some sense, these soils could be considered 100% clay.
Water dispersible clay ratio (percentage of total clay) was significantly and dramatically higher in pre-mined Ap horizons (mean of 68.9%) than in post-mined ^Ap horizons (mean of 17.2%) and pre- and post-mined Bo and Bob horizons (mean of 1.1%) (Fig. 2
). When grouped by site, mean water dispersible clay (WDC) ratios for post-mined ^Ap horizons ranked nearly in order of age of reclamation. Battersea (the oldest site) had the highest mean WDC ratio (41.1%) and Trinity (reclaimed 1 yr) had the lowest mean WDC ratio (1.4%). It is unclear why this trend occurs, but apparently something was binding and/or flocculating clay more strongly in the oxic subsoil and more recently reclaimed post-mined ^Ap horizons than in pre-mined and older reclaimed ^Ap horizons. More recently reclaimed ^Ap horizons had more oxic subsoil material mixed into them than older ^Ap horizons, so it was the horizons with greater amounts of Bo material that had lower WDC ratios. This correlates with previous studies indicating that oxic horizons generally have very low WDC (Van Wambeke et al., 1983). Bartoli et al. (1992) found that the lower Bo horizons of a gibbsitic Oxisol from Brazil had the lowest WDC ratio when the soil pH was closest to the point of zero charge. If Bo material in the Jamaican soils was at a pH near the PZC, which was quite likely, that would have contributed to the low WDC ratios.
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Fig. 2. Mean water dispersible clay ratio for pre- and post-mined soils. Letters indicate significant differences by Fisher's least significant difference with p  0.05; numbers in parentheses indicate n values.
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The horizons with higher WDC ratios also had higher organic matter. Organic C was weakly (R2 = 0.40), but statistically significantly (p = 0.003) positively correlated with WDC ratio. El-Swaify (1980) suggested that organic polymers can act as peptizing (that is, dispersing) agents when the soil sample is rigorously agitated. However, they apparently enhance aggregation in the undisturbed condition. Therefore, the higher organic matter horizons (with higher WDC ratios) may have more stable aggregates in situ than WDC ratios suggest.
Mean 33-kPa bulk density was 0.99 Mg m–3 in pre-mined Ap horizons, 1.19 Mg m–3 in pre-mined Bo horizons, and 1.22 to 1.29 Mg m–3 in all post-mined horizons. There are very few studies with bulk density data for Jamaican bauxite soils, but Ahmed et al. (1990) reported values of 0.88 to 1.05 Mg m–3 for Ap horizons and 1.05 to 1.19 Mg m–3 for Bo horizons of two pedons. Bulk density was significantly lower in pre-mined Ap horizons than in post-mined Ap horizons. This difference could mean that the pre-mined Ap horizons with lower bulk density have greater water retention and less restricted water movement.
Water Retention
Mean 33-kPa gravimetric water contents for pre- and post-mined horizons ranged from 0.250 to 0.605 (Table 7). The highest value was for two 2Bob horizons just above undulating limestone bedrock at the post-mined Trinity site. Unlike most Bob horizons, these were quite friable and presented no difficulties for root penetration, so they likely were originally formed near the surface. Values for 33-kPa gravimetric water content varied considerably within each horizon grouping, so significant differences between mean values were seldom found. Mean 1500-kPa gravimetric water contents ranged from 0.200 to 0.257. The variation in values was smaller for 1500-kPa values both within and between horizon groupings. The 2^C horizons had significantly lower 1500-kPa water contents, probably due to higher CaCO3 contents. Mean volumetric water retention difference (WRD) between 33 and 1500 kPa (accounting for volume of rock fragments) ranged from 0.121 to 0.334. Significant differences in volumetric WRD were also difficult to establish because values for post-mined samples were highly variable. Scholten and Andriesse (1986) reported 33-kPa gravimetric water contents of 0.346 and 0.328 for the Ap and Bo1 horizons of a red bauxite soil near Mandeville, and 1500-kPa gravimetric water contents of 0.247 and 0.245 for the same horizons. These values are comparable with those found for pre-mined soils in the present study.
The volumetric WRD values most likely underestimated water available to plants because Oxisols have been reported to hold up to 40% of available water between 10- and 33-kPa (Sanchez, 1976; Van Wambeke et al., 1983). Scholten and Andriesse (1986) found about 10% of available water held between 10 and 33 kPa in a red bauxite soil near Mandeville. For the present study, 33- to 1500-kPa WRD gave an estimate that allowed comparison between samples for which 10-kPa water contents were not available. Pre- and post-mined samples without significant CaCO3 exhibited the classic water retention characteristics of well aggregated clayey Oxisols. These Oxisols tend to behave like sands (or silts) at tensions near field capacity, due to the very stable sand-sized (or silt-sized) microaggregates, but they behave like clays at higher tensions (Sanchez, 1976; Van Wambeke et al., 1983). The Bob (formed deep) samples, which had very few microaggregates, had a mean 33-kPa gravimetric water content of 0.250 and a mean 1500-kPa gravimetric water content of 0.248 (Table 7). Although the 33- and 1500-kPa values did not come from the same samples, the Bob horizons that originally formed deep probably had negligible WRD.
To examine factors affecting water retention, linear correlations were determined for water content values and selected soil properties (Table 8). The 33-kPa gravimetric water contents were significantly correlated with bulk density, but not with clay or OC content. Water retention differences were also significantly correlated with bulk density, and not with any of the other properties. This was due primarily to the great variation in volumetric WRD at low rock fragment contents. The 1500-kPa gravimetric water contents were most significantly correlated with CaCO3 equivalent. This negative correlation was probably due to a dilution effect and accounts for the lower 1500-kPa gravimetric water contents found in 2^C horizons.
USDA Classification
Pre-mined (natural) soils which were >125 cm to limestone bedrock (12 pedons) were classified as very fine, gibbsitic, isohyperthermic Anionic Acrudoxs (Soil Survey Staff, 2006). Anionic is a subgroup introduced in the ninth edition of Keys to Soil Taxonomy (Soil Survey Staff, 2003) that requires pH in KCl to be equal to or greater than pH in water (1:1), thus indicating a net zero or positive surface charge at soil pH. Pre-mined soils <125 cm deep to limestone bedrock (eight pedons) were classified as Lithic Eutrudoxs due to considerable CaCO3 in the system.
Classifying disturbed soils with a system designed primarily for natural soils is inherently problematic, and revisions to Soil Taxonomy have been under consideration for some time (Sencindiver and Ammons, 2000; Galbraith et al., 2002, 2003). One of the key issues has been horizon designation. After much debate and discussion, the 10th edition of Keys to Soil Taxonomy (Soil Survey Staff, 2006) introduced several new horizon designations for disturbed soils. The caret (^) symbol, used as a prefix to indicate horizons formed from human transported material (HTM), is the only new designation that applies to Jamaican post-mined soils since there are no human manufactured or modified artifacts present. Recent proposed changes in taxonomic classes to account for disturbed soils have focused primarily on great group, subgroup and family divisions (Sencindiver and Ammons, 2000; Galbraith et al., 2002; Galbraith et al., 2003). Among several possible designations to indicate amount and type of artifacts, the proposed Spolic materials, indicating HTM with <2% artifacts of any kind, would best fit the Jamaican bauxite fill material. For this study, soils were classified using the 10th edition of Keys to Soil Taxonomy (Soil Survey Staff, 2006), which did not introduce any new taxonomic classes for disturbed soils.
Post-mined soils that consisted of replaced topsoil (^Ap) directly over bedrock (18 pedons) were classified as clayey-skeletal, mixed, isohyperthermic Lithic Udorthents. Post-mined soils with Bob horizons (residual bauxite still in situ) and without 2^C horizons (fill material) were designated very fine, gibbsitic, isohyperthermic Lithic (<125 cm to bedrock, four pedons) or Rhodic (>125 cm to bedrock, four pedons) Eutrudoxs. The Bob horizons that formed deep were so root restrictive that they acted, in a sense, as "oxipans," but that term does not exist in any classification system. However, Oxisols that are not in a Lithic subgroup, but are <100 cm deep to a root limiting layer, are designated "shallow" as part of the family. So, post-mined soils without fill material or bedrock within 125 cm, but with a dense Bob horizon that formed deep (two pedons) were designated very fine, gibbsitic, isohyperthermic, shallow Rhodic Eutrudoxs.
Most post-mined soils had 2^C horizons (41 pedons) and were classified as clayey-skeletal, mixed, isohyperthermic Haplic Udarents. They are Arents rather than Orthents due to the considerable amount of oxic material mixed in the fill. Of these 41 pedons, five had dense Bob horizons within 50 cm and, therefore, also had "shallow" in the family designation. Although the family designation gives some information, Haplic Udarents is such a broad subgroup that it does not adequately reflect the soil properties or origin of materials for post-mined soils (Schafer, 1979). Addition of the subgroups Oxic and Lithic or Lithic Oxic for Udarents would allow more informative classification of these soils. An Oxic subgroup could apply to other disturbed Oxisols, and a Lithic subgroup would likely be useful for many disturbed soils.
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CONCLUSIONS
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Tropical soils have different properties, limitations, and desired land uses than most temperate soils. As the concept of soils expands to include disturbed soils, it is important to include disturbed tropical soils and their undisturbed counterparts in studies to evaluate properties and assess land use and classification recommendations. Bauxite soils in Jamaica had low bulk density and low surface charge that was primarily pH dependent. Post-mined soils had higher bulk density in ^Ap horizons than pre-mined Ap horizons and higher pH throughout due to the incorporation of limestone rock fragments. For post-mined ^Ap horizons, organic matter and granular structure increased with time after reclamation and vegetation with grass. The new caret (^) symbol, which indicates horizons formed from HTM, was helpful in describing pedons of post-mined soils. It is proposed that the subgroups Oxic, Lithic, and Lithic Oxic be added for Udarents in Soil Taxonomy. This would allow for a more informative classification of these disturbed Oxisols.
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ACKNOWLEDGMENTS
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The authors thank Alcan Jamaica Company and the Jamaica Agricultural Development Foundation for their financial and personnel support of this project.
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NOTES
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Abbreviations: CEC, cation exchange capacity; ECEC, effective cation exchange capacity; HTM, human transported material; OC, organic C; PET, potential evapotranspiration; PZC, point of zero charge; WDC, water dispersible clay; WRD, water retention difference.
Received for publication December 7, 2004.
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ABSTRACT
INTRODUCTION
