Skip to main content

Characteristics of BCI soil classes

Morphological features

The main morphological features of the soil classes are summarised in Table 5.3.  There are fuller descriptions of representative profiles in Appendix B.  There are descriptions of additional profiles in the databases held by Institut für Geoökologie, Universität Potsdam, STRI, the authors, and soon to be posted on the STRI website.

The features in Table 5.3, including textures, are as seen in augerings.  Some features look slightly different in pit profiles, particularly depths of dark topsoils, consistence, and the distinction between discrete weathered clasts and continuous in situ saprolite.

The numbers of auger sites for the classes in Table 5.3 are for the dedicated soil survey fieldwork. Where several augerings were done at one site, the site is counted only once. The numbers do not include the augerings for non-survey soil studies (e.g., Barthold, F., R.F. Stallard, and H. Elsenbeer. 2007. Soil nutrients and landscape relationships in a lowland tropical rainforest in Panama. Submitted to Forest Ecology &. Management) by the soil survey team, although their data have been used in the class characterisations and soil mapping   Because of more intensive sampling in areas of research interest and in the variability testing, some are strongly represented in the auger data.

Within the well-drained fine loams and light clays, there is a strong association between colour, texture and depth.  Most red clays are deep and most brown loams are shallow.  Only on the Bohio formation are there sufficient shallow red soils to warrant a separate class (Fairchild).  Similarly, there are sufficient deep brown soils for a separate class (Chapman) only on the Caimito volcanic facies.

The shallowness of most brown fine loams is often due to thin sola, with the paralithic contact to saprolite in the upper metre.  In other profiles augering is blocked, even with repeated attempts, by high densities of stones and boulders.  The relative importance of saprolite/clast shallowness varies between classes.  Shallow saprolite is encountered in many StandleyMarron and Wetmore augerings, but Hood is more bouldery.

The topsoils of Standley and Marron are dark brown, often with reddish tinges, friable, silty loams and silty clay loams.  They have strong medium and coarse crumb structures, most of which are worm casts.  Many were moderately–highly cracked when seen at the end of the dry season, and some, in topographically stable sites, also showed slight micro-gilgai.  The brown and reddish brown subsoils are stony but many of the stones are brightly rubefied, soft and highly weathered.  Subsoil fine earth textures are similar to those in the topsoil, but are stonier.  Subsoil structures are compound, with moderate subangular blocky breaking to moderate crumbs, many of which appear to be formed by endogeic worm casting. There are cutans in some subsoils, but these are not always associated with systematic increase in clay content and many appear to be shrink-swell pressure faces. The upper few decimetres of the saprolite are often varicolored, soft and well rooted, so that these soils are edaphically deeper than indicated by their shallow sola.

Most Wetmore soils are morphologically indistinguishable from Standley and Marron, with shallow, reddish brown fine loam over saprolite.  However, the stratigraphic heterogeneity of the Caimito marine formation means that there are also grey soils that have inherited weak textural layering, including horizons with significant sand. (e.g. Profile PF11 in Appendix B)

Hood has reddish brown colours and fine loam texture. However, its shallowness is more often due to unaugerably and undiggably hard and dense stones and boulders than to saprolite (e.g. Profile PR07).  These soils are common in the extensive boulder fields on the upper northern part of the dipslope on the Caimito volcanics.

The only deep brown clay class is Chapman, which is of limited extent on the Caimito volcanics, and is similar to Hood, but can be augered to below 1 m.  Profile PF07 in Appendix B shows how a soil that augers as a brown bouldery fine loam actually has a reddish lower light clay subsoil over a pale mottled fine textured saprolite.

The contents of black ferrimanganiferous concretions vary considerably between the different classes of brown fine loam (Table 5.4a).  Concretions are infrequent and scattered in Standley and Wetmore, which occur mainly on relatively steep scarp topographies.  They are plentiful in Marron and Hood, which are dipslope soils.  The main reason for the difference may be palaeo-hydrological, with the dipslope soils being more stable, older, and subject to seasonal redox fluctuations for longer (Hue et al., 2001).  Alternatively, they may be due to geochemical variability and higher Mn contents in the andesite and Caimito volcanics.

Ferrimanganiferous concretions in BCI fine loams and light clays

Soil class

n (= number of augerings)




Many or common concretions

Few or rare concretions

Zero concretions

(a)     Brown fine loams




















(b)    Red light clays




















The dark fine loams are defined as having chroma/value 3/3 or darker at 20 cm.  The separation of these soils on depth of melanised topsoil can be blurred by augering.  Thus, the type profiles for the andesite brown fine loam (Marron, PF03) and its corresponding dark fine loam Nemesia (PF12) were dsitinct when augered, but more similar in profile. The dark fine loams on Bohio (Miller) and Caimito marine (Oscuro) are morphologically similar to Nemesia on andesite. Insufficient deep humic clays were seen on the Caimito volcanic facies to warrant a separate class.  The subsoils of these soils are similar to those of the ordinary brown fine loams, with many stones in a matrix of brown or reddish fine loam or light clay.  Systematic increases in clay content with depth are uncommon. Saprolite or unaugerable stones are usually encountered within the upper metre.

The red light clays have thin (< 10 cm) slightly darkened (ochric) topsoils over deep and uniformly and brightly coloured red or orange silty clay loam – clay subsoils.  Clay contents increase from silty clay loam topsoil to silty clay in the upper subsoil, and then vary erratically between clay and silty loam in the lower subsoil.  Topsoils have moderate or strong crumb structures, with high proportions of worm casts.  The subsoils have compound structures, with weak or moderate blocky breaking readily to moderate porous crumbs.  These also appear to be of mainly faunal origin, with common clusters of endogeic casts. There are diffuse layers of weathered stones and occasional floating boulders in some subsoils, but weathering otherwise appears to be well advanced and there are no visible primary minerals.

The depth, homogeneous red colours, advanced weathering, and subsoil micro-aggregation makes these soils look oxic/ferralic/ferrallitic. However there are some morphological features, which suggest that weathering is incomplete.  These include coherent networks of surfaces cracks that are up to 10 mm wide and 15 cm deep by the end of the dry season.  Also many of the subsoils are compact and firm, despite visible micro-aggregation, and have air-dry bulk densities up to 1.2.  These suggest the persistence of some expansible 2:1 aluminosilicate clay minerals, and a transitional fersiallitic – ferrallitic stage of weathering.

The classes within the red light clays show considerable morphological overlap.  Ava has somewhat wider surface cracking and more compact subsoils.  Harvard and Balboa tend to be slightly more orange, with hues commonly 5YR, than, whereas nearly all subsoils in Ava and Poacher have 2.5YR subsoils.  At one stage we contemplated a separate orange class (Lathrop – see Profile PR06 in Appendix B) on the Bohio, but it is now subsumed into Balboa.

The bright red colours and absence of significant mottling indicate that the red clays are well drained, with profile pits retaining water for only a few days, even after heavy rainfall.  However, the compaction of the subsoils counteracts the micro-aggravation and porosity, and may temporarily impede infiltration during high intensity rainfall.  This accounts for the tendency to intermittent, patchy, shallow and ephemeral puddling on the Ava red clays on the andesite dipslope plateau during the wet season.  It may also lead to subsurface throughflow, by which water infiltrating the topsoils is constrained to flow off laterally where it meets less permeable subsoil horizons. Throughflow supplementation from upslope is a possible explanation for the generally higher dry season moisture contents in soils on the scarp risers than on the upper tread surface of the plateau (Becker et al., 1988; Daws et al.2002).

Ferrimanganiferous concretions are common in the red clays (Table 5.4 (b)).  As in the brown clays, they appear to be most frequent on the Caimito volcanics, but occur in similar densities and frequencies in the red clays on other formations

Fairchild is the only class of shallow red fine loams and clays.  Apart from colour, it is morphologically similar to Standley.

There are five classes of pale swelling clays.  The two most extensive are on the larger outcrop of the Caimito marine sedimentary facies in the west and southwest of the island.  Zetek has dark brown cracking clay or fine loam topsoil over a reddish or dark brown blocky clay or fine loam upper subsoil.  At somewhere between 30 and 80 cm, there is a clear boundary to pale mottled heavy clay lower subsoil. Common colour combinations in the lower subsoil are prominent coarse port-red, mauve and purplish mottles in a slightly bluish light grey matrix, or distinct medium orange mottles in a greenish light grey or very pale yellow matrix.  Lower subsoils hand texture as heavy clay because of their mineralogy, but sand and/or silt contents vary in bands, presumably inherited from the sedimentary stratification.  The other pale swelling clay on the main outcrop of the Caimito marine is Barro Verde.  It lacks the reddish brown upper subsoil of Zetek and its profile consists of a very dark cracking clay topsoil directly overlying pale mottled heavy clay, similar in matrix and mottling colours, structure, hydrature and texture to the lower subsoil of Zetek.

The predominantly smectitic clay minerals result in surface cracking, slight micro-gilgai and dark topsoils.  They are also the cause of the impermeability of the subsoils when wet.  Once these soils are thoroughly wetted, profile pits remain flooded for the rest of the wet season. The shrink-swell capabilities of the smectites also make the subsoils unstable.  Unconfined pit faces spalled and slumped in the 2005 wet season, leaving the rootbound topsoil and (in Zetek) reddish brown upper subsoil as overhangs.  The degradation of pits in the pale swelling clay contrasts with the stability of those in the red light clays and brown fine loams.  However, despite the high smectite contents and wide surface cracking, the pale swelling clays are not vertic as they do not have subsoil wedge structures and slickensides.

The soils of LakeBarbour and Gross are the analogues of Zetek on andesite, Caimito volcanics and Bohio formations respectively (Table 5.2)They all have reddish brown well-structured blocky upper subsoils.  They are morphologically fairly similar, although the Barbour soils seen have particularly firm and sticky lower subsoils.  These classes are of limited extent.  There are no equivalents of Barro Verde on these geologies.  It is not clear if LakeBarbour and Gross form on intra-formational bands of marine sediments in their respective bedrocks, on outliers of the Caimito marine, or where local drainage conditions impede leaching and weathering irrespective of local lithology.

The smaller northern outcrop of the Caimito marine is on steeper slopes than most of this formation.  This gives rise to the mottled heavy clays.  These are mostly shallow, with saprolite at < 1m.  The subsoil is predominantly brown or reddish heavy clay, with hydromorphic grey and rust coloured mottles mixed in with dark and bluish grey fragments of incompletely weathered rock.  The main class of these soils is Lutz.  There are also small patches scattered patches of similar soils of Weir class on the Bohio formation,

Swamp soils are poorly drained gleys.  Their subsoils have grey matrix colours with rust mottling, and are wet throughout the year.  As they are formed in local alluvium, subsoil textures show depositional layering.  Some subsoil horizons have significant contents of coarse sand and grit, but textures are mainly fine loams and clays, because of the andesitic lithology of the upper Conrad catchment.

Chemical and mineralogical features

The laboratory data for selected samples in 24 of the described profiles are summarised in Table 5.5.  There are more details in Appendix B.  The complete data for all horizons are in the database soon to be posted on the STRI website.

Because the data are from a limited number of profiles, Table 5.5 groups the soils by form rather than classes.  The premise underlying our classification, i.e., that parent material lithology affects soil chemical attributes, cannot be tested with these few and subjectively sited cases. In places we speculate about inter-class differences but these are not statistically validated.  However, additional non-survey studies (e.g., Barthold, F., R.F. Stallard, and H. Elsenbeer. 2007. Soil nutrients and landscape relationships in a lowland tropical rainforest in Panama. Submitted to Forest Ecology & Management) do systematically quantify and test class differences for soil potassium and soil organic matter

Many analytical features of the brown fine loams confirm the morphological indications of incomplete weathering and pedogenetic immaturity.  The dominant clay mineral is kaolinite, but there are substantial contents of montmorillonite (smectite) in most profiles and it is the dominant clay mineral in one of them.  The montmorillonite accounts for the moderate shrink-swell features in these soils. Although substantial systematic increases in clay content with depth are uncommon, several profiles have cutans in the upper subsoil, but these are probably pressure coatings from clay swelling, rather than argillans.  The montmorillonites also account for the high cation exchange capacities of these soils, with CEC and ECEC > 24;-1 clay throughout most profiles.  The exchange complexes are highly base saturated, and BS and EBS are close to 100% in all topsoils and remain high down most profiles, and labile Al is negligible. Calcium is the dominant exchangeable cation, and there is also much exchangeable Mg.  Exchangeable K is extremely low, even undetectable, in some subsoils. Most of these soils are neutral or slightly acid, and all topsoils have pH (water) > 5, with two > 6.  SOM is high in the topsoil of the single profile analysed for OC.

Two profiles deviate from the generally high base status, incomplete weathering and limited leaching.  They are PF07, in the deeper (Chapman), and PR12, in the redder (Fairchild), classes.  Both these profiles have very acid subsoils and low CEC, ECEC and base saturation.  The Chapman profile also has kaolinite and gibbsite as its main clay minerals, and no significant montmorillonite.  The increased depth (Chapman) and rubefaction (Fairchild) suggest more intense leaching and more advanced desilication.

Our analytical data for the inextensive dark fine loams come from only two profiles. These confirm the morphological indications that these soils are similar to the brown fine loams.  They have high cation exchange capacities, with CEC and ECEC > 24;-1 clay throughout.  The exchange complexes are highly base saturated, and BS and EBS are close to 100% and labile Al is negligible. Both profiles are neutral or slightly acid. There are large contents of exchangeable Ca and substantial exchangeable Mg.  Exchangeable K is variable and extremely low in some subsoil samples.  SOM is high in the topsoil of the single profile for which we have data.

Analytical features of the red light clays confirm the morphological indications of advanced weathering and pedogenetic maturity.  In all of the profiles with X-ray diffraction (XRD) data, kaolinite is the dominant and gibbsite the main subordinate clay mineral.  There are no XRD indications of significant montmorillonite, which accounts for limited shrink-swell but does not corroborate morphological suggestions of limited weathering.  These soils are quite heterogeneous with respect to their exchange complexes and base status.  Some have low cation exchange capacities, with CEC and ECEC < 24;-1 clay, but others are more active. All topsoils are highly base saturated, with BS and EBS at or near 100%.  The subsoils with small exchange complexes remain highly base saturated throughout the subsoil and extractable Al is negligible.  The subsoils with larger exchange complexes and higher CEC and ECEC values are base depleted, with minima < 20% for BS and < 50% for EBS. These minima are often mid-profile, with higher values both above and below. These soils have significant contents of extractable Al.  Ca is the dominant exchangeable base, with substantial exchangeable Mg.  Exchangeable K contents vary, but are low – extremely low in most subsoils. The soils with small but base-saturated exchange complexes are moderately acid, with pH (water) in the range 4.5 – 6.5.  The base deleted soils are more acid, with pH (water) < 4 in one subsoil. In the two profiles analysed for SOM topsoil OC contents are higher than the ochric colours suggest, with one almost 7%.  The weak melanisation is attributed to masking by the intense reds of ferruginous oxidic minerals.

The analytical features of the pale swelling clays confirm the morphological indications that their pedogenetic development is different from the light clays.  Montmorillonite is the dominant clay mineral and kaolinite is only subordinate. The predominance of montmorillonite accounts for strong seasonal shrink-swell in these soils and their impermeability when wet.  The dominance of the clays by smectites gives high cation exchange capacities, with topsoil CEC and ECEC > 24;-1 clay.  All topsoils have EBS > 90% and appear highly base saturated.  However the BS values in some topsoils are < 50%, possibly due to methodological inflation of CEC.  The subsoil CEC and ECEC are all > 16;-1 clay, with some > 50.  These substantial exchange complexes are variably base saturated. The Barro Verde subsoil has BS and EBS > 90%, with Ca as the dominant exchangeable cation. However the subsoils of the other classes have low base saturations, and very high extractable Al, with > 30;-1 clay in one profile. Topsoil exchangeable K contents are moderate  – high, at 0.2 – >1;-1 clay, and slightly better than in the other soil forms.  However subsoil values are low – extremely low.

Our limited data for the inextensive shallow mottled clays confirm that they are less weathered and less developed precursors of the pale heavy clays.  They have very large and active exchange complexes, with some CEC values > 100;-1 clay, The ECEC values are also very high > 50;-1 clay.,  We have no clay mineralogy data for our profiles, but stream sediment sampling in Lutz Creek indicate that these soil contain large quantities of montmorillonite (Johnsson & Stallard, 1989). The large cation exchange capacities are wholly base-saturated and contents of extractable Al are negligible in the sola, although the saprolite of one profile has about 3;-1clay.  These soils are slightly acid with pH (water) values > 5.5 in the topsoils but dropping to 4.4 in the subsoil of one profile.  Ca is the dominant exchangeable basic cation, and these soils contain up to > 50 earth exchangeable Ca, which are equivalent to > 70;-1 clay and are among the highest values encountered under tropical forests.  The quantities of Ca leached from these soils are sufficient to form tufa on boulders in the creek (Johnsson & Stallard, 1989). Exchangeable Mg is also high, up to  > 10;-1 clay.  These soils appear to be well endowed with K by BCI standards, with moderate  – high topsoil contents of exchangeable K,  > 0.8, but they decrease with depth, to < 0.2   in one subsoil.