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  1. This is the report of a 2005-6 semi-detailed soil survey of the Smithsonian Tropical Research Institute’s 1571 ha research reserve on Barro Colorado Island in the man-made Gatun Lake, Republic of Panama.
  2. This was a conventional free survey, in which the field observations were subjectively sited according to landscape indicators.  There were almost 500 soil survey sites plus an additional 250 sites where members of the soil survey team identified soil classes whilst working on other projects.  The density of observations qualifies the survey as 2nd order by USDA, and detailed by FAO, criteria.  30 soil profiles were described in detail; 24 of them have laboratory analytical data.
  3. The climate of the island is seasonal wet tropical with a mean annual rainfall of about 2300 mm and a dry season of 2-4 months. There are four volcanic and associated marine sedimentary geological formations, all with intermediate–mafic lithology, with varying admixtures of calcareous materials. The topography of the island is dominated by two low cuestas.  The dipslope of the larger and higher cuesta in the west runs down a very gently SW-sloping plateau on andesite in the centre of the island.  The dipslope of the eastern cuesta stretches southwards from the Thomas Barbour trail to the Harvard peninsula.  The dissected scarps of the cuestas form the moderately steep topography in the north of the island.  There is another strip of moderately steep terrain along the main N-S fault system in the centre of the island.
  4. All of the soils are fine textured. Even where deeply weathered, many of the soils have substantial clast contents.
  5. Stony and shallow brown fine loams predominate on steeper terrains and are the most extensive soils.  Their moderate cracking, brownish colours and high cation exchange capacities support X-ray diffraction data in indicating that these soils contain subordinate smectites as well as dominant kaolinitic clay minerals. This and their shallowness/stoniness indicate that weathering is incomplete. These soils are not intensively leached, are only slightly acid, and their high CEC’s are mostly highly base-saturated. Some have deep dark topsoils
  6. There are extensive red light clays on the much of the gentler topography, especially on the central andesite dipslope/plateau.  These soils are well weathered, and the main clay minerals are kaolinite and ferric sesquioxides, commonly with subordinate gibbsite.  These soils are deep, with the paralithic contact often > 2m, but there are floating boulders in the sola.  Below thin darkened topsoils, the subsoils are uniformly bright red.  There are no active hydromorphic mottles but there are variable contents of ferrimanganiferous concretions and occasional dark red or pale coloured flecks of incomplete weathering. The textures are silty clays and clays, often with gradual increases in clay content with depth.  The clays are micro-aggregated into pseudo-silt and –sand particles, giving moderate porosity and free drainage, but many subsoils are compact.  These soils are leached and acid, and their low to moderate CEC’s are variably base-saturated, with significant extractable Al in some subsoils.
  7. There are extensive pale swelling clays, especially on the Caimito marine sedimentary facies in the southwest and west of the island. Smectites are the dominant clay minerals.  The swelling of these minerals when wet impedes water movement in the rainy season and makes these soils imperfect-poorly drained, with very pale matrix colours, often slightly bluish or greenish, and prominent bright orange, red and purple mottles. All of these soils have high cation exchange capacities. In some profiles the exchange complex is highly base-saturated, giving very high total exchangeable bases.  Others are acid and their subsoil exchange complexes are dominated by very high contents of extractable Al.
  8. In the Lutz Creek catchment in the north of the island, the combination of substantial calcareous components in the Caimito marine parent materials and steep slopes give moderately shallow mottled clays that are morphologically intermediate between the brown fine loams and the pale swelling clays.  Their very high cation exchange capacities are completely base-saturated, with very high contents of exchangeable Ca.
  9. There are small areas of poorly drained gleys in the small swamp in the centre of the andesite plateau and in ephemeral ponds elsewhere.
  10. Local soil classes are defined according to profile form and the geology of the parent material.  The lithogenic differences are not readily apparent in the field but are used as pedotaxonomic differentiae at this stage, as regolith lithology may prove to be ecologically significant.  If deemed irrelevant, the geological criteria and subdivisions can be dropped in the future.
  11. The soil classes are correlated with the Panamanian multi-attribute coding system and the two main international soil classification systems.
  12. In the FAO World Reference Base, the brown fine loams and other shallow soils are Eutric Cambisols, with Lithic and Mollic variations.  In USDA Soil taxonomy these soils are Eutrudepts. The pale swelling clays are vertic gleyic variations of Luvisols or Alisols in World Reference Base, and Dystrudertic variations of Udalfs or Udults in Soil Taxonomy. We provisionally designate the deep red light clays as Luvic, Lixic, Alumic, and Acric variations of the Ferralsols in the World Reference Base.  In Soil Taxonomy we designate them as Kandi- and Hapl- Alfic and Ultic variations of Oxisols
  13. The soil mapping units are consociations, which are dominated by one soil class but include small areas of named classes. Ten of the 14 consociations account for 98% of the area of the island.  The spatial pattern of the soils reflects the geological structure, as expected from the importance of lithological differentiae in the soil classification.
  14. The soils of BCI differ from the soils of other research sites in wet tropical forests on intermediate-mafic volcanic parent materials in that they develop to kanditic clay minerals through intermediate smectites rather than via allophane and halloysite.  The extensive Al-saturated smectitic soils appear to be theoretically unstable and they make BCI pedologically distinct.Although texturally fairly unform, the soils of BCI are variable with respect to the physical aspects of soil fertility, i.e. water supply, root aeration and site stability.  Stoichiometric comparisons with soils on some CTFS tropical forest research sites in Asia indicate that BCI soils are relatively well endowed with Ca and Mg, but not K.