Classification of BCI soils

Classification options

We considered three alternatives for classifying the soils of BCI:

1. Use one or both of the main international systems of soil classification, i.e. World Reference Base for Soil Resources (FAO, 1998) and/or the USDA Soil Taxonomy (Soil Surve Staff, 1999 & 2006).
2. Use a Panamanian soil classification system, if available and appropriate.
3. Formulate a local soil classification specifically for BCI.

We prefer local to international systems, because they define taxa polythetically on locally appropriate criteria, and better accommodate overlaps and transitions (Baillie, 2001).  Furthermore, experience at other tropical forest research sites shows that local soil classifications are relatively stable, whereas new data tends to lead to substantial reclassification in the international systems.  Also, the World Reference Base (Deckers et al., 2005; FAO, 1998) explicitly recommends that it be used in conjunction with, not instead of, local classifications.

A Panamanian system was formulated for the Catapan (1970) 1:20 000 soil maps, using a multi-attribute coding system.  It is agriculturally oriented and complex, and we decided against it as the main system for BCI, although we correlate the BCI soil classes with the Catapan criteria, in order to facilitate intra-Panamanian comparisons (Table 5.7).

We therefore devised a local classification system specifically for BCI.  Its classes are taxonomically equivalent to soil series, but are not series sensu stricto, as they are as yet insufficiently characterised (Clayden & Hollis, 1984; Soil Survey Staff, 1999).  Also, they are not wholly defined on solum attributes.

Soil classification system for BCI

Bennett (1926 & 1929) could not satisfactorily differentiate between extensive red clays of the Canal Zone on solum morphology, and defined the Frijoles, Gatun and Arraijau red clays on the basis of their parent materials.  Similarly, Woodring (1958) was unable to morphologically differentiate the red clays weathered from BCI’s different geological formations.

Eighty years on, and we have reached similar conclusions and adopt a similar approach to Bennett.  We subdivide the soil forms into classes on basis of the geology of their soil parent materials (Table 5.2), as determined from saprolite, stones, topography, and geological maps.

As our classification uses parent material geological criteria, it deviates from the principle of wholly pedophenetic differentiation.  However, this is the first soil systematic survey of the whole island, and we do not yet know the extent to which morphologically similar soils from the various lithologies differ with respect to nutrients, hydrature, site stability and other ecologically important attributes.  Yavitt (2000) indicated that andesite soils on BCI have slightly but significantly better P status than those on sedimentary and volcanic rocks, but the three types are similar with respect to N and S.

If some or all of the rego-lithological differences are later deemed to be irrelevant, soil classes can be amalgamated.  Experience elsewhere shows that it is relatively straightforward to group soil classes.  For instance, three Sarawak soil series were aggregated into a single group of sandstone soils in an analysis of humus-nutrients relationships on the Lambir CTFS plot in Malaysian Borneo (Baillie et al., 2006).  Similarly, West Malaysian soil series have been grouped at the Pasoh CTFS plot (Yamashita et al., 2000). It is usually simpler to lump classes that were initially over-split than to split classes that were initially over-lumped.

Separate classes have been named if found in four or more augerings.  Cells in the form/lithology matrix (Table 5.2) with < 4 augerings are not named, and their augerings are incorporated into pedotaxonomically similar classes.  We distinguish shallow (Fairchild) red clay classes for the Bohio Formation, but shallow red clays are scarce on the other lithologies.  Similarly, we distinguish deep (Chapman) from shallow (Hood) brown fine loam/clay on the Caimito volcanic facies, but deep brown soils are rare on other lithologies.  We distinguish deep humic topsoil classes from normal brown loams on all lithologies except the Caimito volcanic facies.

Most of the classes are named after trails where the soils are common or were first seen

BCI soil classes