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BCI soil forms and pedogenetic trends

As the name Barro Colorado implies, there are many bright reddish soils of medium and fine texture on the island.  BCI soils are texturally limited, and virtually none have high contents of sand.  However, they vary with respect to colour, depth, stoniness and drainage, and a number of soil forms can be distinguished (Table 5.1).  These are similar in concept and taxonomic level to the Soil Forms of the South African Binomial system (MacVicar, 1991)

The most extensive soil form is shallow brown immature stony clay and fine loam overlying saprolite (soft weathered rock) or stone and boulders.  On the andesite, Bohio and Caimito volcanic formations these soils mature to deep, red kanditic and oxidic clays, by prolonged leaching, deep and intensive weathering, aluminosilicate desilication and rubefaction.
There are also some deep red clays on the Caimito marine facies, mainly in the western part of the Poacher Peninsula and on steeper slopes elsewhere. However, most of the mature soils on the gentle topography of the main Caimito marine outcrop in the west and southwest of the island are deep, imperfectly drained heavy silty clays, with pale matrix colours, some of which have distinct bluish or greenish grey tinges, with variable but often prominent red-purplish mottling.  The swelling smectitic clay minerals give substantial surface cracking in the dry season and severely limited permeability when the cracks close in the wet season.  Similar pale swelling clays occurs in smaller patches on the other geological formations, where impeded drainage limits leaching and maintains soil solution cations at levels high enough for 2:1 minerals to survive.  These soils may develop from brown fine loams.  However, there are some shallow immature-looking mottled clays in the Lutz Creek catchment, probably precursors of the swelling clays

Other less extensive soil forms on BCI are:

  • Fine loams, with deep dark humic topsoils;
  • Gleyed clays and loams in the limited areas of swamps and ponds, especially in the mid-plateau swamp on the andesite dipslope.

Where soils combine features of several forms, they are assigned according the dominant features in the top metre in the preferential sequence; gley > pale clay > dark loam > red clay > brown loams.
We cannot discern any substantial differences in soil morphology that can be attributed to human disturbance.  Similarly, there are no immediately obvious examples of differences in vegetation affecting pedogenetic trends.

Soil forms on BCI

Soil form Morphological features Extent on BCI
Brown1 fine loam Dark fine loam/clay topsoil, with little or clay increase with depth in stony or bouldery brown subsoil; mostly < 1m to saprolite Extensive
Dark1 fine loam Similar to brown fine loam but dark1 topsoil is > 20 cm deep. Scattered patches
Red1 light2 clay Shallow dark brown fine loam topsoil over deep red – reddish brown clay or fine loam subsoil; gradual increase in clay with depth; mostly > 1m to saprolite Extensive
Pale1 swelling clay Dark clay or fine loam topsoil over variable reddish brown clay or fine loam over greenish or bluish light grey heavy clay, intensely mottled with red/purple; depth > 1m. Extensive on Caimito sedimentary, limited elsewhere
Mottled heavy clay Fine loam topsoil < 5cm, over firm heavy2 clay; mixed red, brown & yellow colours with some grey mottles. Mostly < 1m depth to firm, heavy-textured saprolite. Limited; mainly on Caimito sedimentary facies in Lutz Creek catchment
Gley Grey, variably mottled, wet clays and loams. Limited; mid-plateau swamp & few small seasonal ponds

1  Colour groups: Brown = 5YR -10YR, & chroma/value darker than 4/6, but lighter than 3/3; Red = everything redder than Munsell  5YR 4/6; Dark = chroma/value 3/3 or darker (any hue); Pale = hue 7.5YR or yellower & chroma/value paler than 6/3

2  Light clays are predominantly kanditic and oxidic, with micro-aggregated macroporous soil structure and friable – firm consistence; heavy clays are predominantly smectitic with blocky soil structures and firm – very firm consistence.