Differentiation of upland soils on the Palawan ophiolitic complex, Philippines

The morphological and chemical features of upland soils derived from ophiolitic parent materials in Palawan, Philippines, are summarised from soil survey data, and compared with those on geosynclinal sedimentary rocks. Although the ophiolitic crystalline rocks are lithologically heterogeneous, the soils have similar morphologies, with reddish colours, clay textures, and friable crumb structures. However, their chemical characteristics differ considerably, with a pH range of over one unit, and exchangeable base status ranging from 95% dominance by Mg in soils derived from ultramafic rocks to 75% dominance by extractable Al in soils from felsic rocks. There are also differences in the total contents of most heavy metals, but less for the extractable forms of Mn and Cu. The soils on nearby geosynclinal sedimentary parent materials are also finely textured but are morphologically distinct, with more yellowish subsoil colours, blockier structures, some clayskins and ‘corned beef’ saprolite. They are also more acid and base-deficient, with Al occupying up to 95% of their exchange complexes. In view of the morphological similarities, chemical criteria are needed to separate the edaphically different groups of ophiolitic soils. Exchangeable Mg/ECEC is the best simple criterion. Soil Taxonomy and the FAO World Reference Base fail to distinguish between the mafic and ultramafic ophiolitic soils at subgroup level. Interactions between droughtiness, cation imbalances, phosphate immobilisation, and heavy metal toxicities may determine the stunted vegetation characteristic of some of the ultramafic soils.