Classifying soils at the ultimate stage of weathering in the tropics

Oxisols cover ≈ 23% of the land surface in the tropics and are utilized extensively for agricultural
purposes in the tropical countries. Under the variable input types of agricultural systems practiced
locally, some of these soils still appear to have problems in terms of proper soil classification
and subsequently hinder attempts to implement sustainable agro-management protocols.
The definition for Oxisols in Soil Survey Staff (1999) indicates that additional input is still required
to refine the definition in order to resolve some of the outstanding classification problems. Therefore,
the objective of this study is to examine the properties of some Oxisols and closely related
soils in order to evaluate the classification of these soils. Soils from Brazil, several countries in
Africa, and Malaysia were used in this study. Field observations provided the first indication that
some of the presently classified kandi-Alfisols and kandi-Ultisols were closer to Oxisols in terms
of their properties. Water-retention differences and apparent CEC of the subsurface horizons
also supported this idea. The types of extractable Fe oxides and external specific surface areas
of the clay fractions showed that many kandic horizons have surface properties that are similar
to the oxic horizons. Micromorphology indicated that the genetic transition from the argillic to the
oxic involves a diminishing expression of the argillic. Properties, such as CEC, become dominant.
The kandic horizon is therefore inferred as a transition to the oxic horizon. It is proposed
that the Oxisols be keyed out based only on the presence of an oxic horizon and an iso–soil temperature
regime. The presence of a kandic horizon will be reflected at lower levels in Oxisols.
The Oxisols will now be exclusive to the intertropical belt with an iso–soil temperature regime.
The geographic extend of the Oxisols would increase and that of kandi-Alfisols and Ultisols
would decrease. A few kandi-Alfisols and Ultisols in the intertropical area will have low CEC which
would fail the weatherable mineral contents. The kandic subgroups of some Alfisols and Ultisols
will be transitional between the low (< 16 cmolc [kg clay]–1)- and high (> 24 cmolc [kg clay]–1)-
activity clay soils. The proposed changes to classification will contribute to a better differentiation
of the landscape units in the field. Testing of the proposed classification on some Malaysian soils
showed that the new definition for Oxisols provides a better basis for the classification of the
local soils and the development of meaningful soil-management groups for plantations.