Fertility capability soil classification: A tool to help assess soil quality in the tropics

The soil quality paradigm was originally developed in the temperate region with the overarching objective of approaching air quality and water quality standards. Although holistic and systems oriented, soil quality focused principally on issues arising from large nutrient and energy inputs to agricultural lands. Soil quality in the tropics, however, focuses on three overarching concerns: food insecurity, rural poverty and ecosystem degradation. Soil science in the tropics relies heavily on quantitative attributes of soils that can be measured. The emotional, value-laden and 'measure everything' approach proposed by some proponents of the soil quality paradigm has no place in the tropics. Soil quality in the tropics must be considered a component of an integrated natural resource management framework (INRM). Based on quantitative topsoil attributes and soil taxonomy, the fertility capability soil classification (FCC) system is probably a good starting point to approach soil quality for the tropics and is widely used. FCC does not deal with soil attributes that can change in less than one year, but those that are either dynamic at time scales of years or decades with management, as well as inherent ones that do not change in less than a century. FCC attributes can be positive or negative depending on the land use as well as the temporal and spatial scales in question. Version 4 is introduced in this paper. The main changes are to include the former h condition modifier (acid, but not Al-toxic) with no major chemical limitations because field experience has shown little difference between the two and to introduce a new condition modifier m that denotes organic carbon saturation deficit. Additional modifiers are needed for nutrient depletion, compaction, surface sealing and other soil biological attributes, but there is no sufficient evidence to propose robust, quantitative threshold values at this time. The authors call on those actively involved in linking these attributes with plant growth and ecosystem functions to provide additional suggestions that would enhance FCC. The use of diffuse reflectance spectroscopy (DRS) shows great potential on a wide range of tropical soils. The evolution of soil science from a qualitative art into a quantitative science has progressed well in the tropics. Regressing to qualitative and vaguely defined soil quality attributes would be a step backwards.