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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 systemsoriented, 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 1 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 0016-7061/03/$ -see front matter D

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Subsoil samples from selected North Carolina Ultisols and Brazilian Oxisols were analyzed to determine how the colors of these materials were influenced by the nature and distribution of their constituent iron oxides. The effects of extraneous variables, i.e., other than the iron oxides, were minimized by utilizing pairs of red and yellow soils that were otherwise similar in their physical, morphological, and mineralogical properties.The iron oxides were found to be concentrated in the <0.2‐µm fractions, and the colors of these clays were the same as or similar to those of the parent soils. The spectral properties of the <0.2‐µm clays were primarily influenced by iron mineralogy. Goethite or mixtures of goethite and hematite were identified in all of the clays; however, Mössbauer analyses indicated that the red members of all sample pairs contained larger proportions of hematite than did their yellow counterparts. In addition, as the clays became redder in hue, the ratio of hematite to goethite generally increased. Calculated surface areas for the iron oxides ranged from 60 to 200 m2/g; values from the yellow clays were consistently higher than those obtained from their red counterparts. The yellow clays were also more efficient adsorbers of phosphate.

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Biotite transformations in 10 pedons representing a developmental sequence of soils formed from coarse‐grained regional metamorphic rocks in the Piedmont and lower elevations of the Blue Ridge provinces of North Carolina are reported. Kaolinite is common to abundant in sand fractions of six of the pedons and is found to some extent in nine of the 10 pedons. Most of the kaolinite is derived from the pseudomorphic (isomorphic and largely isovolumetric) alteration of biotite. Biotite first alters to interstratified biotite‐vermiculite in which the vermiculite is hydroxy‐Al interlayered. The kaolinization of biotite extends throughout the grain, the pseudomorphs retaining the morphology of the biotite precursor but having the optical and structural properties of kaolinite. The koalinite pseudomorphs can be considered as single crystals because unit cells within the individual grains are all aligned crystallographically. A transformation model is proposed that accounts for most of the properties observed in weathering biotite. The degree of kaolinization of biotite increases with nearness to the soil surface and with increased degree of soil development. In the saprolite of the shallowest Dystrochrept biotite is relatively unweathered. In the sola of deeper Dystrochrepts and fine‐loamy Hapludults most of the biotite is kaolinized. Sand‐sized kaolinite pseudomorphs are less elastic and tenacious than fresh biotite and begin to physically disintegrate in the upper sola of fine‐loamy soils under the influence of processes of pedoturbation. In deeply developed clayey soils, kaolinite pseudomorphs may be absent from the sola. The increased clay content of clayey soils over that of fine‐loamy soils is largely a result of claysized kaolinite added from the comminution of sand‐sized kaolinite pseudomorphs of biotite.

Excluding fertilizer P, a finite quantity of soil P exists for plant uptake. To improve our understanding of sinks and sources of long‐term plant‐available P, soil P fractions to a depth of 30 cm were determined in soils under a continuous cropping system prior to fertilization (1975), after 10 yr of P fertilization (1985), and 6 yr after the last P fertilization (1992). Soil types for two study sites were Norfolk loamy sand (fine‐loamy, siliceous, thermic Typic Kandiudult) and Davidson clay loam (clayey, kaolinitic, thermic Rhodic Paleudult). Superphosphate was applied from 1975 to 1986 at rates of 0, 10, 20, and 40 kg P ha−1 yr−1. Average removal of P, via corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] cropping, was between 10 and 20 kg P ha−1 yr−1 from 1975 to 1992. Resin‐extractable soil P increased in the Norfolk soil with annual P applications that were in excess of crop removal and decreased with annual P applications that were less than crop P removal. Resin‐extractable soil P decreased to below 3 mg P kg−1 in the Davidson soil regardless of P application or removal rate. Inorganic soil P extracted with NaHCO3 and NaOH increased with excess P additions and decreased with deficient P additions for both soils. Organic soil P in the Norfolk soil extracted with NaOH represented a P sink at the 40 kg P ha−1 yr−1 treatment in 1985, but subsequently mineralized by 1992. In both soils, inorganic P extracted in the resin, NaHCO3, and NaOH fractions, and organic P in the NaOH fraction of the Norfolk soil, represented the biologically dynamic P.