Quantifying soil morphology in tropical environments: Methods and application in soil classification

We tested the hypothesis that readily observed and easily measured morpholo gical variables can be used to characterize the soils sampled and described in southeastern Nigeria for purposes of land use and management. Field tes ts were developed for estimating soil texture and amount of ironstone nodul es. Two new soil color indices provided an immediate means of diagnosing th e soil drainage regime in case of the color index (CI) and soil forming pro cesses in tropical soils in case of the redness index (RI). The indices cor related negatively with organic C content (R = -0.39) and positively with d ithionite-extracted Fe2O3 (0.44) and Al2O3 (0.51). Inexpensive field tests for color, texture, and ironstone can be quantified using color indices and laboratory measurements. The local soil classification was quantified by m eans of color indices (RI, CI) and percentages of ironstone, sand, silt, an d clay measured in the A horizon. A classification based on soil texture, i ronstone, and color was used to define classes for the B horizon. The two f irst principal components (PC) extracted Corn soil morphological variables measured on the upper three horizons of 72 pedons explained 64.7% of the to tal variance. Nonhierarchical clustering performed on the two PCs produced seven clusters that compare well with the great groups of U.S. soil taxonom y. Principal component analysis on 20 soil chemical and morphological varia bles confirmed that soil texture, ironstone, and soil color account for mos t of the variation of the soils and provide an efficient means of character izing tropical soils derived from sedimentary parent material.