TOPOSEQUENCE OF OXISOLS FROM THE CENTRAL PLATEAU OF BRAZIL

Landscape position is a critical factor influencing many co-varying soil properties even in highly weathered soils that tend to have a high degree of homogeneity. An attempt was made to learn how the position of a soil in a toposequence influences soil color, mineralogy and some chemical and physical properties. In a toposequence of Oxisols developed from basalt in the Central Plateau of Brazil (Goias State), moist soil color changed from dark reddish brown (2.5YR 3/4) in upper slope positions to dark yellowish brown (10YR 4/4) in lower slope positions and the following trends were observed: hematite content decreased and goethite content increased, the amount of Al substitution in the goethite structure increased, gibbsite content decreased and kaolinite content increased, and magnetic susceptibility and maghemite content of fine clay decreased. Also, the yellower soils down slope adsorbed more P and desorbed less P than the redder soils upslope, possibly because the yellower soils contain more goethite, which was found to have a smaller cystallite size and more DCB[sodium dithionite-citrate-bicarbonate]-extractable Al than hematite. Soil color is a reliable indicator of iron oxide mineralology which reflects the genesis of the soil and influences properties that affect plant growth. Thus, color should be used at a relatively high categorical level in soil classsification systems to define classes of Latosols and Oxisols.