PEDOGENIC FORMS OF IRON IN SOILS DEVELOPED OVER FOUR PARENT MATERIALS

Abstract

Information on the distribution of various forms of iron in soils are valuable in the study of soil genesis. The objective of this study was to to evaluate pedogenic forms of iron in soils developed over basement complex and basaltic parent materials of the study area. Geologic units considered in the basement complex ares were granite gneiss, biotite granite and migmatite. Stratified random sampling formed the basis for field work.  Soil sampling was carried ou by digging at least two soil profile pits in each geologic. Organically bound, armouphous and total iron oxides were estimated using 0.1M sodium pyrophosphate, acidified (pH 3) ammonium oxalate, and citrate-bicarbonate-dithionite method repectively. The total Fe in the soil samples were determined after a mixed acid digestion. In general, total iron fractions were statistically similar among soils of basement complex geologic units but significantly lower than those of soils derived from basalt. However, distribution of iron oxide fractions was similar among basement and basaltic geologic units. Furthermore, there was significant differences in the distribution of iron oxides in the B horizons of basement complex derived soils. Consequently, the quantity of iron oxides in the B horizon was in the order migmatite > biotite granite > granite gneiss. 70% of total iron xides in soils over granite gneiss, biotite granite and basalt was amorphous in nature. Furthermore, total iron oxides constitute less than 20% of total clay both in basemt complex and basaltic soils.