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 the 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 area were granite gneiss, biotite granite and migmatite. Stratified random sampling formed the basis for field work. Soil sampling was carried out by digging at least two soil profile pits in each geologic unit. Organically bound, amourphous and total iron oxides were estimated using 0.1 M sodium pyrophosphate, acidified (pH 3) ammonium oxalate, and dithionite-citrate-bicarbonate method, respectively. The total Fe in the soil samples were determined after a mixed acid digestion. In general, total iron fractions were statistically similar among the soils of basement complex geologic units but significantly lower than those of soils derived from basaltic rocks. However, the distribution of iron oxide fractions was similar among the basement and basaltic geologic units. Furthermore, there were 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. About 70% of total iron oxides in the soils over granite gneiss, biotite granite and basaltic rocks was amorphous in nature. Furthermore, total iron oxides constitute less than 20% of total clay both in the basement complex and basaltic soils.
Highlights
Data analysis on the forms of Fe in soils are the major tools used to study soil genesis
The objective of this study is to evaluate the pedogenic forms of iron in soils developed over basement complex and basaltic parent materials of the study area
Total iron fractions were statistically similar among soils of basement complex geologic units but significantly lower than those of soils derived from basaltic rocks
Summary
Data analysis on the forms of Fe in soils are the major tools used to study soil genesis. Porras et al (2016) observed that Fe-(hydr)oxide minerals and Fe-organo-mineral complexes, despite being present in low concentrations, influenced the amount and stability of dense soil organic carbon fractions in some forest soils. In a study of rain forest, valley bottom soils, Fagbami and Ajayi (1990) observed that high values of Fe active ratios (0.5 – 3.11) indicate considerable hydration of crystalline oxides to amorphous forms in the soils, while decreasing values with depth indicate crystallinity of the oxides with depth. Bera et al (2005) reported that the crystalline iron oxides increase at the expense of the poorly crystalline forms with increasing soil age. Lateritic soils have been observed to contain significant higher amount of crystalline iron oxides compared to the poorly crystalline forms (Ko 2014). Olatunji et al (2015) observed higher proportions of crystalline iron oxides in soil of a toposequence under semi humid tropical conditions
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