The content of various forms of free iron (Fe) oxides in ten soil profiles, sampled to represent different locations in upper Egypt (Tushka, Wadi El-Nuqra, El-Kharga and El-Dakhla), was studied using selective extraction methods for whole soil samples and their size fractions (clay, silt and very fine sand). The effect of removing free iron oxides on soil color was also examined.
The studied soils have a texture class, in most locations differs that from sand to loamy sand with a finer texture in El-Dakhla and a coarser one in other locations. Most of these soils are free to slightly saline with low calcium carbonate and organic matter (OM). Soil reaction (pH) tends to be mildly to moderately alkaline in most locations. According to Soil Taxonomy, the studied soils are classified as: Typic Torripsamments, Typic Torriorthents, and Typic Haplotorrerts.
The results reveal that the oxalate-extractable iron (Feo), the dithionite-citrate-bicarbonate-extractable iron (Fed) and total Fe (Fet) in the whole soil samples range from 0.05 to 2.41, from 2.73 to 13.60 and from 4.37 to 42.84 g/kg, respectively. In the clay fraction they ranged from 0.34 to 16.41, from 13.64 to 45.82 and from 45.85 to 139.02 g/kg, respectively, whereas in the silt fraction, they vary from 014 to 19.22, from 3.35 to 66.52 and from 12.78 to 80.24 g/kg, respectively. In the very fine sand fraction, they ranged from 0.04 to 2.48, from 0.63 to 19.30 and from 5.10 to 32.92 g/kg, respectively. The silt fraction contains the highest values of (Feo) for El-Dakhla samples followed by the clay fraction, whereas Tushka samples show the lowest values. In soil profiles of Tushka and El-Dakhla, the silt fraction followed by the clay fraction contains the highest values of Fed, while Wadi El-Nuqra and El-Kharga soils have its lowest values. The clay fraction of Wadi El-Nuqra shows the highest amount of Fet. On the other hand, Tushka whole soil samples and their fractions have the lowest values of Fet. The amount of Fet increases with decreasing the size of soil particles.
The activity ratio (Feo/Fed) ranges from 0.01 to 0.46 in the whole soil samples, from 0.01to 0.44 in the clay fraction, from 0.005 to 0.45 in the silt fraction and from 0.01 to 0.33 in the sand fraction. El-Dakhla soils contain the highest activity ratios in the studied soils, resulting in more domination of amorphous Fe oxides than other locations. Tushka and Wadi El-Nuqra samples contain the highest amounts of crystalline Fe oxides in the studied soils. High amounts of silicate Fe are found in Wadi El-Nuqra and New Valley samples compared with those of Tushka ones.
Generally, the removal of free iron oxides from the whole soil samples and their fractions resulted in changes in their color and redness rating (Rr), especially for Tushka samples.