properties (such as organic matter) in inhibiting the activation of non-crystalline and poorly-crystalline Fe oxides to crystalline Fe oxides. Rezapour et al. (2014) found a positive and significant correlation between Feo and organic carbon probably due to the strong adsorption capacity of amorphous and poor crystalline Fe oxides (with small size and structural disorder) with organic matter to bind them in stable organo-metal complexes. Second, accelerated alteration of Fe-bearing minerals resulted from daily and seasonal fluctuations in temperature and moisture in the topsoil. This data are comparable to those of reported by others (Li and Richter, 2012; Rezapour et al. 2014).
The Feo/Fed ratio, identified as active Fe ratio, almost varied in all of the studied soil profiles (Table 4.8). Several authors (Schwertmann and Taylor 1989; Singer et al. 1998, Rezapour et al. 2010) remarked that the amount of pedogenic iron oxides and their crystallinity were increased with an increase in weathering rate, leading to decline in the pattern of the Feo/Fed ratio.
As was shown in Table 4.8, the Feo/Fed ratio generally was high for A horizons than the B horizons in the majority of the soil profiles which may be interpreted as follows; organic matter impeded crystallization of Fe-oxides in surface horizons as represented by Schwartzman and Taylor (1989). For example, profiles of 2 and 3 with the highest organic matter (Table 4.8), as a key factor for inhibition of crystallinization of Fe oxides, exhibited the highest Feo/Fed ratio. This assertion was highlighted by evidence from Blume and Schwartzman (1969), who suggested that the crystallinization of amorphous Fe oxides might be promoted by low organic matter content, and their ageing speeded up by relatively high pH. Some authors (McKeague and Day 1966; Moore 1973) employed the active Fe ratio to recognize well-drained from poorly drained soils; soils containing ratio of more than 0.35 were classified as well-drained, while poorly drained soils have the content less than 0.35. In this study, this interpretation cannot separate well-drained soils from their poorly drained counterparts in all of the soils, which is according to Rezapour et al. (2010).
4.4 Clay Mineralogy
The XRD patterns of the clay fraction from all of the soil profiles (Fig. 4.9-4.15) exhibited a similar composition of phyllosilicate minerals, including illite, smectite, chlorite, vermiculite, and kaolinite.
X-ray diffractogram of the clay samples exhibited that minerals were similar in type but some different were observed in the intensity, position, and peak figure of them except kaolinite. This pattern cold be attributed to the change in drainage condition and ground water table depth. Such results are comparable with the finding of Abtahi and Khormali (2001) and Rezapour et al. (2009) who observed that clay minerals (such as illite, smectite and chlorite) were conditioned, relatively, under influence of drainage condition and physiography.
Illite was recognized by X-ray diffraction lines at 10.0, 5.0 and 3.33 ? in all of treatments. The strong XRD peaks at 14.2 to 14.6 ? of the Mg-saturated specimens shifted to 16.9 – 17.3 ? by the ethylene glycol treatment, to 10-1.2 ? by K-saturation, and to 10 ? in the K-saturated specimens heated at 550 oC, showed the presence of smectite in all of the soils. The14.0 to 14.5 ? peaks that was unchanged by any treatments identified as chlorite. In some clay samples, K-saturation treatment at 25°C and 550 oC showed that the 14 ? XRD peak shifted to lower d-spacing clay minerals and the intensity of the 10 ? peak increased. This indicates the presence of vermiculite. The presence of kaolinite was recognized by the peak of 7.1 -7.2 ? in the treatment of Mg-saturation and K-saturation that unaffected by ethylene glycol and disappears after heating at 550 oC.
Fig.4.9 The XRD pattern of clay minerals for profile 1
Fig. 4.10 The XRD pattern of clay minerals for profile 2
Fig. 4.11 The XRD pattern of clay minerals for profile 3
Fig. 4.12 The XRD pattern of clay minerals for profile 4