Assessing Parent-Material Homogeneity of Soil Pedons Around Mosul Dam Lake Using Sand-Size Distribution and Chemical Weathering Indices

3.1 Morphological properties of the study soil pedons

Table 3 shows the results of morphological properties for the studied soil pedons based on information from the American Soil Survey staff [40]. The results show some differences between soil pedons of the studied sites, and between soil horizons of a single pedon, because the variations in soil nature, parent material, and pedogenic processes affecting soil formation and development, soil color is one of the most important properties and easily observable morphological characteristics. The (Hue) values for all studied soil horizons were in page (10YR) of the Munsell Soil Color chart. These Hue values represent most soils in arid and semi-arid areas, as noted in research [41]. For the surface and subsurface soil horizons, the color intensity (Value) and color purity (Chroma) values were 7/4–5/3 in the dry state and 4/6–3/3 in the moist state. These values are related to soil moisture content, as well as mineral and organic components, as confirmed by research [42]. The type of soil structure was fine to moderate granular and semi-angular in the surface horizon of P1, and moderate to coarse angular blocky in the subsurface horizons, while in P2, P3, and P4, it was moderate angular and semi-angular blocky for surface horizons, and it was moderate to coarse angular prismatic in the surface horizon of P2.

Table 3. Some morphological characteristics of the studied soils

This variation of soil structure between horizons of studied soil pedons is a result of the clay loamy and silty clay soil texture of surface horizons because the organic matter on the soil surface, as well as human activities (plowing and sheep grazing), while the clay texture of subsurface horizons of studied soil pedons is a result to pedogenic sediments processes and the nature of cementing materials such as lime. The soil Consistency ranged from slightly hard to friable in surface horizons of all the studied sites' pedons, and plasticity to sticky in most of the subsurface horizons of the pedons. This difference may be due to the nature of the parent material for soils, as well as the fine texture of the soil and the size of particles for the studied soils.

3.2 Some physical and chemical properties of the study soil pedons

Table 4 shows some of the physical and chemical properties of soil particles of the studied sites. The soil particle size distribution was varied between samples for the same pedon and different sites. The soil texture of all studied soils was loamy clay and silty clay; moreover, clay was predominant in most sites. The lowest sand content (158 g/kg) was in horizon C in soil pedon P3, and the highest was 298 g/kg in horizon C1 in pedon P1. The lowest silt content (272 g/kg) was in horizon C1 in soil pedon P2, and the highest was (414 g/kg) in horizon C2 in pedon P2. The lowest clay content (390 g/kg) was equally found in horizon C1 in pedon P1 and horizon B in pedon P3, while the highest clay content (496 g/kg) was in horizon B in pedon P4. The reason for these differences relates to the nature of the soil parent material in the studied sites [43]. Bulk density values of the studied sites were varied between the horizons of the same pedon and between different sites. The lowest bulk density value was (1.4 Mg·m^-3) in surface horizon A of soil pedon P2, and the highest value was (1.55 Mg·m^-3) in subsurface horizon B of pedon P4. Bulk density values are affected by soil texture, soil organic matter content, and other soil components. Low bulk density values have an impact on soil water retention and porosity, which fosters chemical weathering. Generally, bulk density is a useful measure of soil growth and degree of weathering, especially in arid and semi-arid regions [44]. Soil pH values were neutral and slightly alkaline in all soil pedon horizons, with the lowest value (7.3) in surface horizon A of pedon P2 and the highest value (7.9) in horizon C for soil pedon P4. The reason for the variation between these values is that surface horizons contain soil organic materials, which leads to a reduction in soil pH values, while subsurface horizons are affected by the parent material of soil, which is CaCO₃ and basic elements present in the composition of primary and secondary soil minerals [45]. Soil EC values were noticeably low in most of the soil pedons' horizons, ranging between 0.32 and 1.70 ds·m^-1. This indicates that the soils in the study area are not affected by salts, and they are based on rain during the rainy seasons in the study area. The content of soil organic matter exhibited significant variability across the studied sites in relation to different soil horizons, it ranging between (18.6 g·kg^-1) in surface horizon A of pedon P1, and (6.5 g·kg^-1) in horizon C for soil pedon P4, this variability can be ascribed to the relative disparities in the characteristics of the vegetative cover present in the study area, in conjunction with the biological activities of soil microorganisms, alongside the concomitant variation in organic matter quantities. The surface horizons of the soil demonstrated an elevation in organic matter values, whereas a decline in these values was observed with increasing depth, thereby illustrating the inherent distribution patterns of organic matter within the soil body. CaCO₃ content showed a decrease in the surface horizons and began to increase with depth for all pedons of the studied soils. Values of CaCO₃ were between 176 and 243 g.kg^-1 for all studied soils. Similarly, the soil CaSO₄ content followed the same pattern as the soil CaCO₃ content; the values of CaSO₄ were between 8.30 and 17.7 g·kg^-1 for all soil pedons. This difference in soil CaCO₃ and CaSO₄ content between surface and subsurface horizons is perhaps the reason for the continuous leaching of calcium carbonate and gypsum from soil surface horizons, which then accumulates in subsurface horizons, as well as in the parent material of these soils in the studied sites [46]. While the soil cation exchange capacity CEC, values ranged between (19-28 cmol·kg^-1 soil) for all soil pedons, this results may be because the organic matter and clay texture of soil has a clear effects on increasing soil CEC, since organic colloids carry a negative charge on their surfaces due to the presence of carboxyl, phenol, hydroxyl, and carbonyl groups, as well as humus. Therefore, these colloids have the ability to adsorb cations.

Table 4. Some physical and chemical properties of soils

3.3 Chemical elements oxides for the studied soil pedons

Results of Table 5 showed differences in percentages of chemical elements oxides that were analyzed using an XRF device in very fine sand particles (0.05 mm) for soil pedons. The percentage of CaO was the highest percentage for all soil studied sites, ranging between (33.88-37.44%) in the C2 horizon of P1 and the B horizon of P3 respectively. While the percentage of TiO₂ was the lowest compared to other elements' oxides, it ranged between (0.65-0.84%) in the C2 horizon of P1 and the C1 horizon of P2, respectively. Other element oxides showed variation in all soil pedons. Fe₂O₃ ranged between (4.01-6.64) in B horizon of P2 and A horizon of P1 respectively, K₂O value was between (1.11-1.77) in C2 horizon of P2 and A horizon of P3, while percentage of SiO₂ ranged between 19.13 and 23.26 in C2 horizon of P2 and C1 horizon of P2 respectively, Al₂O₃ value ranged between 6.11 and 7.63 in B horizon of P3 and B horizon of P1. This difference in oxide values may be due to the difference in parent material and minerals present in the soil in the study area. Additionally, the environmental conditions surrounding the study area control the intensity of weathering processes to which these minerals were exposed during different time periods.

Table 5. Percentage of some element oxides in soil study sites

3.4 Weathering indices for the studied soil pedons

The results of Table 6 showed clear differences between chemical weathering indices that depended on this study. The WI showed some variations between soil horizons and pedons; its value was 3.94 in the surface horizon A of P1, and it was 5.16 in the subsurface horizon B of P2. The differences between WI values indicate weak to moderate weathering in some soil pedons, represented by the surface and subsurface horizons of the P1, since the lower value indicates more weathering has occurred due to the loss of mobile CaO from surface soil horizons, while an increase of concentration of relatively immobile oxides such as A1₂O₃, Fe₂O₃, and TiO₂. On the other hand, a high value of WI indicate very weak weathering, as the case in horizon B of the P2, which attributed to continuous loss of CaO from horizon A and its accumulation in horizon B of P2. The CIA values indicate a very low intensity of chemical weathering that happened within soil horizons of P1, P2, P3, and P4, ranging between 15.87 and 12.12, because the lower value of CIA indicates a very weak weathering in the sites, so these results were consistent with the study [47]. Values of the WI (Si/Ses) were almost close for all soil pedons horizons, this may be due to the relatively similar percentages of the weathering-resistant oxides, that is, Fe₂O₃, A1₂O_3, and SiO₂, and values of this index ranged between (1.85-1.46). CIW values showed slight variation among the studied soil pedons' horizons. This result may be attributed to the factors that effect on studied soils, which have undergone weak weathering, and are attributed to the climate and natural factors surrounding the soil pedons, such as rainfall and temperature variations between seasons, as well as anthropogenic activities, erosion, and precipitation. It is observed from these results that the values were slightly high in soil pedons P1, P2, and P3, ranging between 13.95 and 16.22. These values then decreased in soil pedon P4, reaching 12.59 in the surface horizon A of P4. High CIW values indicate different degrees of weathering, meaning that the mobile oxides, such as CaO, were lost through leaching within the soil pedon horizons, compared to relatively weathering-resistant and immobile oxides such as Fe₂O₃, A1₂O₃, and SiO₂. Conversely, low values indicate weak weathering as a result of increased concentration of CaO and Na₂O. The weathering ratio (WR) values showed slight variation between soil horizons in different locations. A significant increase in WR values was observed in horizon C2 of P1, compared to horizon C of P3. These differences are attributed to the lower (TiO₂) content in some locations. The highest WR value (69.63) was recorded in P1C2, while the lowest value (51.91) was recorded in horizon C of P3.

Table 6. Weathering indices values of soil study sites

3.5 Homogeneity curves for sand particle accumulations

Figures 2-5 show the relationship between sand particle sizes and cumulative percentage of sand particle diameters for each horizon of the studied soil pedons, which were calculated in very fine sand particles (0.05 mm) for soil pedons, to determine homogeneity or heterogeneity of parent material for each soil pedon. The results showed there was homogeneity of soil pedon P1 with its parent material (Figure 2), which is evident from the parallel curves for all horizons. There is a clear convergence in the cumulative values of sand particle diameters in four horizons in very coarse to coarse sand particles, and a slight variation is observed in fine to very fine sand particles between the horizons. The homogeneity of parent material for this pedon may be because it’s near the lake, which allowed for weathering and soil-forming processes that affected the size similarity of non-clay soil particles of this pedon horizons [48], or because it’s uncultivated land, so there isn’t effected for plowing and irrigation. Figure 3 showed heterogeneity of horizons with parent material in soil pedon P2, through curves intersection of surface (0-8 cm) and subsurface horizon (8-30 cm) with the deeper soil horizons (30-57 cm) and (57-100 cm), and the curves intersection was in the medium and fine sand particles (0.5-0.125 mm). The reason may be attributed to the transfer and deposition of fine sand particles due to plowing, machine movement, and grazing. These major additives continuously affect the mixing of soil components. Figure 4 shows that the surface horizon curve A of soil pedon P3 with a depth of (0-12 cm) intersected with horizons B and C in very coarse sand particles (2 mm) as well as coarse sand particles (1.18 mm). This may be attributed to the exposure of surface soils to erosion and sedimentation processes; furthermore, the influence of parent material and geological factors that would redistribute the soil components. While, the soil pedon P4 in Figure 5, its noted that the curve of subsurface horizon B with a depth of 15-48 cm has intersected with the horizons A and C in medium sand particles (0.5 mm), whereas the results for the remaining parts were similar, this may be attributed to different weathering processes as well as rainwater, which washed away the surface and subsurface horizon components, and the cost of the calcareous, gypsum, and pedogenic processes.