*3.5. X-ray Fluorescence Analysis*

The major oxides present in the soil's FF, CF, and OPBS are presented in Table 4. The percentage of oxides decreased in the sequence of SiO<sup>2</sup> > Al2O <sup>3</sup> > Fe2O<sup>3</sup> > CaO > MgO > K2O > TiO<sup>2</sup> > Na2O > P2O<sup>5</sup> > SO<sup>3</sup> in the three analyzed soils for both the fine fraction (FF) and the complete fraction (CF) of the soils. The results showed that the samples were predominately silicate soils rich in iron and aluminum. According to Chong et al. [66], the high content of exchangeable Al and Fe ions is mainly due to the high temperatures and heavy rains resulting in low soil pH, low nutrient availability, and low organic matter content. A significant difference in SiO<sup>2</sup> percentage was noted between the FF and CF, confirming quartz particles' predominance in the R'mel soils. In contrast, the SiO<sup>2</sup> percentage has decreased while other oxides have increased parallelly in the soil clayey fraction. The mineral matrix present in the soil ranges between 83.3% and 89.62% for the FF, whereas it ranges from 89.04 to 92.13% for the CF, indicating the higher rate of soil cultivation in the R'mel area. Considering the heterogeneity of the situation in soils, normally the denser and more friable/loose mineral fraction (less oxide/silica content) is found in the FF of the soil representing higher OM, CEC, and specific surface area (SSA). On the contrary, the less friable (or gangue) material is typically composed of oxides, often the majority being silica. In addition, the higher percentage of mineral matrix signifies a more dominant effect of hydrogen bonding on the adhesion of inorganic oxide particles. The size distribution and mineralogy of the clayey and silty fractions associated with sand grains are also responsible for variations in the physical properties of tropical sandy soils [67]. The XRF analysis of OPBS reveals a distinct composition dominated mainly by SiO2, CaO, and K2O accounting for more than 66% of the total OPBS. Oxides such as CaO, K2O, MgO, P2O5, and SO3 were present at negligible amounts in both soil fractions. In contrast, these oxides were found in high concentrations in the OPBS. Consequently, the OPBS can provide essential nutrients needed by crops when amended with OPBS. In addition, the higher alkali oxides represent a great advantage due to the liming characteristics of this material.

**Table 4.** Chemical composition of the three soil samples and Olive Pomace Biomass Slag (OPBS).


(Note: F.F = Fine-soil fraction (<63 µm); C.F = complete-soil fraction; OPBS = Olive Pomace Biomass Slag).
