*3.1. Soil Physicochemical Characterization*

The results of the physicochemical properties of the soil are shown in Table 1. The granulometric classification made it possible to classify the three types of soil according to their particle size into three groups: sand, silt, and clay. The grain-size distribution showed that the soils have a sandy texture (>95% of sand) with low silt and clay content. This granulometric classification highlights the infertility of the R'mel soils, which leads farmers to intensify the use of inorganic fertilizers and manures in order to increase soil fertility. In consequence, the intensive irrigation rates applied to crops and soils have caused the leaching of these amendments towards groundwater due to the soil's low capacity to adsorb fertilizers.


**Table 1.** Physicochemical analysis of soil samples.

\* analyzed in the fine fraction (FF) of soil.

These results were also confirmed by analyzing the organic matter available in the three soils. Indeed, analyses of the organic matter in the complete fraction, CF, of the soil showed that the R'mel soils were characterized by a low OM content of 2.8, 2.36, and 1.57% for the samples S01, S02, and S03, respectively. On the contrary, the fine fraction, FF, which represents the fraction under 63 µm mainly composed of silt and clay, has a higher OM content. Soil 1 was characterized by the highest OM rate of 9.11%, whereas percentages of 7.5 and 7.65% were measured in S02 and S03, respectively. The pH of the R'mel soils varied between 6.33 and 8.7, with a mean value of 7.55. The soil S02 was alkaline in nature with the highest pH value of 8.75; soil S03 was slightly acidic with a pH value of 6.33, whereas soil S01 had a neutral pH value of 7.57. The measured cation exchange capacity (CEC) showed values of 9.28, 9.11, and 8.18 meq/100 g for the samples S01, S02, and S03 with a mean value of 8.86 meq/100 g of soil. This value confirms the low percentage of soil organic matter and clay in the R'mel soil. The electrical conductivity, EC, of the soil indicated the salinity of the sampled soils and ranged between 0.23 and 0.32 mS/Cm, thus indicating a low EC. Phosphorus in soils is almost entirely in the form of orthophosphate, with total P concentrations typically ranging from 500 to 800 mg/kg dry soil [45]. The presence of orthophosphate in the soil is closely related to organic matter and clay minerals. The analysis of PO3<sup>−</sup> <sup>4</sup> in the soil's fine fraction showed a concentration ranging from 0.81 to 2.04 g/Kg, which indicates that despite the dominance of sandy texture, the clayey fraction tends to adsorb higher amounts of orthophosphate ions. Nutrient availability in soils is affected by many interconnected variables; examples include parental rock composition, particle size, humus and water content, pH, aeration, temperature, root surface area, and fungal growth [46]. The lack of nutrients in sandy soils is frequently resulting in decreased water-holding capacity, soil pH, cation exchange capacity, and soil organic matter [47]. The concentrations of nutrients in the fine fraction of the R'mel soils were in the following order: Fe > Ca > Mg > K> Mn > Na, with values ranging from 46.19 to 47.41, 5.24 to 13.37, 3.42 to 4.86, 2.05 to 3,48, 1.35 to 1.62, and 0.18 to 0.33 g/Kg, respectively. The lack of OM in the R'mel soils probably influences the plants' nutrient availability since the dominant texture is coarse. The higher Ca2+ content (13.37 g/kg) recorded in Soil 2 indicates that calcium cation influences the pH of the soil. This relatively higher alkalinity in Soil 2 could be originating from the application of free lime (CaO) to increase soil pH for optimal plant growth in this area known for its low alkalinity. The concentrations of these nutrients in the fine fraction of the R'mel soil indicate that their presence is barely at the level recommended for agricultural soils. Furthermore, this fraction accounts for less than 5% of the total soil fraction, although the remaining fraction is notably coarse particles with low CEC and low water- and organic-matter-holding capacity, and therefore poor in nutrients.

During their assimilation, certain trace elements, such as Cu, Zn, Ni, Fe, Co, Se, and Ba, are essential for the functioning of plants [48]. They intervene in processes such as photosynthesis, biosynthesis of proteins, amino and nucleic acids, and chlorophyll, as well as the production of substances made by plants that make them competitive in their environment [49]. However, soil pollution by heavy metals is one of the world's major environmental problems [50]. The accumulation of heavy metals in soil could originate from geogenic or/and anthropogenic sources. Several studies have shown that agricultural practices could be a source of heavy metal accumulation in agricultural fields. Several studies [51–53] reported that the applications of fertilizers and pesticides were responsible for the accumulation of Cr, Cd, Cu, Zn, Ni, Mn, and Pb in agricultural fields.

Table 1 represents the concentrations of trace elements analyzed in the fine fraction of the R'mel soils. The heavy metals were well absorbed by the FF of the soil, which was evident from the As, Cr, Cu, and Ni contents surpassing the allowable limits set at 20 ppm, 0.8 ppm, 30 ppm, and 35 ppm, respectively. Zn concentrations varied between 114 and 224 mg/kg in the soil samples. Zn is generally present in soils at background concentrations of 10–100 mg Zn kg−<sup>1</sup> [54]. Arsenic is a ubiquitous element that can be found in every Earth compartment; arsenic derives naturally from geogenic rocks and/or can originate from anthropogenic activities such as the use of fertilizers and pesticides. R'mel drinking water was already reported as being contaminated by arsenic. This contamination has been attributed to both geogenic and anthropogenic sources [39]. From these results, it is possible to mention the crucial role of the fine fraction (clays and silts) in the retention of pollutants and also to shed light on the impact of low organic matter on the release of pollutants towards groundwater. By comparing the amount of clay in the three samples, it was consistent that Soil 1 had retained a higher concentration of trace elements than Soils 2 and 3; this higher concentration is closely related to the higher amount of OM and clay contained in the soil which is manifested by the relatively higher adsorption capacity of trace elements. Despite this finding, the R'mel soils contain negligible amounts of silt and clay and consist largely of coarse particles, which affects the leaching of trace elements and other agricultural pollutants.
