Improving the Physical Properties and Water Retention of Sandy Soils by the Synergistic Utilization of Natural Clay Deposits and Wheat Straw

Abstract

Improving the physical properties and water retention of sandy soils is of critical importance in arid and water-scarce regions such as Saudi Arabia. The impacts of organic amendments of different particle sizes coupled with natural clay deposits on improving the soil physio-chemical characteristics, nutrient availability, and growth of Sudan grass were investigated in this study. A loamy sand soil was amended with natural clay deposits at 2.5%, 5.0%, and 10% (w/w) application rates, and in combination with 1.0% (w/w) wheat straw of different particle sizes. The water infiltration, evaporation, and retention characteristics of the amended soil were studied for 9 weeks, and then Sudan grass was grown for 7 weeks. The impacts of the particle size of wheat straw on soil properties and nutrient availability were significant (p < 0.05) when combined with clay deposits. The highest application rate of clay (10%) demonstrated the highest water content (20.63–21.73%), and increased P and K availability to 35.54 and 6980 mg kg⁻¹, respectively, in soil, which were 33% and 88% higher, respectively, compared to the control. Plant N, P, and K concentrations were increased to 0.95%, 0.26%, and 4.33%, respectively, which were 2–3.5-fold higher than the control. Therefore, the integrated application of natural clay deposits and wheat straw of fine particle size could be an effective strategy in improving plant production in water-scarce regions.

1. Introduction

Sandy soils are characterized by lower clay content, organic matter, and nutrient retention, with higher pH, CaCO₃, and infiltration [1,2]. In terms of their physical soil properties, sandy soils have a low moisture retention capacity, a low aggregate stability, and a high infiltration rate. These characteristics are responsible for poor fertility and negative effects on physical, chemical, and biological soil properties, which eventually result in a lower crop productivity in sandy soils [3]. The population of the planet Earth will increase from 8.0 to 10.4 billion by 2050, and to meet the demand to feed this ever-increasing population efficiently, there is a need to increase food production by up to 50–70% [4]. Therefore, there is a dire need to enhance arable land and boost crop productivity. Although the production of food seems difficult in arid and semi-arid regions, by improving crop varieties, pest management, and soil and water conservation practices, crop production could be increased. Therefore, improving the physical characteristics, nutrient cycling, and water retention of sandy soils is a great challenge for researchers in this era.

The addition of various organic amendments in sandy soils has been suggested as an economic and feasible strategy to enhance soil fertility. This includes improvements in physical and chemical soil properties, such as bulk density, total porosity, water retention, hydraulic conductivity, aggregate stability, organic matter, and cation exchange capacity, as proposed by various researchers [3]. However, harsh climatic conditions in arid and semi-arid regions, such as extreme temperatures, dry weather, and low precipitation, have led to the impoverishment of these soil amendments owing to the fast decomposition and mineralization of organic matter [3]. Moreover, some of the organic amendments, such as municipal solid waste, manure, and compost, could contain toxic elements (Pb, Ni, Zn, Cd, Cr, Cu, polychlorinated bi-phenyls, polychlorinated di-benzo-p-dioxins/furans, and antibiotics), which may contaminate the soil after application [5]. Therefore, under such conditions, the addition of clay to the soil could protect organic materials, improving physical and chemical soil properties. Clay particles could serve as an effective medium to retain water and nutrients [6]. Previously, it has been reported that soils with a high content of clay particles possess higher levels of organic matter as compared to sandy soils with lower clay contents, which could be owing to the protection of organic compounds (via binding and complexation with clay) from microbial mineralization and breakdown [7]. Furthermore, soils with a higher clay content could enhance aggregate formation, which physically preserves soil organic matter from microbial breakdown [2].

Previously, it has been reported that the addition of clay minerals as soil conditioners to sandy soils leads to the improvement of soil properties [8,9]. In a study conducted by Croker et al. [10] to investigate the influence of recycled bentonite on soil properties and plant growth, it was demonstrated that applying waste bentonite enhanced the soil cation exchange capacity and plant growth. Similarly, Al-Busaidi et al. [11] found that applying synthetic zeolite to saline sandy soil improved water-holding capacity, nutrient balance, and plant growth, making it an effective amendment to ameliorate salinity stress. Therefore, the application of clay in combination with organic materials might add value in terms of improving physical and chemical soil properties as well as soil organic matter maintenance through the protective action of the clay fraction. However, the binding of organic matter with clays and their impacts as soil additives on soil properties may be governed by their physical and chemical properties, which can be mainly altered by their size fractions [12]. In this context, very little research has been devoted to exploring the role of the combined application of clay and organic amendments at different particle sizes in improving the chemical and physical properties of sandy soils. Most earlier studies related to the particle sizes of added organic residues have focused on their effects on soil biochemical properties but not on physical properties [13,14]. In the literature, the role of particle sizes of organic residue in improving biochemical soil properties is controversial [15,16]. Thus, we hypothesized that applying smaller particle sizes of plant residues (wheat straw) in combination with natural clay deposits to sandy soils could be more efficient in enhancing organic matter, nutrient availability, and water content. This, in turn, could lead to increased nutrient and water supply to plants, ultimately contributing to improved crop production. Saudi Arabia is rich in many natural resources, including clay deposits, which are found in geological formations [17]. It is important to maximize the utilization of these local clay deposits through their use as environmentally safe and cheaper materials in different ways that increase the desired benefit of their application. However, to the best of our knowledge, no studies have investigated the effects of natural clay deposits (enriched with montmorillonite and kaolinite) combined with wheat straw on the physical and chemical properties, nutrient availability, and plant productivity of sandy soils. Therefore, the objective of this study was to evaluate the interactive effects of natural clay deposits (enriched with montmorillonite and kaolinite) coupled with wheat straw of different size fractions on improving physical (bulk density, water evaporation, infiltration, and water retention) and chemical (pH, electrical conductivity, and organic matter) soil properties, and nutrient availability. This study also aimed to assess the impact of these factors on the growth of Sudan grass (Sorghum drummondii L.) in sandy soils.

2. Materials and Methods

2.1. Collection of Soil and Clay Deposit Samples

A composite soil sample from a depth of 0–30 cm was collected from the Agricultural Experimental and Research Station at King Saud University, located in Dirab, Riyadh, Saudi Arabia (24°42′ N and 46°65′ E). Soil samples were collected randomly from five different places with the help of a soil auger and mixed to form a composite sample. The sample was brought to the Department of Soil Science, King Saud University, dried in air for two days, and passed through a 2 mm sieve. The prepared soil sample was subjected to basic physio-chemical characterization.

For this study, local natural clay deposits and wheat straw waste were used as soil amendments. The natural clay deposits (C) were collected from Riyadh province, brought to the laboratory, and sieved through a 53-micron screen. Then, the clay sample was subjected to basic physio-chemical characterization, mineral composition, and surface area analyses. Wheat straw was brought from wheat residues from the Agricultural Experimental and Research Station in Dirab. The wheat straw was dried, ground, and passed through different sieves to achieve various particle sizes of <0.5 mm (W1), 0.5–1.0 mm (W2), and 1.0–2.0 mm (W3).

2.2. Characterization of the Soil and Natural Clay Deposits

The electrical conductivity (EC) and pH of the soil samples were determined in saturated paste in deionized water with the help of an EC and pH meter, respectively. The particle distribution of the samples was analyzed using the hydrometer method, while the soil texture was estimated using the USDA (United States Department of Agriculture) textural triangle by following the method reported by Koehler et al. [18]. The soil organic matter was analyzed using the Walkley and Black method, as reported by Hesse [19]. The soil bulk density was determined by following the procedure reported by Amoakwah et al. [20]. The mineralogical composition of the collected natural clay deposits was analyzed using an X-ray diffractometer (model: MAXima XRD-7000, Shimadzu, Tokyo, Japan), while the surface area was analyzed using the Brunauer–Emmett–Teller (BET) procedure (TriStar II 3020, Micromeritics, Norcross, GA, USA) [21].

2.3. Set-Up of the Pot Experiment

Plastic soil pots measuring 9 cm in internal diameter and 15 cm in height were used in the experiment. The pots were sealed from the bottom using two filter papers. The pots were packed with 1.0 kg of collected soil. Natural clay deposits were added to the soil at application rates of 2.5, 5.0, and 10% on a w/w basis (referred to hereafter as C1, C2, and C3, respectively). Wheat straw at three different particle sizes of <0.5, 0.5–1.0, and 1.0–2.0 mm (referred to hereafter as W1, W2, and W3, respectively) was added to the soil at an application rate of 1.0% (w/w). Different combinations of wheat straw and natural clay deposits were formulated and mixed into the soil. In total, sixteen treatments were prepared: six treatments for single applications of clay deposit (C1, C2, and C3) and wheat straw (W1, W2, and W3), and nine treatments of a combination of both clay deposit and wheat straw (C1W1, C1W2, C1W3, C2W1, C2W2, C2W3, C3W1, C3W2, and C3W3). In addition, one control treatment (CK) was used with no addition of either clay deposit or wheat straw. In all treatments, the wheat straw and clay deposits were mixed thoroughly with the soil, and the pots were arranged in a completely randomized design (CRD). Each treatment was replicated three times. The pots were kept on the bench inside the laboratory at a controlled room temperature of 22 ± 2 °C. One hundred milliliters of water (ECw of 0.73 dS m⁻¹) were added weekly for nine wetting/drying cycles. The cumulative evaporation versus time was measured daily by weighing each soil pot. At the end of the ninth week, the amount of water retained in each treatment was calculated based on the cumulative evaporation and water content data. The cumulative infiltration and infiltration rate were also estimated.

After 9 weeks, the soil pots were transported to a greenhouse, planted with Sudan grass (Sorghum drummondii L.), and arranged in a CRD layout. The pots were irrigated daily to maintain field capacity. The plants were grown for 7 weeks in the greenhouse before being harvested. At the time of harvesting, the plant height and fresh biomass were recorded. After harvesting, the soil and plant samples were brought to the laboratory, dried at 70 °C for 24 h, and subjected to further analyses.

2.4. Evaporation and Infiltration

The evaporation from the soil pots was calculated as follows:

$$\mathrm{E}=\frac{{\mathrm{W}}_{\mathrm{c}\mathrm{n}}-{\mathrm{W}}_{\mathrm{d}}}{{\mathsf{\pi }\mathrm{r}}^2}\ast 10$$

(1)

where E is the weekly evaporation (mm) for each soil pot, ${\mathrm{W}}_{\mathrm{c}\mathrm{n}}$ is the weight (g) of each pot during the week, cn is the number of cycles (8 cycles + 4 days), ${\mathrm{W}}_{\mathrm{d}}$ is the initial dry weight (g) of the pot, and ${\mathsf{\pi }\mathrm{r}}^2$ is the area of the pot.

Infiltration was measured using a mini-disk infiltrometer (model M12, 2 cm suction; Decagon Devices, Pullman, WA, USA). The disk infiltrometer was submerged in water to ensure that the mini-disc was saturated, and was then carefully filled with water to avoid air bubbles. The disk infiltrometer was vertically placed on the surface of the soil in the pot, ensuring full contact with the soil surface. Infiltration readings were recorded every 30–60 s and continued for 10–15 min depending on each treatment. The volume of water infiltrating and the position of the wetting front were recorded every minute, and the cumulative infiltration was calculated according to Philip’s equation [22]:

$$\mathrm{I}={\mathrm{S}\mathrm{t}}^{0.5}+{\mathrm{A}}_1\mathrm{t}$$

(2)

where I is the cumulative infiltration (cm), S is the sorptivity (cm∙min^−0.5), A₁ is a constant related to the hydraulic conductivity, and t is the time (min). Equation (2) was mathematically represented by plotting cumulative infiltration versus the square root of time, and the values for S and A₁ were obtained by fitting a second-order polynomial equation to the measured data. The first derivative of the cumulative infiltration was used to calculate the rate of infiltration (i) according to the relation:

$$\mathrm{i}=0.5{\mathrm{S}\mathrm{t}}^{-0.5}+{\mathrm{A}}_1$$

(3)

Equation (3) was mathematically represented by plotting the infiltration rate versus 1/(2 t^0.5), and the fitting of a linear equation to the measured data.

2.5. Hydraulic Conductivity and Soil Water Retention

The saturated hydraulic conductivity (K_s) was measured using the constant head method technique. The measurements of the hydraulic conductivity of saturated soils in the laboratory were based on the direct application of Darcy’s equation to a saturated soil column of a uniform cross-sectional area [23]. A hydraulic-head difference was imposed on the soil column, and the out-flux of water was measured. The saturated hydraulic conductivity (K_s) was calculated as follows:

$${\mathrm{K}}_{\mathrm{s}}=\frac{\left(\mathrm{Q}\mathrm{L}\right)}{\left(\mathrm{A}\mathrm{t}\mathrm{H}\right)}$$

(4)

where K_s is the saturated hydraulic conductivity (cm s⁻¹), Q is the volume of water (cm³), L is the length of the soil column (cm), A is the cross-sectional area (cm²), t (s) is the time required for the volume of water Q to be discharged, and H is the water head (cm).

Several soil water parameters were measured, including soil water content at 100 and 15,000 hPa retention, representing field capacity and permanent wilting, respectively. In addition, the water retention curve was measured in the collected soil sample using a pressure plate extractor at water potential values of 0.1, 0.3, 0.5, 1, 5, 8, 10, and 15 bar. The RETC program was used for fitting the parameters ($\mathsf{\theta }$s, $\mathsf{\theta }$r, $\mathsf{\alpha }$, and n) for the van Genuchten model [24]:

$$\mathsf{\theta }\left(\mathrm{h}\right)={\mathsf{\theta }}_{\mathrm{r}}+\frac{({\mathsf{\theta }}_{\mathrm{s}}-{\mathsf{\theta }}_{\mathrm{r}})}{{\left[{1+\left(\mathsf{\alpha }\mathrm{h}\right)}^{\mathrm{n}}\right]}^{\mathrm{m}}}$$

(5)

where $\mathsf{\theta }$ is the soil water content (cm³·cm⁻³), ${\mathsf{\theta }}_{\mathrm{r}}$ is the residual water content (cm³·cm⁻³), ${\mathsf{\theta }}_{\mathrm{s}}$ is the saturated water content (cm³·cm⁻³), h is soil water potential (kPa), $\mathsf{\alpha }$ is a scale parameter inversely proportional to the mean pore diameter (cm⁻¹), and n and m are the shape parameters of soil water characteristics, m = (1 − 1/n), 0 < m < 1.

2.6. Soil and Plant Analysis

The soil EC and pH were measured in saturated soil paste with the help of an EC and pH meter, respectively. The organic matter content in the soil was determined using the Walkley–Black method. The available contents of soil phosphorus (P), potassium (K), micronutrients (Zn, Cu, Fe, and Mn), and heavy metals (Cd, Co, Cr, Mo, Ni, and Pb) were extracted via the AB-DTPA (ammonium bicarbonate-diethylene triamine Penta acetic acid) extraction procedure, as reported by Soltanpour and Schwab [25]. The contents of P, K, nitrogen (N), micronutrients, and heavy metals in the plant tissues were extracted by digesting the samples using the Wolf method [26]. The concentrations of P in the plant and soil extracts were determined via the colorimetric method with the help of a spectrophotometer (Lambda EZ 150, PerkinElmer, Waltham, MA, USA), while the concentrations of K were analyzed using a flame photometer. The concentration of N in the plant samples was determined through the micro-Kjeldahl apparatus, while the concentrations of micronutrients and heavy metals in the soil and plant extracts were analyzed through the inductively coupled plasma optical emission spectrophotometer (ICP-OES) (PerkinElmer Optima 4300 DV, Waltham, MA, USA).

2.7. Statistical Analysis

The obtained data were statistically analyzed using the Microsoft Excel and Statistics 8.01 programs. All the graphs were constructed using the SigmaPlot 12.5 program. The least significant difference (LSD) test was used to compare various treatments used in this study by using a 5% probability level [27].

3. Results and Discussions

3.1. Soil and Clay Characterization

The physio-chemical properties of the soil and clay deposits used in this study were determined and are presented in

Table 1. Selected physio-chemical characteristics of the soil and natural clay deposits used in the experiment.

Sample	Sand	Silt	Clay	pH	EC	Bulk Density	Surface Area *	Organic Matter
	(%)	(%)	(%)	(−)	(dS m−1)	(g cm−3)	(m2 g−1)	(%)
Clay deposit	3.33 ± 1.18	25 ± 4.08	71.67 ± 3.12	7.55 ± 0.02	4.03 ± 0.04	1.38 ± 0.10	62.72 ± 0.42	1.02 ± 0.09
Soil	86 ± 4.05	11.5 ± 1.19	2.5 ± 0.21	8.19 ± 0.05	2.05 ± 0.06	1.46 ± 0.08	5.31 ± 0.28	0.90 ± 0.07

* Determined using the BET method.

. The soil was characterized by an alkaline pH (8.19), and a low EC of 2.05 dS m⁻¹. The bulk density of the soil was observed as 1.46 g cm⁻³, while it contained very low organic matter content (0.90%). The pH of the clay deposit (7.55) was slightly lower than the pH of the soil sample; however, the EC of the clay deposit was slightly higher than that of the soil and reached 4.03 dS m⁻¹. The particle size distribution showed that there were 71.67% of clay sized fractions in the clay deposit sample, whereas, this fraction was only 2.5% in the soil sample. Based on the particle size distribution, the soil was characterized as loamy sand in texture. The mineralogical composition of the clay deposits, as determined with the help of XRD, is given in Figure 1. It was observed that these clay deposits were mainly composed of kaolinite, montmorillonite, quartz, and feldspar. The peaks observed around 11° and 24° 2θ were designated as kaolinite, while the peaks that appeared around 31° were ascribed to montmorillonite [28]. The peaks representing quartz were recorded at various locations on the XRD pattern, such as at 2θ of 20.6°, 27°, 37.2°, 39.8°, and 60°. Likewise, the peaks representing feldspar were found at 2θ of 29.6° and 50.2°. Therefore, it can be observed from these results that kaolinite, montmorillonite, quartz, and feldspar were the dominant minerals in these clay samples. The dominant contents of these minerals in the clay deposits could be attributed to their higher stability in weathering [29]. Previous studies have demonstrated similar minerals in natural clay deposits collected from different locations in Saudi Arabia (eastern region: Al-Hassa; central region: Marrat-Riyadh, Al-Kharj, and Al-Summan; Western region: Jeddah, Khulays, and Harrat-Medina) [30].

3.2. Impacts of Amendments on Soil Chemical Characteristics

The impacts of soil amendments on the variation in soil pH, EC, and organic matter contents are shown in Figure 2. The application of natural clay deposits at varying rates individually as well as in combination with wheat straw significantly (p < 0.05) affected the pH of soil (Figure 2a). The highest pH (8.19) was observed in the control soil, where no amendment was applied. However, the pH tended to decrease with the application of clay and wheat straw individually or in combinations.

Overall, the application rate of clay deposits affected the pH of soil. It was observed that the soil pH was reduced with increasing the amount of applied clay. However, the impacts of the particle size of wheat straw on the soil pH were non-significant. Therefore, treatment C3W3, which is the combination of 10% clay (C3) with the smaller particle sized wheat straw (W3), resulted in the lowest soil pH (7.79) among all tested treatments. Overall, it was observed that with the application of C3W3, the pH of the soil was reduced by 5% as compared to the control (CK).

The application of natural clay deposits and wheat straw-based amendments significantly increased the soil EC (Figure 2c). Overall, an increasing trend was observed with increasing the amount of applied clay and/or decreasing the particle size of the applied wheat straw. This could be attributed to the large EC values observed in the clay deposit (Table 1). The lowest EC (2.05 dS m⁻¹) was observed in the control (CK). The increase in EC was observed with both the single application and the different combined applications of clay deposits and wheat straw amendments. The highest soil EC was observed in C3W3 (6.30 dS m⁻¹), followed by C3W2 (6.08 dS m⁻¹), which were almost 3-fold higher than the control (CK). Likewise, significant variations were observed in organic matter contents with the application of the soil amendments (Figure 2e). The control treatment exhibited the lowest organic matter contents, which increased with the application of amendments. It was observed that the application of clay deposits alone did not change the organic matter significantly, while when combined with wheat straw, a substantial increase in organic matter contents was observed. Moreover, the reduction in wheat straw particle size resulted in higher organic matter contents. The increase in the percentage of organic matter content was almost doubled as compared to the control with the application of the C2W3, C3W1, C3W2, and C3W3 treatments, and reached 1.91%, 1.81%, 2.00%, and 2.14%, respectively. Therefore, our findings demonstrated that the application of natural clay deposits in conjunction with wheat straw has improved the chemical properties of the soil, which could be reflected in improved crop productivity and soil health. Previously, Sarkar and Naidu [31] demonstrated significant variations in soil characteristics after the application of clay-based soil amendments. Moreover, several researchers demonstrated the beneficial impacts of organic amendments in improving the physio-chemical characteristics of the soil. Ahmad et al. [32] applied poultry manure-based organic amendments to sandy loam soil and observed improved nutrient cycling, organic matter, and P availability. Likewise, Roldán et al. [33] reported that the application of crop residue as soil amendment has improved the soil quality and nutrient availability. Therefore, a combination of natural clay deposits and wheat straw could be an effective soil amendment to improve soil properties such as pH and organic matter, which could subsequently enhance nutrient availability.

3.3. Impacts of Amendments on Soil Physical Properties

The variations in some of the soil’s physical properties including bulk density, water content, and hydraulic conductivity as affected by the application of soil amendments are shown in Figure 2. No significant (p < 0.05) differences in soil bulk density were observed with the application of either natural clay or wheat straw or their combinations (Figure 2b). Overall, the largest bulk density (1.46 g cm⁻³) was observed in the control soil (CK), whereas the lowest bulk density (1.18 g cm⁻³) occurred with treatment C1W1.

Unlike bulk density, a significant increase in water content was observed after the addition of amendments to the soil (Figure 2b). The addition of clay deposits or wheat straw alone as well as their different combinations resulted in improving soil water retention and water content. The results revealed that the control soil (CK) had the lowest water content (3.72%) among all treatments, whereas treatments C3W2 and C3W3 showed the highest water content of 20.63 and 21.73%, respectively. Moreover, the results showed that the amount of water retained was increased with increasing the amount of added clay deposits either alone or in combination with wheat straw. The increased water retention could be due to the increased surface area of the clay. The water content was also increased with the decrease in the particle size of wheat straw; however, this increase was non-significant when wheat straw was applied alone.

Changes in soil hydraulic conductivity as influenced by soil amendments are presented in Figure 2f. Overall, the hydraulic conductivity of soil decreased with the application of amendments. The largest decrease was observed with a higher application rate of the clay deposits. In contrast, the particle size of wheat straw did not significantly affect the hydraulic conductivity. The results indicated that the highest soil hydraulic conductivity (1.33 × 10⁻³ cm s⁻¹) was observed in the control soil (CK), followed by the W1 treatment (1.19 × 10⁻³ cm s⁻¹), whereas, treatments C3W2 and C3W3 exhibited the lowest soil hydraulic conductivity of 5.19 × 10⁻⁴ and 5.08 × 10⁻⁴ cm s⁻¹, respectively.

The positive impacts of clay application on improving soil properties, moisture conservation, and crop production enhancement have previously been reported [28]. Our results are in agreement with the findings of Hassan and Mahmoud [34], where a 7.2% reduction in soil bulk density was reported with the application of natural clay as compared to un-amended soils. Moreover, these results were in agreement with the findings of Al-Omran et al. [6], who found a significant reduction in soil hydraulic conductivity, and an enhancement in water retention and conservation with clay application. Similar results were previously reported by Hassan and Mahmoud [34], where a significant decrease in soil hydraulic conductivity was noticed with natural clay application. The significant increase in water retention and availability with the application of natural clay deposits, along with the improvement in water retention, could be due to the increase in the fine particles of soil with the application of clay deposits, which can increase the ability of the soil to hold more water by more than 55% [34]. Soil water retention is an important factor influencing soil chemistry and the availability of nutrients [31]. Therefore, the application of natural clay in combination with wheat straw of a smaller particle size could effectively be used to increase water retention, which can eventually improve soil properties and enhance crop productivity.

3.4. Impacts of Amendments on Evaporation and Infiltration

The impacts of the single and combined application of natural clay deposits and wheat straw on water evaporation from the soil were investigated for nine cycles of wetting and drying. The results revealed that cumulative evaporation was significantly decreased with amendment application (Figure 3). This decrease was more pronounced with the higher application of the natural clay deposits, while it did not change substantially when decreasing the particle size of wheat straw. The highest cumulative evaporation was noticed in the control soil (CK), and a substantial decrease was observed with the various soil amendments. Treatments C3W3, C3W2, and C3W1 exhibited the lowest cumulative evaporation among all treatments, which was 19–22% lower than that of the control (CK) (Figure 3B). Equations (2) and (3) were fitted to the experimental data using a second-order polynomial, and the first derivative of the cumulative infiltration was used to assess the infiltration rate (Figure 4). Similar to cumulative evaporation, the addition of soil amendments resulted in a decreased cumulative infiltration from the soil (Figure 4a,b). These variations were independent of the particle size of wheat straw when applied alone; however, cumulative infiltration increased with increasing the application rates of natural clay deposits alone, and in combination with wheat straw. The highest cumulative infiltration was observed in the CK, followed by the C1, W1, and W3 treatments, whereas the lowest cumulative infiltration was exhibited with the C3W3, C3W2, C3W1, and C2W3 treatments, which ranged from 2- to 3-fold lower than the control (CK). Similarly, the infiltration rate was also decreased with the application of the various soil amendments used in this study (Figure 4c,d). The decrease in the infiltration rate was less pronounced with the individual application of amendments, but it increased when natural clay and wheat straw were combined. The lowest infiltration rate was observed with the C3W3, C3W2, and C3W1 applications.

The reduction in water infiltration and evaporation with the wheat straw and clay applications was due to the absorption and retention of water molecules on organo–mineral complexes. In this regard, Al-Omran et al. [6] reported similar results by observing a significant reduction in cumulative evaporation and cumulative infiltration after adding natural clay deposits in soils. Moreover, the application of wheat straw helped in improving the physical properties and water relations in soil as it can serve as a carbon source, which is a critical factor in maintaining soil structure, nutrient cycling, and water availability [35,36]. The application of soil amendments such as clay deposits and crop residues can reduce evaporation and water leaching, consequently increasing water availability in soil [37,38]. The significant reduction in evaporation and infiltration could improve soil structure with the application of a carbonaceous amendment (wheat straw) and the adsorption of water into silicate interlayers of clay deposits [39]. Moreover, the electrical charges on soil and clay minerals as well as organo–mineral complexes also help in retaining water molecules, subsequently reducing evaporation and infiltration [37]. Therefore, a combination of clay deposits with crop residues such as wheat straw can be used to reduce water evaporation and infiltration in sandy textured soils in arid and semi-arid regions.

3.5. Effects of Soil Amendments on Available P and K

The impacts of the clay deposits and wheat straw amendments on the available P and K in the soil are presented in Figure 5. The application of natural clay deposits and wheat straw significantly increased the available P contents in the soil (Figure 5a). The control soil (CK) had the lowest P content (23.89 mg kg⁻¹). The P content values increased significantly with the single application of either clay, or wheat straw, or their different combinations. When clay and wheat straw were applied separately, the overall increase in P content was comparable to when both treatments were applied together. Among all treatments, C3W3 exhibited the highest available P content (35.54 mg kg⁻¹), which was 33% higher than that of the control (CK). Other treatments, i.e., C2W2, C2W3, C3W1, and C3W2, showed similar results with an increase in available P content that ranged between 31.08 and 32.93 mg kg⁻¹, which represents an increase in available P by 23–27% as compared to the control (CK). No significant differences in available K content, as compared to the control, were observed with the single application of either clay deposit or wheat straw amendments (Figure 5b). However, the combined application of both amendments (with the exception of C1W1) resulted in a significantly higher available K content. The highest K content among all treatments was observed in treatments C3W3 and C3W2, which amounted to 6980 and 5590 mg kg⁻¹, respectively, representing an increase in available K by 85% and 88%, respectively, as compared to the control (CK).

3.6. Effects of Soil Amendments on Micronutrients and Heavy Metals

The available concentrations of micronutrients as affected by the soil amendments are shown in

Table 2. Concentrations of available micronutrients (mg kg⁻¹) in the soil as impacted by the clay deposit and wheat straw amendments.

Treatment *	Zn	Mn	Fe	Cu
CK	5.11 ± 0.45 a	2.54 ± 0.21 a	15.67 ± 1.12 a	6.07 ± 0.21 a
C1	4.84 ± 0.30 ab	2.41 ± 0.31 a	15.33 ± 1.00 a	5.79 ± 0.21 a
C2	4.69 ± 0.31 ab	2.12 ± 0.12 a	14.33 ± 1.02 a	5.78 ± 0.31 a
C3	4.43 ± 0.18 ab	1.87 ± 0.30 b	12.58 ± 1.03 a	5.50 ± 0.21 a
W1	4.11 ± 0.20 ab	2.09 ± 0.21 ab	12.64 ± 1.21 a	5.53 ± 0.24 a
W2	4.05 ± 0.03 b	2.00 ± 0.13 b	12.00 ± 0.82 a	5.34 ± 0.17 a
W3	3.60 ± 0.12 bc	2.01 ± 0.38 b	11.42 ± 0.51 a	5.33 ± 0.11 a
C1W1	3.86 ± 0.50 bc	2.25 ± 0.32 ab	13.11 ± 1.17 a	5.19 ± 0.09 a
C1W2	3.54 ± 0.09 bc	2.12 ± 0.31 ab	12.07 ± 1.14 a	4.86 ± 0.13 a
C1W3	3.40 ± 0.18 bc	1.83 ± 0.27 b	11.15 ± 0.85 a	4.66 ± 0.21 a
C2W1	3.83 ± 0.12 bc	2.05 ± 0.27 ab	13.18 ± 0.50 a	4.29 ± 0.26 a
C2W2	3.47 ± 0.11 bc	1.90 ± 0.21 b	11.33 ± 0.47 a	3.89 ± 0.20 ab
C2W3	3.60 ± 0.13 bc	1.82 ± 0.01 b	10.00 ± 0.30 ab	3.74 ± 0.15 ab
C3W1	3.13 ± 0.09 bc	1.91 ± 0.13 b	11.67 ± 0.47 ab	3.99 ± 0.14 ab
C3W2	3.10 ± 0.37 bc	1.64 ± 0.01 b	10.67 ± 0.46 ab	3.93 ± 0.11 ab
C3W3	2.90 ± 0.30 c	1.55 ± 0.05 b	9.89 ± 0.30 b	3.20 ± 0.19 ab

* See Section 2.3 for the explanation of the treatment symbols. Different superscript letters indicate significant differences (p < 0.05) among mean values in each column, while the use of the same letter shows non-significance.

. The concentration of available Zn, Mn, Fe, and Cu reduced after amending the soil with natural clay deposits or wheat straw. The control treatment (CK) demonstrated the highest concentrations of available Zn (5.11 mg kg⁻¹), Mn (2.54 mg kg⁻¹), Fe (15.67 mg kg⁻¹), and Cu (6.07 mg kg⁻¹). These values were reduced with the application of clay deposit and wheat straw amendments, and the lowest concentrations of Zn, Mn, Fe, and Cu were observed in the C3W3 treatment, which reached 2.90, 1.55, 9.89, and 3.20 mg kg⁻¹, respectively. The concentrations of available heavy metals in the soil as influenced by the applied amendments are shown in

Table 3. Concentrations of available heavy metals (mg kg⁻¹) in the soil as impacted by the clay deposit and wheat straw amendments.

Treatment *	Cd	Co	Cr	Mo	Ni	Pb
CK	0.37 ± 0.19 a	0.38 ± 0.02 a	0.09 ± 0.01 a	0.20 ± 0.02 a	0.16 ± 0.02 a	0.24 ± 0.02 a
C1	0.35 ± 0.04 a	0.35 ± 0.00 a	0.08 ± 0.01 a	0.19 ± 0.04 a	0.14 ± 0.00 a	0.24 ± 0.02 a
C2	0.34 ± 0.03 a	0.37 ± 0.01 a	0.08 ± 0.01 a	0.17 ± 0.07 a	0.15 ± 0.01 a	0.22 ± 0.01 a
C3	0.34 ± 0.03 a	0.33 ± 0.04 a	0.07 ± 0.04 a	0.17 ± 0.07 a	0.14 ± 0.01 a	0.21 ± 0.01 a
W1	0.34 ± 0.06 a	0.33 ± 0.04 a	0.08 ± 0.03 a	0.18 ± 0.08 a	0.13 ± 0.01 a	0.20 ± 0.02 a
W2	0.31 ± 0.03 a	0.33 ± 0.04 a	0.08 ± 0.06 a	0.17 ± 0.02 a	0.14 ± 0.01 a	0.18 ± 0.01 a
W3	0.31 ± 0.04 a	0.31 ± 0.04 a	0.08 ± 0.01 a	0.15 ± 0.01 a	0.14 ± 0.01 a	0.17 ± 0.01 a
C1W1	0.34 ± 0.03 a	0.32 ± 0.03 a	0.08 ± 0.03 a	0.13 ± 0.05 a	0.15 ± 0.01 a	0.18 ± 0.01 a
C1W2	0.32 ± 0.03 a	0.30 ± 0.01 a	0.08 ± 0.01 a	0.12 ± 0.03 a	0.13 ± 0.01 a	0.15 ± 0.01 a
C1W3	0.31 ± 0.04 a	0.31 ± 0.04 a	0.07 ± 0.02 a	0.14 ± 0.02 a	0.13 ± 0.01 a	0.16 ± 0.02 a
C2W1	0.32 ± 0.03 a	0.34 ± 0.02 a	0.08 ± 0.02 a	0.15 ± 0.01 a	0.13 ± 0.02 a	0.13 ± 0.01 a
C2W2	0.31 ± 0.03 a	0.33 ± 0.06 a	0.08 ± 0.02 a	0.15 ± 0.06 a	0.13 ± 0.01 a	0.14 ± 0.01 a
C2W3	0.31 ± 0.06 a	0.31 ± 0.06 a	0.08 ± 0.01 a	0.14 ± 0.02 a	0.12 ± 0.01 a	0.17 ± 0.01 a
C3W1	0.33 ± 0.03 a	0.27 ± 0.05 a	0.08 ± 0.01 a	0.17 ± 0.03 a	0.12 ± 0.01 a	0.21 ± 0.02 a
C3W2	0.31 ± 0.02 a	0.24 ± 0.04 a	0.08 ± 0.01 a	0.15 ± 0.05 a	0.11 ± 0.01 a	0.20 ± 0.02 a
C3W3	0.28 ± 0.07 a	0.22 ± 0.02 a	0.07 ± 0.02 a	0.09 ± 0.06 b	0.10 ± 0.01 a	0.13 ± 0.01 b

* See Section 2.3 for the explanation of the treatment symbols. Different superscript letters indicate significant differences (p < 0.05) among mean values in each column, while the use of the same letter shows non-significance.

. The results showed that As was not detected in the soil, whereas traces of Cd, Co, Cr, Mo, Ni, and Pb were detected in all treatments including the control. The concentrations of available heavy metals decreased with the single and combined application of natural clay deposits and/or wheat straw. However, this decrease was in general not statistically (p > 0.5) different as compared to the control. The control soil (CK) showed the highest available heavy metal concentrations for Cd, Co, Cr, Mo, Ni, and Pb (0.37, 0.38, 0.09, 0.20, 0.16, and 0.24 mg kg⁻¹, respectively), whereas the lowest available heavy metal concentrations for Cd, Co, Cr, Mo, Ni, and Pb were observed in the C3W3 treatment (0.28, 0.22, 0.07, 0.09, 0.10, and 0.13 mg kg⁻¹, respectively).

The improvements in nutrient retention after clay application has previously been reported by Beusch et al. [40]. It was reported that the application of clay to the soil resulted in a reduction in the leaching of NH₄⁺−N and K⁺ by 79 and 51%, respectively. The higher nitrate availability could be due to the retention of NO₃ within the pore spaces of the clay deposits, which is not accessible by nitrifying bacteria [41]. Therefore, the retention and protection of N and P within the pores of clay result in higher nutrient availability. However, this nutrient retention is largely dependent on the pore size and prevailing soil conditions. Likewise, Hall et al. [42] reported an increment of 0.2% in organic carbon, 1.3 cmol kg⁻¹ in soil exchange capacity, and 47% mg kg⁻¹ in K concentration with clay application. The reduction in available micronutrients could be attributed to the sorption capacity of clay deposits towards the aforementioned elements.

Previously, Sheta et al. [43] stated that natural clay deposits with zeolite and bentonite clays possess higher efficiency for the sorption of Zn and Fe. This higher sorption efficiency of clay deposits has also resulted in reduced concentrations of available heavy metals, consequently reducing their pollution hazard. Various reports have previously described the stabilization of heavy metals with clay and organic amendment application [41,44]. On the other hand, the application of crop residues such as wheat straw could be helpful in enhancing nutrient availability as it releases essential nutrients upon mineralization and decomposition [45]. Moreover, when combined with organic amendments such as wheat straw, the efficiency of clay deposits for nutrient availability and soil health improvement could be enhanced many-fold [44]. Organic amendments help improve microbial growth which subsequently enhances mineralization processes. The presence of clay deposits helps in the retention of the released mineral nutrients as well as sorbing metals onto ion-exchange sites [31]. Thus, a combined application of natural clay deposits with smaller sized wheat straw could be an efficient soil amendment for enhanced nutrient availability.

3.7. Impacts of Soil Amendments on Plant Growth and Nutrient Content

The impacts of soil amendments on plant growth parameters including plant height, fresh biomass, and dry biomass are shown in Figure 6. All amendments significantly (p < 0.05) improved plant growth over the entire duration of the experiment. The results demonstrated an up to 82% increase in plant height with the highest rate of application of natural clay combined with the smaller particle sized wheat straw (C3W3) (Figure 6a). The control treatment exhibited the lowest plant height (11.33 cm). The single application of clay deposit and wheat straw resulted in an increase in plant height by 11–32% and 37–43%, respectively. However, the combined application of natural clay and wheat straw increased the height of plants by 36% to 82%. Similarly, plant fresh and dry biomass was significantly increased with the application of the soil amendments (Figure 6b,c). The lowest values of fresh and dry biomass were observed in the control soil (1.53 and 1.07 g, respectively). The weight of both fresh and dry biomass increased substantially with the application of the various rates of amendments. Treatment C3W3 showed the largest plant fresh and dry biomass (5.14 and 3.30 g, respectively), followed by C3W2 (4.39 and 3.07 g, respectively).

It has been reported that the application of natural clay deposits enhances plant growth and produces more biomass by retaining more water and available nutrients [34]. Bernardi et al. [46] stated that nutrient-enriched clay deposits (zeolite) behaved as slow release fertilizers and enhanced plant growth and nutrient availability. In another study, Sonmez et al. [47] used a combined application of natural zeolite and turf and observed a significant improvement in height and stem diameter, as well as in the fresh and dry biomass of tomato seedlings. Similarly, Uher [48] applied natural clays in soil and used pepper as a test crop. He reported significant improvements in soil fertility, structure, moisture, and plant growth with the application of clay-based amendments. Hall et al. [42] observed a 102% increase in the yield of lupin and canola with clay application. They attributed the higher plant production and yield with clay application to the improvement in nutrient availability, plant emergence, and water distribution. In another study, Wei et al. [49] reported an up to 30.5% increase in available N, an up to 69.5% increase in available P, and a 27.3% increase in available K concentrations with wheat straw application into soil as compared to untreated soil. The higher nutrient availability resulted in an up to 26.75% increase in wheat crop yield as compared to the untreated soil. Therefore, wheat straw coupled with natural clay deposits could be used as an efficient and economical strategy to enhance crop productivity.

3.8. Plant Nutrient and Heavy Metal Concentrations

The influence of soil amendments on plant nutrient concentrations including N, P, and K is shown in Figure 7. The concentrations of all nutrients increased with increasing the application rate of clay deposits and decreasing the particle size of the applied wheat straw. More prominent improvement in nutrient concentrations were recorded with the combinations of clay and wheat straw as compared to clay and straw applied individually. The highest N concentration (0.95%) was observed in the C3W3 treatment, followed by C3W2 and C3W1 (0.85 and 0.81%, respectively) (Figure 7a). These N concentrations were 3-fold higher than that of the control soil (0.30%). The concentration of P in the plant tissues increased with the application of soil amendments; however, there were no significant differences among various treatments (except CK, C1, C2, and C3W3) (Figure 7b). The lowest P contents were recorded in the control soil (0.14%), followed by C1 and C2 (0.16% and 0.17%, respectively), while the highest (0.26%) was observed with the C3W3 treatment. All other amendments applied either individually or in combination showed statistically similar concentrations of P. Similar results were obtained with the plant concentrations of K, where a slight increase in the concentration of K was observed with the application of soil amendments; however, most of the differences in mean values among the different treatments were not significant, except with the C3W3 (4.33%) and CK (1.21%) treatments (Figure 7c).

Table 4. Concentrations of micronutrients (mg kg⁻¹) in the plant tissues after amending the soils with various clay and wheat straw combinations.

Treatments *	Zn	Mn	Fe	Cu
CK	36.79 ± 3.45 a	45.21 ± 5.51 a	585.15 ± 32.93 a	7.21 ± 0.84 a
C1	32.45 ± 4.25 a	43.77 ± 8.12 a	760.83 ± 18.34 a	6.24 ± 0.78 a
C2	29.94 ± 2.27 a	41.81 ± 4.43 a	823.41 ± 28.82 a	5.68 ± 1.97 a
C3	29.64 ± 1.70 a	40.00 ± 6.15 a	816.57 ± 21.41 a	4.69 ± 0.69 a
W1	30.85 ± 2.84 a	32.82 ± 2.27 b	804.78 ± 16.64 a	3.41 ± 0.61 a
W2	29.47 ± 2.67 b	36.98 ± 6.35 a	729.57 ± 32.01 a	4.26 ± 0.91 a
W3	31.91 ± 3.21 a	33.46 ± 1.11 b	667.13 ± 19.94 a	4.26 ± 1.49 a
C1W1	31.59 ± 3.21 a	34.42 ± 6.19 b	610.20 ± 30.55 a	4.37 ± 1.42 a
C1W2	33.74 ± 3.75 a	30.25 ± 1.61 b	514.17 ± 18.86 a	4.03 ± 0.16 a
C1W3	31.82 ± 3.19 a	26.09 ± 2.41 b	500.37 ± 17.95 a	3.81 ± 0.54 a
C2W1	31.03 ± 2.87 a	37.11 ± 7.54 a	616.56 ± 19.87 a	3.83 ± 0.78 a
C2W2	29.75 ± 0.48 b	38.27 ± 4.87 a	658.44 ± 26.01 a	4.96 ± 0.23 a
C2W3	29.92 ± 1.71 b	28.42 ± 8.92 b	549.99 ± 14.66 a	3.60 ± 0.63 a
C3W1	29.28 ± 2.67 b	29.48 ± 2.65 b	470.81 ± 13.61 a	6.36 ± 0.49 a
C3W2	28.02 ± 2.87 b	24.70 ± 5.85 b	461.90 ± 16.46 a	5.74 ± 0.83 a
C3W3	26.62 ± 2.89 b	22.79 ± 4.41 bc	394.19 ± 8.06 a	3.99 ± 0.93 a

* See Section 2.3 for the explanation of the treatment symbols. Different superscript letters indicate significant differences (p < 0.05) among mean values in each column, while the use of the same letter shows non-significance.

represents the concentrations of micronutrients such as Zn, Mn, Fe, and Cu in the plant tissues after the application of soil amendments. It was noticed that the concentrations of the aforementioned micronutrients deceased with the application of amendments. The control treatments showed the highest concentrations of all micronutrients (36.79, 45.21, 585.15, and 7.21 mg kg⁻¹ for Zn, Mn, Fe, and Cu, respectively.) among all tested treatments. In contrast, the lowest concentrations of micronutrients (26.62, 22.79, 394.19, and 3.99 mg kg⁻¹, for Zn, Mn, Fe, and Cu, respectively) were observed with the C3W3 treatment. A similar trend was observed with the concentrations of heavy metals in the plant tissues, which decreased with the addition of the soil amendments (

Table 5. Concentrations of available heavy metals (mg kg⁻¹) in the plant tissues after amending the soils with various clay and wheat straw combinations.

Treatments *	Cd	Co	Cr	Mo	Ni	Pb
CK	6.44 ± 1.01 a	15.84 ± 1.49 a	16.66 ± 0.46 a	0.06 ± 0.01 a	12.49 ± 1.32 a	8.71 ± 0.34 a
C1	5.58 ± 0.47 a	14.38 ± 3.01 a	13.60 ± 1.97 a	0.05 ± 0.01 a	8.11 ± 0.13 a	7.67 ± 0.62 a
C2	5.39 ± 1.07 a	14.15 ± 1.73 a	12.02 ± 0.56 a	0.05 ± 0.01 a	8.87 ± 0.91 a	8.03 ± 0.37 a
C3	5.32 ± 0.87 a	12.12 ± 1.60 a	12.34 ± 1.89 a	0.05 ± 0.01 a	4.77 ± 0.31 b	7.00 ± 0.82 a
W1	4.58 ± 0.07 a	13.38 ± 0.59 a	13.76 ± 1.50 a	0.05 ± 0.01 a	10.28 ± 1.91 a	8.00 ± 0.63 a
W2	4.46 ± 0.80 a	13.74 ± 1.47 a	11.33 ± 0.47 a	0.02 ± 0.01 a	9.01 ± 0.62 a	7.67 ± 0.89 a
W3	4.17 ± 0.85 a	11.22 ± 1.79 a	11.17 ± 1.66 a	0.05 ± 0.01 a	9.41 ± 0.90 a	7.33 ± 0.25 a
C1W1	4.44 ± 1.52 a	10.20 ± 0.52 a	11.57 ± 0.45 a	0.05 ± 0.01 a	8.31 ± 0.39 a	7.20 ± 0.33 a
C1W2	4.37 ± 1.16 a	9.80 ± 1.88 a	10.26 ± 1.75 a	0.03 ± 0.01 a	4.59 ± 0.99 b	5.87 ± 0.53 a
C1W3	4.06 ± 1.71 a	11.60 ± 1.71 a	11.27 ± 0.91 a	0.05 ± 0.01 a	7.78 ± 0.57 a	5.60 ± 0.49 a
C2W1	4.01 ± 0.59 a	10.91 ± 0.86 a	10.00 ± 0.84 a	0.05 ± 0.01 a	10.94 ± 1.11 a	4.97 ± 0.17 a
C2W2	3.99 ± 0.81 a	13.23 ± 0.36 a	8.67 ± 0.47 a	0.05 ± 0.01 a	12.52 ± 0.54 a	5.28 ± 0.15 a
C2W3	3.27 ± 0.57 b	13.09 ± 1.74 a	8.00 ± 0.82 a	0.02 ± 0.01 a	10.85 ± 0.45 a	4.97 ± 0.23 a
C3W1	3.50 ± 0.78 a	11.15 ± 1.78 a	8.00 ± 1.13 a	0.04 ± 0.01 a	13.73 ± 0.55 a	4.87 ± 0.26 a
C3W2	2.98 ± 1.16 a	9.67 ± 1.25 a	9.00 ± 0.82 a	0.05 ± 0.01 a	13.98 ± 0.65 a	4.79 ± 0.49 a
C3W3	2.53 ± 0.44 b	8.33 ± 1.49 a	8.48 ± 0.25 a	0.03 ± 0.01 a	9.50 ± 0.83 a	4.13 ± 0.33 a

* See Section 2.3 for the explanation of the treatment symbols. Different superscript letters indicate significant differences (p < 0.05) among mean values in each column, while the use of the same letter shows non-significance.

). In all treatments, the concentrations of As were not detectable. In contrast, the Cr and Co concentrations in the plants were larger as compared to the concentrations of the other heavy metals. The control treatment (CK) contained the maximum heavy metal concentrations (6.44, 15.84, 16.66, 0.06, 12.49, and 8.71 mg kg⁻¹ for Cd, Co, Cr, Mo, Ni, and Pb, respectively), whereas the lowest concentrations of these metals (2.53, 8.33, 8.48, 0.03, 9.50, and 4.13 mg kg⁻¹ for Cd, Co, Cr, Mo, Ni, and Pb, respectively) were recorded with the combined application of clay deposit and wheat straw (C3W3).

The results of the current study demonstrated that the combined application of natural clay deposits and wheat straw produced a significantly enhanced macronutrient concentration in the plant tissues, while the concentrations of micronutrients and heavy metals were reduced with these amendments as compared to the control treatment. The higher concentrations of N, P, and K in the plants could be due to the improved soil properties and nutrients availability as a result of a higher proportion of macro-aggregates formed with the application of clay deposits, which subsequently enhanced the nutrient uptake by the plant roots [37]. Moreover, similar results were found by Shen and Shen [50] who reported a significant increase in the K, Ca, and Mg concentration in mung bean leaves after the application of wheat straw; however, there were no significant impacts of wheat straw application on leaf Zn concentrations. The improvement in soil properties and nutrient availability after the clay and wheat straw application might have resulted in an enhanced nutrient uptake by the plants [42]. Contrarily, the reduction in micronutrient and heavy metal contents in the plants with applied amendments could be due to the higher sorption capacity of such amendments for those metals [43]. Therefore, the regular application of clay coupled with wheat straw of a fine particle size is effective for nutrient cycling, nutrient uptake by plants, and plant productivity enhancement.

4. Conclusions

A loamy sand soil was amended with natural clay deposits at 2.5%, 5.0%, and 10% (w/w) alone and in combination with wheat straw (1.0% w/w) of different particle sizes (<0.5, 0.5–1.0, and 1.0–2.0 mm). The variations in soil physio-chemical characteristics, water retention, and nutrient availability and concentration in the plants and the growth of Sudan grass were explored. All amendments were effective in improving soil physio-chemical properties, nutrient availability, water retention, plant nutrient concentrations, and plant growth. Among all tested treatments, the one involving a 10% clay addition along with 1% wheat straw demonstrated outstanding performance in improving soil properties, conserving more water, and enhancing Sudan grass production. These amendments resulted in the highest P and K availability in the soil (33% and 88% higher than untreated soil, respectively). The cumulative evaporation and infiltration were reduced by 22% and 300% with the treatment involving 10% clay coupled with wheat straw application. In addition, an 82% enhancement in plant height, as well as a 3-fold increase in N, 2-fold increase in P, and 3.5-fold increase in K concentration in the plant tissues were observed with the combined application of clay deposit and wheat straw. Thus, we concluded that the application of natural clay deposits in combination with wheat straw of a fine particle size (<0.5 mm) could be an economical, environmentally friendly, and efficient technique to improve soil characteristics, water conservation, nutrient availability, and plant growth in sandy soils.