Effect of Surface and Subsurface Drip Irrigation with Treated Wastewater on Soil and Water Productivity of Okra (Abemoschus esculentus) Crop in Semi-Arid Region of Tunisia

Abstract

Under semi-arid conditions, irrigated agriculture faces hard competition for water. It is against this backdrop that appropriate management of irrigation techniques and water resources becomes a major concern. This study investigated the effect of surface (SDI) and subsurface drip irrigation (SSDI) with domestic treated wastewater (TWW) and fresh water (FW) on soil water dynamics, salinity, yield, and mineral nutrition of okra. The experimental design was set-up based on two adjacent plots according to the water quality: Fresh Water (FW) T₁ and domestic Treated Wastewater (TWW) T₂. Results showed that measured soil water contents (SWCs), under TWW treatment (T₂), were greater than their corresponding measurements under FW (T₁), and in particular at 35 cm depth. Meanwhile, for both water qualities, soil Electrical Conductivity (EC) registered at 5 cm depth was higher than those measured at 35 cm, with values ranging from 0.14 to 0.36 mS·cm⁻¹ and from 0.20 to 0.47 mS·cm^−m for T₁ and T₂, respectively. Regarding crop yield, a statistically significant increase (p = 0.05) in okra fresh yield was observed when TWW was used. Fresh yield in SDI was 2.55 t·ha⁻¹ and 3.9 t·ha⁻¹ in T₁ and T₂, respectively. Nevertheless, results indicated that lateral depth did not significantly affect okra fresh yield. Moreover, a significant higher irrigation water productivity (WP_irrig) with TWW (1.08 ± 0.26 and 1.23 ± 0.18 kg m⁻¹) was observed, which was nearly double those obtained with FW (0.72 ± 0.33 to 0.78 ± 0.18 kg m⁻¹). Appropriate use of SSDI with TWW stands as an irrigation management technique to improve yield and irrigation water productivity of okra crops.

1. Introduction

Water scarcity is one of the key constraints for agricultural activity in arid and semi-arid regions, as in the case of Tunisia. High competition for water due to population growth and the development of the socio-economic sectors, combined with climate changes and marked by the increase in temperatures and the irregularity of rainfall, have led countries in these regions to turn towards the exploitation of lower-quality irrigation resources. Among these resources, treated wastewater (TWW) is used by farmers and is perceived as a cheap alternative that could reduce pressure on groundwater [1,2,3] under some conditions related to the nature of the crop used and the quality of the water processed. According to [4], the reuse of TWW in irrigation has experienced significant success worldwide over the past 30 years. In Tunisia, the use of TWW in irrigation is an old practice that began in 1965 to safeguard the citrus-growing perimeter of Charguia after the salinization of groundwater. [5,6]. Then, with the creation of the National Sanitation Office (ONAS) in 1974, the number of wastewater treatment plants and the volume of treated water have continually increased. Since 1996 in Tunisia, the reuse of TWW has been an integral part of the strategy for mobilizing the country’s water resources. According to the latest ONAS publications, there are currently 122 steps, including 61 steps directly concerned with water reuse in the agriculture sector. The total volume of TWW produced in 2019 was 284 Mm³, of which 62 Mm³ were reused to irrigate 2734 ha among the 8435 ha actually devoted to this purpose [7]. Further efforts have been made to improve the quality and availability of wastewater, as well as to propose other irrigation techniques and practices, promoting its reuse in irrigation as mandatory. According to [1], it is necessary to integrate appropriate irrigation management and strategies to resolve certain problems linked to the reuse of TWW in agriculture. Drip irrigation remains the safest technic for using TWW in crop production [8,9], as it allows the application of irrigation water directly into the root zone. Similarly, [10,11,12] micro-irrigation has been considered as a solution to conserve water and protect users.

Surface drip irrigation (SDI) systems have become increasingly popular and are considered a solution to conserve water [10,11,13]. However, a few studies [14,15,16] have focused on subsurface drip irrigation (SSDI) using TWWs. It has been demonstrated to be an appropriate tool to improve yields and water use efficiency in comparison with other irrigation systems [17,18]. In addition, it limits weed germination and growth [16]. According to [19,20], these latter advantages are attributed to the better soil moisture distribution and the limiting of salt accumulation in the root zone [17]. Some operational parameters, such as the emitter discharge rate and spacing, the duration and frequency of irrigation, and the placement of the drip laterals [13,21] are very important factors that significantly influence the soil moisture and salt distribution in the soil. In this context, [18] reported that selecting drip tape installation depths for various crops is the most critical decision. Hence, appropriate depths of the emitters and deficit irrigation are important in terms of modeling wetted areas and enhancing water use efficiency [22]. In addition, the quality of irrigation water under drip irrigation has a considerable effect on the distribution of water content, salinity, and root development [23].

Moreover, the choice of irrigation systems and the reuse of non-conventional water sources can be made more efficient with an appropriate choice of plants to grow. Okra (Abelmoschus esculentus L.) is classified as a moderately salt-tolerant plant [24]. In fact, okra is widely cultivated in several regions around the world. It is characterized by a wealth of nutrients beneficial to human health, including potassium, carbohydrates, dietary fibers, unsaturated fatty acids, calcium, and vitamins [25,26]. Limited studies have been devoted to assess the performance of SSDI using treated wastewater on SWCs and salts distribution in loamy-sand soil. Therefore, the aims of this study were: (1) to assess the combined effect of water quality and lateral depth on okra production, as well as the sodium content (Na⁺) and potassium content (K⁺) in different parts of the plant; and (2) to evaluate the effect of treated wastewater on soil water content (SWC) and soil salinity in the root zone, between 5 and 35 cm depth, and particularly at 15 cm lateral depth.

2. Materials and Methods

2.1. Site Description and Experimental Setup

Field experiments were carried out in 2018, at the agricultural experimental unit of the National Institute for Research in Rural Engineering, Water and Forestery (INRGREF) of Oued Souhil, Nabeul, Tunisia (long. 32°37′3′′ N; lat.10°42′22′′ E, altitude 25 m a.s.l.). The area is characterized by a Mediterranean semi-arid climate with a mild winter. The average annual precipitation is about 440 mm and the reference evapotranspiration (ET₀), computed using the FAO-56 PM method, is around 1350 mm.

Some physical and chemical properties of the experimental field soil are summarized in

Table 1. Physical and chemical properties of the experimental soil.

Soil Depth	Sand	Clay	Loam	Bd 1	pH	OC 2	Humus	ECs 3	Mn	Ni	Zn	Fe	Pb
(cm)	%			(g·cm−3)		%		mS·cm−1	(mg·Kg−1)
0–10	79	7.1	13.9	1.40	8.1	0.70	1.4	1.46	106.3	5.5	69.1	14,007	15.8
10–20	76.7	7.5	15.8	1.45	8.0	0.60	1.3	1.24	108.3	7.9	69.1	15,483	7.8
20–30	79.3	7.0	13.7	1.4	8.2	0.60	1.1	1.12	115.9	7.8	78.7	15,900	13.7
30–40	75.1	8.2	16.7	1.57	8.3	0.60	1.1	1.21	126.2	4.9	64.4	17,373	10.8
40–50	73.4	8.5	18.1	1.52	8.4	0.53	<1.1	1.15	95.5	8.1	51.3	13,512	15.9
50–60	77	7.4	15.6	1.50	8.4	0.53	<1.1	1.39	70.4	3.4	19.8	12,555	8.3
Permissible limit 4									2000	50	300	-	100

¹ Bulk density; ² organic carbon; ³ soil electrical conductivity, ⁴ European Union Standards, 2000.

. According to USDA classification, the soil texture is loamy sand. Mean bulk density is 1.47 g·cm⁻³. Mean water content at field capacity and permanent wilting point are equal to 0.20 m³·m⁻³ and 0.09 m³·m⁻³, respectively. The soil is moderately alkaline with a pH value close to 8. The Soil ECs values range from 1.46 to 1.12 dS·m⁻¹. ECs was higher in the top soil layer (0–10 cm) and below 50 cm depth. The soil content of Mn, Ni, Zn, Fe, and Pb in experimental soil was below European Union Standards, 2000.

The okra crop (Abelmoschus esculentus L.) was cultivated on 13 May 2018, at a density of 35 kg·ha⁻¹. Plants were spaced at 30 cm along the rows and 95 cm between the rows. Experimental soil was periodically tilled to control weeds. To ensure optimal growth of the okra, a foliar fertilizer, floriatal N30, was applied at the beginning of the development stage (6–7 leaves) at a rate of 2 L·h⁻¹. To establish the potential performance of the crop, two treatments with pesticides were applied. The picking of okra pods started at 55 DAS and three pickings were made per week. The experimental field was divided into two 11-m long and 5-m large plots. Two adjacent experimental plots were defined according to water quality: Fresh Water (FW) T₁ and domestic Treated Wastewater (TWW) T₂. The first plot (T₁) was irrigated using fresh water (FW) pumped from a deep well located adjacent to the INRGREF experimental unit. The second plot (T₂) was irrigated with Treated Wastewater (TWW) supplied by the Nabeul wastewater treatment plant “SE4”, located approximately 6 km from the experimental site. The TWW was mainly domestic and treated with up to a secondary biological treatment. At the INRGREF experimental unit, TWW was stored in a geo-membrane basin with a capacity of 500 m³ to undergo additional treatment (decantation and filtration) before use, to avoid dripper clogging.

According to the drip line’s depth (0, 5, 15, and 25 cm), the main plots for each water quality, T₁ and T₂, were divided into four individual sub-plots (Figure 1), each designated to one depth. In all, eight treatments were used: T_1-0, T_1-5, T_1-15, T_1-25 and T_2-0, T_2-5, T_2-15, T_2-25 for FW and TWW, respectively. Each plant row was equipped with a drip line with one emitter per plant and were spaced 35 cm apart. For T₂, drippers were characterized by a flow rate equal to 3 L h⁻¹. However, for T1, dripper discharge was equal to 1.5 L h⁻¹. A micro-valve was installed on each lateral to control water supply.

Daily meteorological data relative to minimum and maximum air temperature, relative humidity, wind speed, and solar radiation were collected from a local weather station in order to estimate daily reference evapotranspiration using the standard FAO-56 Penman-Monteith model. Crop evapotranspiration was determined using the FAO-crop coefficient approach, which consists of multiplying ET0 by a pre-determined crop-specific coefficient K_c [27].

$$E{T}_C\left(mm\right)= \mathrm{ET}0\times \mathrm{Kc}$$

(1)

K_c values of okra crop are those proposed by [28,29] (

Table 2. Okra crop coefficients Kc and stage development length.

	Crop Development Stages
	Initial	Development	Mid-Season	Final Season
Period length (d)	10	31	25	20
Kc	0.2	0.2–1	1	0.9

).

For both treatments, irrigation water requirements (I) were determined at weekly time steps using the following equation:

$$\mathrm{I} \left(mm\right)={ \mathrm{ET}}_C-R_e$$

(2)

where ET_c: crop evapotranspiration (mm); R_e: Effective precipitation (mm). At sowing, initial soil water content was equal to zero.

In order to avoid deep percolation, irrigation depths were split equally into three applications from the beginning of the crop cycle (13 May) until harvesting (8 September), for a total of 42 irrigation events. This irrigation scheduling allowed for the replenishment of the soil to field capacity.

2.2. Field Measurements

For each water quality (FW and TWW), samples were taken during okra irrigation and collected in polyethylene bottles. The pH and electrical conductivity were measured using a conductivity meter. Dry residue of water samples was assessed by oven drying at 105 °C until at constant weight. The dry residue (g·L⁻¹) was measured as residue weight resulting from the evaporation of one liter of water. The sodium adsorption ratio (SAR) determines the level of crop damage. SAR was calculated considering the square root of the sodium (Na⁺) to calcium plus magnesium (Mg²⁺ Ca²⁺) divided by two, as cited by [30]. Samples were divided into two parts. The first consisted of mineral analysis (sodium (Na⁺), calcium (Ca²⁺), magnesium (Mg²⁺), chloride (Cl⁻), bicarbonates (HCO₃⁻), sulfate (SO₄²⁻); kept in the dark at less than 4 °C. The second part consisted of adding some drops of nitric acid to obtain the determination of the element trace metallic, such as cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), nickel (Ni), iron (Fe), Zinc (Zn), and lead (Pb). For TWW, additional analysis was performed for ammonia nitrogen (N-NH⁴⁺), nitrate nitrogen (N-NO³⁻), and phosphorus (P).

The dynamic of soil water content (SWC) and soil electrical conductivity (EC) were monitored using capacitive 5TE sensors (METRE Group, Inc., Pullman, WA, USA) after a preliminary calibration. SWC measurements were performed at daily time steps, starting from 24 July, from the T_1-15 and T_2-15 sub-plots. In each sub-plot, two sensors were installed vertically at 5 and 35 cm depth from the soil surface.

For each sub-plot, a total of 12 Okra plants were labeled in order to assess the total fruit yield per plant. Okra yield was measured after every harvest using an electric weighing machine. At final harvest, the labeled plants were separated into roots, shoots, and fruits, and washed with de-ionized water. These different sections were placed in a forced-air circulation oven and dried at 70 °C to a constant weight. After drying, samples were ground and then subjected to acid digestion using a mixture of acids (HClO₄, HNO₃). Then, exchangeable Na⁺ and K⁺ were analyzed by means of flame photometry (PFP7/C Research flame photometer, Jenway, Stone, UK). Total soil heavy metals were extracted by HF-HClO₄ mixture, and were analyzed by flame atomic absorption spectrophotometer (PFP7/C Research flame photometer, Jenway, Stone, UK)

Irrigation water productivity ($W{P}_{irrig})$is determined as the ratio between total harvestable fresh yield (Y) and the total irrigation water (I) supplied from planting to harvest [31].

$$W{P}_{irrig}\left(\mathrm{Kg}.{ \mathrm{m}}^{-3}\right)=\frac{Y}{I}$$

(3)

where Y: total harvestable fresh yield (Kg ha⁻¹); I: total irrigation water (m³·ha⁻¹).

2.3. Statistical Analysis

Data related to Na⁺, K⁺ content in roots, shoots, and fruits, yield and WP_irrig were subjected to two-way ANOVA (irrigation water quality, lateral depths) using IBM SPSS.16 software. Duncan’s Multiple Range Test was used for comparing means estimated at (p = 0.05).

3. Results and Discussion

3.1. Irrigation Water Quality Assessment

The physicochemical quality of TWW and FW used in this study are reported in

Table 3. Chemical properties of TWW and FW.

	Treated Wastewater		Fresh Water	NT 106.03
pH	7.25 ± 0.21		7.4	6.5–8.5
CE (mS/cm)	3.35 ± 0.22		2.77	7
SAR	8.06 ± 0.20		5.09	-
Dry residue (g/L)	2.17 ± 0.26	mg/L	1.94	-
N-NO3−	<5		-	-
N-NH4+	38.70 ± 3.82		-	-
P	5.05 ± 1.68		-	-
K+	42.71 ± 11.06		8.4	-
Na+	487.6 ± 54.53		325	-
Cl−	666.2 ± 93.51		518.3	2000
Ca2+	126.3 ± 5.42		178	-
Mg2+	88.4 ± 7.09		78	-
HCO3−	333.1 ± 34.39		579.5	-
SO42−	490.6 ± 158.4		288	-
Cd	0.0072 ± 0.0032		-	0.01
Co	0.0114 ± 0.0099		-	0.1
Cr	0.0312 ± 0.0302		-	0.10
Cu	0.0084 ± 0.0054		-	0.50
Fe	0.2348 ± 0.1370		0.03	5
Mn	0.0120 ± 0.0140		0.08	0.50
Ni	0.0137 ± 0.0121		-	0.20
Pb	0.0349 ± 0.0142		-	1
Zn	0.0144 ± 0.0090		0.03	5

. For both water qualities, pH means were classified as slightly basic with values equal to 7.25 and 7.4 for TWW and FW, respectively. These values fell within the range of Tunisian Standards NT 106.03 (6.5–6.8). The EC value for TWW (3.35 mS·cm⁻¹) was higher than that recorded for FW quality (2.77 mS·cm⁻¹). The EC value relative to TWW was considered slightly higher than the upper limit set for irrigation use (3 mS·cm⁻¹). The EC indicated a high and a very high salinization risk for TWW and FW; while the SAR indicated a low sodicity risk. According to the classification by [32], this result indicated that both TWW and FW fell in the C4-S1 category and were suitable for irrigation in almost all soil and crop types. Dry residue values in TWW (2.17 g·L⁻¹) were slightly higher than those found in FW (1.94 g·L⁻¹). Based on water quality classification relative to its potential for drip emitter clogging [33], FW and TWW presented moderate hazard ratings. Moreover, chloride (Cl⁻) and sodium (Na⁺) ion contents in TWW were relatively higher than those measured in FW. Concerning metallic trace elements, the values were below Tunisian standards for agricultural use.

3.2. Agro-Meteorological Characterization of the Study Area

Patterns of daily average temperature, reference evapotranspiration (ET0), and precipitation (P) for the entire growing period are shown in Figure 2. As can be seen, T_avg and ET0 follow similar trends over the growing season. Daily T_avg increases from approximately 18 °C at the beginning of the growth season to 29.5 °C in mid-July (63 DAS). Then, T_avg values present a general tendency to decrease. Regarding precipitation, limited events occur during the entire crop season, with a total amount equal to 87.6 mm.

The total irrigation amount applied during the crop growth cycle and total crop water requirements were equal to 365.4 mm and 401.4 mm, respectively (

Table 4. Total applied water and crop evapotranspiration (mm).

Treatment	ETc (mm)	P (mm)	I (mm)
T1
T1-0	401.4	87.6	365.4
T1-5	401.4	87.6	365.4
T1-15	401.4	87.6	365.4
T1-25	401.4	87.6	365.4
T2
T2-0	401.4	87.6	365.4
T2-5	401.4	87.6	365.4
T2-15	401.4	87.6	365.4
T2-25	401.4	87.6	365.4

ETc: Okra crop evapotranspiration. P: Precipitation. I: Irrigation.

). Based on the daily evolution of cumulative okra crop evapotranspiration ETc and amounts of water supply (I + P) (Figure 3), it can be concluded that okra plants were grown under optimum water conditions from sowing till harvest. Starting from 86 DAS, the total amount of supplied water (I + P) largely exceeded ETc as a result of the particular heavy precipitation event that occurred.

3.3. Soil Water and Electrical Conductivity Monitoring

Daily patterns and magnitudes of the soil water content measurements at 5 and 35 cm depth under T_1-15 and T_2-15 treatments are shown in Figure 4.

For both treatments, SWCs recorded at a 5 cm depth followed the same trend as those recorded at 35 cm. As can be observed under T₁, except for fewer overlaps, SWCs measurements at 5 cm are relatively higher than their corresponding measurements at 35 cm. During the considered period, both patterns (under T₁) tend to decrease with local peaks following a watering or precipitation event. In addition, the daily SWCs were not much different from the field capacity value for the root zone, insuring optimum water conditions for the okra crop during the whole growing period. Contrary to T₁, SWCs recorded at 35 cm were higher than those recorded at 5 cm, with an average value of 0.22 ± 0.021 and 0.17 ± 0.015 (cm⁻³ cm⁻³), respectively. Furthermore, the average SWCs under Treatment T₂ seemed to be greater than under T₁, particularly at a 35 cm depth where values were higher than field capacity. As expected, the high rates of chemical elements and suspended matter (SM) in TWW resulted in an obstruction of soil micro-pores [34] and impeded deep percolation. Therefore, it promotes waterlogging and subsequently higher water content in the deep layer. These results are in good agreement with those obtained by [35,36], who found that the water infiltration rate decreased with increasing water SM load. In addition, their results evidenced that clogging occurred on the soil surface when fine textured soils were considered. However, this behavior was observed in depths of sandy soils, which corresponds to our case study.

Figure 5 shows the daily measurements of soil bulk electrical conductivity (mS cm⁻¹) for both sub-treatments, T_1-15 and T_2-15, at 5 and 35 cm depths. As can be seen, EC values recorded for T₂ were higher than their corresponding measurements for T₁. Similar to the SWC curves, different peaks occurred after watering or precipitation events were detected.

EC values, measured at 5 cm depth, vary from 0.14 to 0.36 mS.cm⁻¹ and from 0.20 to 0.47 mS cm⁻¹ for T1 and T2, respectively. However, at 35 cm depth, values ranged from 0.11 to 0.16 and from 0.15 and 0.44 mS cm⁻¹ for T₁ and T₂, respectively. Thus, for both water qualities, EC registered at 5 cm depths were greater than those at 35 cm depths. These patterns are relatively different from those observed for SWCs, in particular those under TWW treatment. These results could be explained by the fact that the top layer is more subjected to water evaporation, inducing an ascension of water through a capillarity rise, leading to salt accumulation near the soil surface. In this context, [37,38] stated that salinity at the top layer increased as the depth of the drippers increased. Furthermore, [39] indicates that the highest EC values were in the top 3 cm in both cases of tape placement at 18 and 25 cm in loamy sand soil conditions.

3.4. Okra Response to Water

The total irrigation amount supplied during the okra growing season, total yield, and WP_irrig are summarized in

Table 5. Yield and WP_irrig of okra crop under different treatments.

Treatment	P (mm)	I (mm)	Yield (t ha−1)	WPirrig (Kg m−3)
T1
T1-0	87.6	365.4	2.55 ± 0.93 a	0.72 ± 0.33
T1-5	87.6	365.4	2.7 ± 0.37 a	0.75 ± 0.23
T1-15	87.6	365.4	2.6 ± 0.46 a	0.73 ± 0.30
T1-25	87.6	365.4	2.8 ± 0.22 a	0.78 ± 0.18
T2
T2-0	87.6	365.4	3.9 ± 0.34 b	1.10 ± 0.26
T2-5	87.6	365.4	4.4 ± 0.22 b	1.23 ± 0.18
T2-15	87.6	365.4	4.1 ± 0.09 b	1.16 ± 0.23
T2-25	87.6	365.4	3.9 ± 0.6 b	1.08 ± 0.26

(P = precipitation; I = irrigation; WP_irrig = irrigation water productivity). Means in the same row followed by the same letters are not significantly different at p < 0.05, according to Duncan multiple range test (DMRT).

.

As can be observed, total fresh yield was significantly affected by irrigation water quality. For treatment T₁, total fresh yield varied from 2.55 ± 0.93 to 2.8 ± 0.22 t ha⁻¹. However, higher yield values were recorded under treatment T₂, which varied from 3.9 ± 0.34 to 4.4 ± 0.22 t ha⁻¹. These results might be attributed to the high concentration of nutrient elements (nitrogen, potassium, calcium, magnesium, and phosphorus) in TWW compared to FW [26,40,41]. When separately considering both treatments, detailed analysis evidenced that total fresh yields of the okra crop were not significantly affected by lateral depth. In this context, [42] revealed that the maximum root density of okra is located in the top 0–20 cm soil layer. In addition, they highlighted that optimal crop yield and irrigation efficiency could be reached with a dripper depth not exceeding 20 cm. Regardless of lateral position, higher WP_irrig values were achieved under TWW compared to those recorded under treatment T₁. WP_irrig values varied from 0.72 ± 0.33 to 0.78 ± 0.18 kg m⁻³ for T₁, and between 1.08 ± 0.26 and 1.23 ± 0.18 kg m⁻³ for T2. These results were considered below those achieved by [43]. The authors found that the WP of okra crops can reach 2.84 ± 0.1 kg m⁻³ under conventional furrow irrigation (CFI) and 5.29 ± 0.1 kg m⁻³ under alternate furrow irrigation (AFI). This observed difference might be attributed to the okra variety used, which resulted in lower yield, and in the difference in the adopted technical package.

3.5. Sodium and Potassium Concentration in Okra Crop

Effects of irrigation water quality and lateral position on Na⁺ and K⁺ concentration, as a percentage of dry matter (% DM), are presented in

Table 6. Na⁺ and K⁺ concentration in roots, shoots, and fruits under different treatments (% DM).

Emitter Depth (cm)	Na+ Contents						K+ Contents
	Roots		Shoots		Fruits		Roots		Shoots		Fruits
	T1	T2	T1	T2	T1	T2	T1	T2	T1	T2	T1	T2
0	1.37 bc	1.11 a	0.22 a	0.41 cd	0.74 ab	0.64 a	1.79 cd	1.72 c	2.87 a	3.09 b	4.03 a	4.13 bc
5	1.19 ab	1.41 c	0.27 ab	0.49 d	0.62 a	1.52 a	1.40 b	1.65 c	2.99 ab	3.21 c	3.43 a	4.17 bc
15	1.19 ab	1.89 e	0.39 c	0.44 cd	0.63 a	1.14 c	1.06 a	2.03 d	3.06 ab	3.03 ab	3.93 ab	4.13 bc
25	1.44 c	1.73 d	0.31 bc	0.56 e	0.75 b	0.84 a	1.18 a	2.51 e	3.09 b	3.02 ab	4.10 a	4.27 c
average	1.30	1.54	0.30	0.48	0.69	1.04	1.36	1.98	3.00	3.09	3.87	3.89

Mean value of ± standard deviation of yield three replicate (n = 3) with same letters are not significantly different according to Duncan multiple range test (DMRT) at significance level 5% (p < 0.05). DM = Dry matter.

. Results revealed that irrigation with TWW resulted in higher Na⁺ and K⁺ content in different okra plant organs (fruits, roots, and shoots) compared to those recorded with FW irrigation. Relatively similar results were obtained by [44,45]. The authors reported that irrigation with TWW significantly increased Na⁺ and K⁺ content in plant fruits compared to those irrigated with fresh water.

The highest contents of Na⁺ were observed in plant roots with average values equal to 1.3 and 1.54% DM for T₁ and T₂, respectively. Meanwhile, the highest amounts of K⁺ were recorded in okra fruits, then followed by shoot and root sections. These results highlighted that roots act as a barrier against the transition of Na⁺ to the aerial region, while fruits act as storage for K⁺. For both treatments, shoots were characterized by the lowest Na⁺ content, with average values of 0.3 and 0.48% DM for T₁ and T₂, respectively. These low Na⁺ levels might be explained by the fact that shoots act as organs of transition for the solutes from the roots to the fruits. Therefore, they are subjected to continuous changes which impede the effect of water quality or the irrigation system. In fact, Na⁺ content increased in the root zone with increasing dripper depth when TWW was used. The values increased from 1.11 to 1.89% DM for T_2-0 and T_2-15, respectively. Regarding K⁺ content, higher values were recorded in plant fruits and shoots compared to the roots. Obtained values ranged from 3.34 to 4.27% DM in the fruits, and varied from 2.8 to 3.21% DM in the shoots. These values were in the same order of magnitude as those obtained by [46,47] for okra crop fruits. Independent of irrigation water quality, the values of Na⁺ and K⁺ recorded in the different parts of the plant suggested that the Na/K pump in the roots acts in favor of K. Indeed, higher values of K⁺ were found in the fruits and the shoots than in the roots.

4. Conclusions

Under semi-arid and water scarcity conditions, improving water management practices in agriculture is widely recommended. The appropriate use of SDI and SSDI techniques and non-conventional water resources stand as useful options to save and better valorize water. The results of this study evidence that adopting subsurface drip irrigation with TWW, as an alternative to water saving through irrigation, allows for better retention of soil water contents, particularly in the root zone. Based on electrical conductivity measurements, salt concentrations tend to increase near the soil surface than in dipper layers. In the investigated soil, using both irrigation water qualities resulted in significant effects on okra crop yield and WP_irrig. In fact, the nutrient-rich TWWs significantly improved the okra yield and the WP_irrig. However, for both water qualities, the SDI and SSDI systems did not significantly affect okra crop yield and WP_irrig. In terms of Na⁺ and K⁺ concentrations, our findings showed that irrigation with TWW resulted in higher Na⁺ and K⁺ absorption by okra compared to fresh water. This result was closely related to the TWW composition characterized by high Na⁺ and K⁺ elements.

Regardless of the lateral depth, the highest Na⁺ content was noted in the roots acting as barriers preventing passage to the aerial part. This behavior was related to the intrinsic physiological specificity of okra crops. At the same time, a significant amount of K⁺ was recorded in okra fruits, which are reputed to be very rich in potassium nutritional value. Considering the limited studies on okra crops, and beyond the regional implications of this study, our findings could be considered as a contribution to the irrigation management of okra crops under local conditions in terms of water quality and irrigation systems. Further field trials are needed to assess the impact of the lateral depth on crop response to water, soil water dynamics, and the mineral nutrition of okra crops. Similarly, the long-term effects of TWW on soil should be assessed in the future.