Sustainable Strategies Based on Reused Leachates and Hydrogen Peroxide Supply to Fertigate Cordyline fruticosa var. ‘Red Edge’ Plants

Abstract

Leachate reuse is a helpful tool that contributes to the sustainability of agricultural systems, but it requires previous disinfection. Hydrogen peroxide can be found among the disinfectants frequently applied in ecological production systems. Moreover, it can improve the oxygenation of the root system. The objective of this work was to study its effect on C. fruticosa plants fertigated with leachates. A split-plot design with six treatments, three without an H₂O₂ supply (S₀) and three with an H₂O₂ supply dosage at 2% (S_H2O2), was arranged: raw leachate from C. lanatus (L₁₀₀), raw leachate from C. lanatus diluted with tap water until EC of 2.5 dS m⁻¹ (L_WD), and raw leachate from C. lanatus diluted with standard nutrient solution until EC of 2.5 dS m⁻¹ (L_NSD). The results produced data about the evolution of the nutrient and leachate solutions throughout the cultivation period. Morphological (height, leaf number, leaf area, total fresh weight, relative water status, and dry weight) and physiological (chlorophyll a, chlorophyll b, carotenoids, chlorophyll a+b, and proline) parameters were studied to reveal the plant response. The efficiency of nutrient utilization was higher with the L_WD treatment, and water and nitrogen utilization efficiency decreased under the H₂O₂ supply. In conclusion, the reuse of diluted leachate is advised for nutritionally undemanding crops, such as C. fruticosa; moreover, the H₂O₂ supply improved tolerance to salinity and enhanced root growth and Red-Green-Blue (RGB) values.

1. Introduction

Currently, three objectives are related to sustainability in intensive production: (1) to cultivate without deteriorating the agricultural environment, (2) to get rid of the food insecurity generated by the contamination of products, and (3) to obtain a production that allows owners of small and medium-size farms to have a decent income in disadvantaged areas [1].

In the province of Almería (located in the southeast of Spain), there are 30,600 ha of protected crops [2], of which 2000 ha are occupied by biological and ecological crops. The progressive increase of organic farming during the last 10 years has allowed producers to obtain a higher yield free of pollutants and food of high nutritional and organoleptic value, thanks to an environmentally friendly production system based on biological interactions and beneficial natural processes [3].

In this area, the reuse of leachates is an advantageous technique for saving water, allowing a more rational utilization of resources and contributing to the sustainability of the intensive production system in the Mediterranean area [4,5,6]. Nevertheless, more research on the disinfection of leachates and the effects of disinfectants (which prevent the development of pathogens) on crops is needed [4,7].

The disinfection of effluents can be achieved by a variety of methods, mostly using chlorine, hypochlorite, chlorine dioxide, ozone, and UV-light; other means employed to a lesser extent are treatments by chloramines, bromine, bromine chloride, iodine, hydrogen peroxide, ozone, metal ions, heat, ultrasound, electrostatic processes, gamma- and X-rays, etc. All these methods were developed primarily for their application with potable water [5,8].

Nevertheless, the need of achieving greater efficiency in production systems compels us to focus this research on aspects that may limit production, such as those generated by deficient aeration in the rhizosphere, which causes serious problems in plants due to radical hypoxia and can be avoided by utilizing oxygen releasers [6,9]. In the presence of hydrogen peroxide, a greater absorption of oxygen takes place, as has been demonstrated with horseradish [10,11].

Peroxides, which have been widely used for more than 50 years in the pharmaceutical and food industry, hospitals, agriculture, wastewater treatment, water purification, etc., act as oxidants. Peroxides have a proven disinfectant activity against all types of microorganisms (bacteria, viruses, protozoa, coccidia, algae, fungi, and spores), causing bacterial proteins to lose their functionality and attacking the cell membrane, DNA, and other components, causing cell death [12,13,14]. The decomposition products of peroxides are water and oxygen, so they are highly biodegradable products that leave no residue of any kind [15,16]. Hydrogen peroxide has the additional advantage of releasing oxygen in the disinfection process, and its use is allowed in biological agriculture in Europe [1].

In terms of water and nutrient requirements, leachates from horticultural crops can be reused for the fertigation of other tolerant crops, thereby achieving a more sustainable production system for growers by reducing the inputs of water and fertilizers [9,10]. The aim of this work was thus to study the effect of oxifertigation (H₂O₂ supply) on Cordyline fruticosa var. ‘Red Edge’ plants fertigated with pure or diluted leachates, as well as with a standard nutrient solution, in a soilless culture of rock wool of Citrullus lanatus in south-eastern Spain. To this point, there is a dearth of research on the addition of H₂O₂ in fertigation in terms of plant growth, a lack that this study aims at remedying.

2. Materials and Methods

2.1. Greenhouse Experiment

The trial was performed in two greenhouses over a 2-month period from April to June. The first site was a commercial plastic greenhouse located on a farm called “Coto Espinosa” in La Cañada de San Urbano, Almería, Spain (36°89′ N; 2°36′ W), with C. lanatus Thunb. (watermelon) plants cultivated in a rockwool soilless system, where leachates were collected from 4 collection trays (each one with 2 slabs). The trays were placed randomly in the greenhouse, and the leachates were mixed and analyzed weekly during the cultivation cycle. The second site was a tunnel greenhouse located at the University of Almería, Spain, (36°83′ N; 2°41′ W), where C. fruticosa L. var. ‘Red Edge’ plants were cultivated with peat moss and perlite (3:1 v/v) in 10 cm pots. The average daily temperature was 22.15 °C, with a relative humidity (RH) 70.43% and photosynthetically active radiation (PAR) 2.74 mol m⁻² d⁻¹.

2.2. Treatments and Experimental Design

The experimental design was a split plot (3 × 2 factorial). The different factors assessed were as follows: the type of fertigation treatment (L₁₀₀: raw leachate from C. lanatus, L_WD: raw leachate from C. lanatus diluted with tap water until EC of 2.5 dS m⁻¹; L_NSD: raw leachate from C. lanatus diluted with standard nutrient solution until EC of 2.5 dS m⁻¹) and levels of supplies of H₂O₂ as OX-VIRIN in the leachate (without H₂O₂ supplied to the leachate and with H₂O₂ supplied to the leachate at 2% (v/v)) resulting in a total of 6 treatments (Figure 1) with 4 randomized complete blocks, and 3 plants per block, giving, plus border plants, a total of 94 plants. The threshold of EC of 2.5 dS m⁻¹ during the cultivation cycle was the most suitable for the growth of this species [17]. Irrigation frequency depended on crop water requirements, with an average daily dose of 20 mL per plant. Table 1 shows the chemical composition of the tap water, the ranges of nutrient solution supplied to watermelon, and the nutrient solution supplied to C. fruticosa.

2.3. Nutrient Solutions Analysis

The parameters determined in the nutrient solutions and leachates tested were pH, EC, NO₃⁻, SO₄²⁻, H₂PO₄⁻, Cl⁻, Ca^2+, Mg²⁺, K⁺ and Na⁺. Samples were collected weekly. The values of pH and EC were measured with a Crison MicropH 2001 pH-meter and with a Crison Micro CM 2200 conductivity meter (Crison, Barcelona, Spain). Nitrate was analyzed according to the colorimetric determination by nitration of salicylic acid [18]. Sulphate was determined following the method proposed by Novozamsky and Eck [19]. Phosphate was determined by the vanadomolybdophosphoric colorimetric method [20], using a Helios Gamma spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA). Chloride was analyzed following the method proposed by Kolthoff and Kuroda [21]. Total Ca²⁺ and Mg²⁺ were determined by atomic absorption spectrophotometry [22] utilizing a 3300 atomic absorption spectrophotometer (PerkinElmer Waltham, MA, USA). The total K⁺ and Na⁺ were directly measured by flame spectrophotometry [23] utilizing a flame photometer (Evans Electro Selenium LTB, Halstead, Essex, UK).

2.4. Plant Morphological Parameters

Four random plants per treatment were sampled at 60 days after the beginning of the trial. Plant height (H) was measured with a ruler from the base of the plant to the tip. Leaf area (LA) was measured at the end of the experiment, using an area integrator ΔT Area Meter MK2 (Delta-T Devices, Burwell, UK). The number of leaves (LN) was obtained through manual counting.

Afterwards, the plant material was washed [24] and separated into absorption organs (roots), conductive organs (stems and petioles) and photosynthetic organs (leaves). The different fresh vegetal fractions were weighted (FW) and dried in a forced-air oven at 70 °C for 48 h and weighed on a Mettler Toledo PB-303-S scale (Columbus, OH, USA) to obtain the respective dry weight (DW). The relative water status (RWS) (g of water/g of dry matter) was calculated on an FW basis [25], following the formula where TFW is the total plant fresh weight and TDW is the total plant dry weight.

(1)$\mathrm{R}\mathrm{W}\mathrm{S}=(\mathrm{T}\mathrm{F}\mathrm{W}-\mathrm{T}\mathrm{D}\mathrm{W})/\mathrm{T}\mathrm{D}\mathrm{W}$

The ratio between DW and FW in roots, conductive organs, and leaves (RWR, SWR, and LWR, respectively), was also calculated.

The color index in the leaves for red (R), green (G), and blue (B) values was assessed using an optical scanner and the program Adobe Photoshop.

2.5. Plant Physiological Parameters

At the end of the experimental period, four plants were randomly selected per treatment for the determination of the concentration in the leaves of proline (Pro) and the pigments chlorophyll a (Chla), chlorophyll b (Chlb), chlorophyll a + b (Chla + b), and carotenoids (Ct).

The fresh material of the leaves (0.5 g) was crushed with ethanol (5 mL at 96%) and then washed with ethanol (5 mL at 70%). The alcoholic extract was centrifuged for 10 min (3500× g), and then the supernatant was stored at 4 °C for further analysis. The ninhydrin reagent method was used to determine the free proline concentration (expressed in µg g⁻¹ FW) in the alcoholic extract supernatant, following the recommendations given by Irigoyen et al. [26].

Fresh samples of the leaves (0.2 g) were submerged in methanol for 24 h under dark conditions at room temperature. The extraction procedure does not include any crushing/homogenization steps, and the supernatant was separated by filtration when the leaf samples were completely colorless. The concentration of pigments in the supernatant was recorded spectrophotometrically utilizing the protocol reported by Wellburn [27].

2.6. Water and Nutrient Use Efficiency of the System Crop

At the end of the experimental period, we determined the water and nutrient use efficiency of the system crop, as frequently used in agronomic research, following the procedure established by Dobermann [28] defined as the total dry weight (g) per liter or gram of nutrient applied (g).

2.7. Statistical Analysis

The significance of the effect of the treatments was assessed by standard analyses of variance (two-way ANOVA) and Least Significant Difference (LSD) test carried out using Statgraphics Plus V.4.0 (Statpoint Technologies, Inc. Warrenton, VA, USA). Differences were considered significant at p < 0.05.

3. Results

3.1. Plant Morphological and Physiological Parameters

During the experimental period, neither mortality of plants nor incidence of pests or diseases for the treatments were assessed. The fertigation with diluted leachates did not have significant effects on H, LN, LA, TFW, RGB values, but the RWS increased significantly under dilution (L_WD and L_NSD). On the other hand, H₂O₂ supply significantly decreased LN, LA, and TFW and increased the RGB values (Table 2). Moreover, the application of diluted leachates did not cause noteworthy effects on the TDW and the distribution ratios of biomass, but the H₂O₂ supply decreased the TFW. Different trends for biomass partitioning were found among the different organs assessed. The RWR increased, and the LWR decreased under H₂O₂ supply (Table 3).

Leachate dilutions and H₂O₂ supply did not significantly affect the photosynthetic pigment concentration in C. fruticosa plants (Table 4). Nevertheless, proline concentration presented significant differences for the two studied factors. The highest proline concentration was detected under L₁₀₀ (71.75 µg g⁻¹ FW), followed by L_NSD, and the lowest was found in L_WD (62.00 µg g⁻¹ FW). The application of H₂O₂ also reduced the proline concentration in leaves.

3.2. Nutrient Solution and Leachate Characterization

Figure 2 showed the evolution of the nutritional parameters (pH, EC, NO₃⁻, H₂PO₄⁻, SO₄²⁻, Cl⁻, K⁺, Na⁺, Ca²⁺, Mg²⁺) in the fertigation treatments throughout the experimental period. The highest values of the nutritional parameter’s concentrations were found under L₁₀₀, except for H₂PO₄⁻. Moreover, pH, EC, SO₄²⁻, Cl⁻, Na⁺, and Mg²⁺ presented similar values in L_WD and L_NSD. Nitrate, H₂PO₄⁻, K⁺ and Ca²⁺ were higher in L_NSD. Nevertheless, when nutrient solutions were supplied with H₂O_2, important changes occurred. The pH, EC, SO₄²⁻, Cl⁻, K⁺, Na⁺, Ca²⁺, Mg²⁺ decreased and NO₃⁻ increased.

Figure 3 shows the evolution of the nutritional parameters (pH, EC, NO₃⁻, H₂PO₄⁻, SO₄²⁻, Cl⁻, K⁺, Na⁺, Ca²⁺, Mg²⁺) in the leachates from C. fruticosa over the trial. The highest values of the leached ion concentration were detected under L₁₀₀, except for H₂PO₄⁻, with a behavior similar to that observed in the other fertigation treatments. Nevertheless, important changes occurred under H₂O₂ treatments: pH and Cl⁻ decreased and H₂PO₄⁻ increased.

The fertigation with leachates diluted with a standard nutrient solution and without H₂O₂ supply decreased the accumulated volume of leachate (Table 5). The accumulated volume of the nutrient solution applied throughout the trial was 1.14 mL per plant, so the drainage percentage was 18.00, 17.00, 17.00, 19.00, 21.00 and 17.00% for the fertigation treatments L₁₀₀, L_WD, L_NSD, L₁₀₀ + H₂O₂, L_WD + H₂O₂ and L_NSD + H₂O₂, respectively. There is an interesting interaction between factors because non-significant differences between oxyfertigation and non-oxyfertigation under L_WD treatment were found (Figure 4).

Leachate pH presented a similar behavior to that of nutrient solutions pH (Figure 2). The oxyfertigated nutrient solutions had more acidic values (4.7–4.9) than leachates (6.26–7.23). There was a decline in leachate pH (Figure 3) throughout the cultivation cycle.

The fertigation treatments L_WD, L_NSD, L_WD + H₂O₂ and L_NSD + H₂O₂ had a leachate EC trend similar to that of the nutrient solutions EC (Figure 2), while it was higher in L₁₀₀ and L₁₀₀ + H₂O₂, where pure leachates were applied. The application of H₂O₂ appears to have caused a slight reduction of EC in all the treatments tested from 22 to 43 DAT.

The fertigation treatments L₁₀₀ and L_WD produced leachates with the highest average concentration of NO₃⁻, while the lowest was found in the fertigation treatment L_WD + H₂O₂. L₁₀₀ presented the highest quantity of leachate related to the diluted treatments and oxyfertigated versus non-oxyfertigated (Table 5). Furthermore, nitrate concentrations in leachates were greater than the concentrations in the applied nutrient solutions (Figure 2). During the investigation, the behavior of leachates and nutrient solutions was comparable.

Phosphate concentration in leachates presented a behavior comparable to that found in nutrient solutions (Figure 2), presenting the highest values for the treatments L_NSD and L_{NSD +} H₂O₂. Under these treatments, the leaching of H₂PO₄⁻ increasedsubstantially. Oxyfertigated treatments caused a higher quantity of leachate related to non-oxyfertigated treatments (Table 5).

For the evolution of SO₄²⁻ concentration, all the treatments followed a similar pattern, with the minimum value at 1495 and the maximum at 2490 mg L⁻¹. The leaching trends were similar to the NS applied (Figure 2), but the leaching showed greater concentration. The total SO₄²⁻ leachate was significantly higher in L₁₀₀ treatment. No effect was found in relation to H₂O₂ supply (Table 5).

For Cl⁻ concentrations over time, all the treatments presented a peak at 22 DAT, being higher in the L₁₀₀ treatment, reaching 3100 mg L⁻¹ of Cl⁻, decreasing from this moment on, and stabilizing until the end of the cultivation. In general, the concentration of Cl⁻ in oxyfertigated treatments increased at the beginning and decreased at the end of the cycle for the non-oxyfertigated treatments. L₁₀₀ presented the highest quantity of leachate, L_WD intermediate value and L_NSD the lowest level. Oxyfertigated treatments presented a lower Cl⁻ concentration than non-oxyfertigated treatments (Table 5). Non-significant differences between oxyfertigation and non-oxyfertigation under L_WD treatment were found (Figure 4).

The behavior of cations concentration was similar: the concentrates were higher in the leachate than in the nutrient solution. There was a rise in all the treatments tested at 22 DAT. L₁₀₀ presented the highest K⁺, Ca²⁺, and Na⁺ concentration in leachate. Magnesium concentration was similar under the factors studied. Oxyfertigation led to higher K⁺, Na⁺, Ca²⁺ concentrations than non-oxyfertigated treatments (Table 6). Non-significant differences between oxyfertigation and non-oxyfertigation under L_WD treatment for Na⁺ were detected (Figure 5).

3.3. Water and Nutrient Use Efficiency

The dilution of leachates did not affect WUE. Nevertheless, the H₂O₂ supply significantly decreased this parameter. The highest NUE, PUE and KUE were detected under L_WD (Table 7). Oxyfertigated treatments significantly decreased NUE but did not affect PUE and KUE. Non-significant differences between oxyfertigation and non-oxyfertigation under L₁₀₀ treatment for NUE were found (Figure 6).

4. Discussion

4.1. Nutrient Solution and Leachate Characterization

The nutrient solution coming from leachates had an average pH near to neutrality, but there was an acidification of the diluted and oxyfertigated nutrient solutions due to the acidity of the supply (water, standard nutrient solution or H₂O₂ supply) [29]. Peroxide oxygen is a weak acid that ionizes releasing protons into the medium [30]. Nevertheless, Soto-Bravo [31] did not find significant differences between the pH of nutrient solutions used in non-oxyfertigated and oxyfertigated tomato plants.

The desired pH range for most crops is between 5.0 and 6.2 [32,33]. Nevertheless, the application of H₂O₂ may require regulation of the pH because both alkaline pH (above 8.0) and highly acidic pH (below 4.0) have shown an inhibitory effect on the growth of water hyacinth plants [34]. Values above 7.0 induce deficiencies of P, Fe, Mn, and Cu and sometimes Zn. The pH values below 4.0 cause root damage because of the effects of H⁺. Below a 4.5 pH level, there are increases in the proportion of H₂PO₄⁻ concentration in the nutrient solution in undissociated forms not available to the plant. Several Mn and Al hydroxides are soluble at a pH level less than 5.0; below 5.5 pH levels, the growth of the plant may be affected because it may not absorb Ca, K and Mg [13,35].

Electrical conductivity presented values between 3.4–4.1 dS m⁻¹ in L_100, higher than those found in L_WD and L_NSD, which were around 2.5 dS m⁻¹ due to the higher concentrations of the ions, apart from H₂PO₄⁻ in pure leachate. Correspondingly, Soto-Bravo [31] found that oxyfertigation with H₂O₂ in tomatoes causes modifications in irrigation water EC, changing from 2.9 dS m⁻¹ to 2.5 ds m⁻¹; in L₁₀₀ the values fell slightly to 0.7 dS m⁻¹. Nevertheless, under L_WD and L_NSD, only a slight decrease was found. These results could be due to the ion concentration effect on EC [36,37]. The values presented in L_WD and L_NSD treatments were in the same range as those recommended for the cultivation of C. fruticosa by other researchers [17]. For NO₃⁻ concentration, the values found in L₁₀₀ were higher than those detected in the L_WD and L_NSD treatments because the former consisted of pure leachate (higher EC), while treatments for the latter were obtained by dilution with tap water or standard nutrient solution (with lower EC). The NO₃⁻ concentrations in the leachate from C. lanatus fluctuated throughout the experiment from 1264 to 870.2 mg L⁻¹, which may be related to the differential nitrogen uptake related to the growth of this species over time, as reported by Hershey and Paul [38]. In the L₁₀₀ treatment, a significantly higher concentration of NO₃⁻ was observed at the beginning of the cycle, which coincides with the highest nutrient demand of C. lanatus for vegetative growth [39]. The concentration of NO₃⁻ increased significantly under the H₂O₂ treatments because of the mineralization process, increasing availability of NO₃⁻ [40].

Under non-oxyfertigated treatments, H₂PO₄⁻ followed the same pattern throughout the cultivation cycle. Nevertheless, the supply of the standard nutrient solution enriched the leachates obtained from C. lanatus and improved their quality for ornamental plant production. Under an H₂O₂ supply, the concentration of H₂PO₄⁻ presented similar values. The concentration of H₂PO₄⁻ in our experiment exceeded the reference value (10 mg L⁻¹ of P) proposed for the optimal growth of woody ornamental plants [41], but it was into the range recommended for the cultivation of ornamental plants [42]. Nevertheless, H₂PO₄⁻ concentration in L_DSN treatments was higher from 22 DAT, and it must be considered that high levels can generate phytotoxicity problems [11,43] although in our experiment, C. fruticosa plants did not show any aesthetic damages.

For SO₄²⁻ concentrations, L_100, because it was pure leachate (with higher EC), presented higher concentrations than L_WD and L_NSD, and the other leachates employed for fertigation were diluted with tap water or standard nutrient solution (with lower EC). Sulphate is one of the most important anions in natural waters [44]. The treatments with H₂O₂ caused a decrease of SO₄²⁻ concentration in the nutrient solution due to the precipitation of calcium sulfate [45].

Chloride concentration presented the same trend as SO₄²⁻ concentration. At 29 DAT, Cl⁻ concentration began to rise remarkably in the nutrient solution, reaching 800 mg L⁻¹. In the cultivation of watermelon, there is an increase in the volume of leachate obtained in the phase of fruit fattening, causing a salts wash and a higher Cl⁻ concentration in the leachate. The concentration is toxic for a large number of ornamental crops; however, a good tolerance is observed in rose cultivation with 30 mmol L⁻¹ (1065 mg L⁻¹) of NaCl [46]. In general, until 22 DAT, the addition of H₂O₂ increased the Cl⁻ concentration in the nutrient solution but decreased afterwards. At low pH conditions, Cl⁻ solubility increases; then, when the chlorides oxidate in chlorine oxides, they are eliminated as a gas and therefore Cl⁻ concentration decreases at the end of the cultivation for oxyfertigated solutions [47].

Similar average concentrations of K⁺ were observed in L₁₀₀ and L_NSD due to the dilution with standard nutrient solution. The treatments with H₂O₂ presented lower values, which can be related to the greater K⁺ plant uptake. These concentrations are higher than those recommended by Yeager et al. [48], who propose a range of 30–50 mg L⁻¹.

Calcium and Mg²⁺ concentrations in the nutrient solution presented similar trends, being higher in the L₁₀₀ treatment. The H₂O₂ supply did not have a significant effect on the concentration of Ca²⁺ in the nutrient solution. Our values were lower than the range established by Edwards and Norton [49] in peach plants (10–15 mg L⁻¹).

It is relevant to mention that Na⁺ concentration presented a peak at 43 DAT in all tested treatments, which could be due to the increase of transpiration and water uptake of C. lanatus plants, as suggested by Savvas et al. [50] in cucumber plants. The highest value was detected in L₁₀₀. The H₂O₂ supply did not have a significant effect on the concentration of Na⁺ in the nutrient solution.

4.2. Plant Morphological and Physiological Parameters

Plants fertigated with leachates were shorter than those fertigated with a standard nutrient solution, which may be explained by the decrease in the concentration of available nutrients in leachates and the positive effects of these nutrients in plant growth, as has been widely reported by Mengel and Kirkby [35]. However, this result disagrees with those obtained by Boyle et al. [51] and Westervelt [52], who reported an increase in plant height under low nutrient concentrations.

Flowers and Yeo [53] consider that the increase of salinity decreases vegetative growth. Parry et al. [54] consider that the promotion of growth might be attributed to an increase of carbon assimilation under H₂O₂ spraying, since even small increases in the leaf net photosynthesis rate can translate into large increases in biomass. Nevertheless, in our experiment, the morphological parameters studied did not show significant differences caused by the modifications in pH associated with dilution. This could be due to the combined effects of the potential growth increase caused by the higher NO₃⁻ concentration in L₁₀₀ [35] and the potential growth decreases under higher EC [55]. The H₂O₂ supply decreased LN (17%), LA (19%), TFW (19%), TDW (15%), and LWR (9%) and increased the RWR (20%) in plants growing in pots filled with peat moss and perlite. These results differ to those obtained by Niñirola [56], who did not find growth and development of watercress (Nasturtium officinale) under nutrient solution aeration treatments, while Soto-Bravo [31] did not find effects in tomato yield under chemical oxifertigation. On the other hand, Semida [57] found an increase of the number of leaves per plant and leaf area per plant in onion plants sprayed with H₂O₂ (1 mM) under salt stress. The total dry weight was lower under H₂O₂ treatments, which may be related to the lower number of leaves and leaf area and, therefore, to their lower capacity to generate bioassimilates. However, Li et al. [58] consider that the exogenous application of H₂O₂ participates in the synthesis of enzymes and antioxidant compounds that induce salinity tolerance in wheat seeds, with a positive effect on growth. It is necessary to highlight that the application of H₂O₂ increased RGB values in leaves of C. fruticosa plants, enhancing their aesthetic value and their market value.

The root weight ratio increased 1.2 times under H₂O₂ supply. These results are consistent with those obtained by Hammed et al. [59], who stated that exogenous H₂O₂ applications provide a more vigorous root system to wheat seedlings because the number of roots was higher, but root length was significantly shorter, indicating suppression of cell elongation, probably by premature initiation of the secondary wall formation. Leaves showed the opposite behavior to roots because the plants treated with H₂O₂ had a lower number of leaves and leaf area; nevertheless, García-Jiménez [60] found a higher stem DW in green bean and melon plants subjected to a chemical oxygenate application to the fertigation solution. Colunje et al. [61] also found a similar behavior in pepper crops supplied with ozonated fertigation. For this reason, this effect could be related to oxygenated nutrient solution more than to a specific effect of H₂O₂.

Relative water status significantly increased in treatments with dilutions (L_WD and L_NSD). Plants under stress often lose some water from their tissues, which can have rapid and major effects on cell expansion, cell division, stomatal opening, and abscisic acid (ABA) accumulation [62]. In our trial no significant effects were found in the H₂O₂ supply, which agrees with the results found by Lorenzen et al. [63], who did not detect any influence on the water content of Typha and Cladium plants under a high oxygen level in the culture solutions. Opposite results have been found by Ishibashi et al. [64] in soybean plants, in which there was an increase of relative water content in the leaves when H₂O₂ was applied. Moreover, Semida [57] found a significant rise of relative water content in onion varieties under H₂O₂ supply.

The concentration of photosynthetic pigments did not show significant differences among the treatments assessed. Nevertheless, Ahmad et al. [65] found a significant increase in Chla concentration with the foliar application of H₂O₂ in corn seedlings under low temperature conditions. Liu et al. [66] also observed that the exogenous application of H₂O₂ increased the antioxidant activity in cucumber leaves and decreased lipid peroxidation, protecting the ultrastructure of chloroplasts and mitochondria under osmotic stress conditions. Foliar sprays in two onion varieties grown under saline soil condition led to an increase in the chlorophyll and carotenoid contents. The low concentration of these parameters in onion plants under saline stress may be due to a decrease in intermediates of chlorophyll biosynthesis [67].

There were significant differences in proline concentration for the two studied factors. L₁₀₀ presented the higher value, which was linked to their higher salinity. Cordyline fruticosa plants synthesize proline as a major compatible solute to adjust the osmotic pressure when Na⁺ and Cl⁻ accumulate in their tissues, and these results coincide with the mechanism to maintain cell homeostasis under salt-stress conditions observed in Casuarina equisetifolia plants [68]. Leaf proline concentration decreased significantly under the H₂O₂ supply in our experiment, a finding that contradicts the results obtained by Ishibashi et al. [64], who noted an increase in proline in soybean plants under water scarcity and foliar application of H₂O₂. The reduction of free proline by H₂O₂ application in onion plants may be attributed to the crucial role of H₂O₂ in mitigating the negative salt effects [57].

In general, the leachate volumes obtained from C. fruticosa in the different treatments are considered normal for the handling of irrigation in crops without soil with regular- quality water [69]. There was a significant decrease in plant growth in the treatments fertigated with leachates that were diluted with standard nutrient solution and without H₂O₂ supply. This is because plants fertigated with the same treatment but supplied with H₂O₂ had a lower leaf area index. The relatively high evapotranspiration is probably caused by the high LAI (Leaf Area Index) [70,71]. Inversely, drainage rises when plant transpiration rate is low [72].

The evolution of leachate pH and fertigation solutions during the trial were similar. The oxyfertigated nutrient solutions have more acidic values (4.70–4.90) than leachates (6.26–7.23). Blond peat is acidic, but limestone and dolomite, normally incorporated to neutralize its acidity, increase the buffer capacity and provide Ca²⁺ and Mg²⁺ [73,74], causing their solution from the substrate not to reach such low pH values [75]. Chen et al. [76] have also demonstrated that substrates containing pine bark, sand, and peat buffer alkaline pH of irrigation water to a measured substrate pH of about 6.5. At 35 DAT, a significant decrease in the leachate pH was observed, probably due to the loss of the buffering capacity of the substrate, because carbonates are depleted. None of the pH values sampled in the leachate fell outside the critical ranges (pH of 6.5–9.5) established by the EEA (2014).

A decline in leachate pH occurs throughout the cultivation cycle. The evolution of leachates pH and nutrient solutions pH was similar in the treatments without H₂O₂; however, in the treatment L_WD + H₂O₂, pH is higher (with values from 7.7 at 15 DAT to 7.4 at 50 DAT) than in L₁₀₀ + H₂O₂ and L_NSD + H₂O₂ (with values from 7.8 at 15 DAT to 5.0 at 50 DAT and from 7.6 at 15 DAT to 4.9 at 50 DAT, respectively). Opposite results were obtained by Betancur and Garcia-Caparros et al. [77,78], who found that when the concentration of H₂O₂ increases, the pH tends to alkalinization.

In drainage water, EC values were slightly lower for H₂O₂ treatments than for non-oxygenated treatments during most of the cultivation cycle of watermelon grown in perlite bags, possibly due to the slightly higher percentage of drainage water [79]. Nevertheless, Soto-Bravo [31] did not obtain significant differences in the EC when injecting H₂O₂ continuously into the irrigation water for the tomato crop.

L₁₀₀ presented the highest NO₃⁻ concentration in the fertigation water, and this was reflected in the leachate. Leachates contain many organic forms of nitrogen; when applying H₂O₂, these forms are oxidized and remain in the form of NH₄⁺ which is nitrified and converted to NO₃⁻, making it more available for plants. This fact must be considered before the application of fertilizer to avoid excessive amounts of NO₃⁻ and the toxicity generated, defects in production of cultivation, and unnecessary expense [76]. The leachates generated exceeded by 20 times the limit imposed by the European Union for NO₃⁻ in groundwater [80].

The elevated H₂PO₄⁻ concentration found in L_NSD and L_{NSD +}H₂O₂ treatments could be due to the dissolution of precipitated forms of calcium phosphates, associated with low pH. In general, lower concentrations of H₂PO₄⁻ were found in drainage solutions, with concentrations between 50% and 100% of the input in soilless culture [81]. From 35 DDT, the supply of H₂PO₄⁻ exceeded C. fruticosa requirements, which were especially high in the oxygenated treatments, so it was leached in large quantities [82].

Although the polluting element par excellence is NO₃⁻, SO₄²⁻ may be taken as a critical parameter in the assessment of groundwater quality [45]. The standard for SO₄²⁻ is 250 mg L⁻¹ in waters intended for human consumption, a quantity 8 times less than the concentration in the leachates of our experiment.

The behavior of Cl⁻ concentration could be due to the joined effects of the low pH, related to the higher solubility of Cl⁻ in the leached solution, and the oxidation of Cl⁻ in chlorine oxides, eliminated in the form of gas, decreasing the concentration of Cl⁻ at the last period of the trial [47]. However, this ion does not cause a problem in crops that have a short growth period and a low transpiration rate, e.g., lettuce (Lactuca sativa) [83] as well as some ornamental plants such as Cordyline fruticose.

The rise of cation concentration in the nutrient solution for the fertigation of Citrullus lanatus can be related to the enrichment in K⁺, Ca²⁺ and Mg²⁺, for it contributes to induce flowering [84,85] and fructification [86,87]. Nevertheless, the requirements of C. fruticosa plants were lower. Baixauli and Aguilar [81] recommend leachate concentrations in the range of 50–100% for K⁺, 100–150% for Ca²⁺ and 100–300% for Mg²⁺, as the inputs in soilless culture.

Sodium concentration increased progressively in treatment with pure leachates; the significant increase at the end of the trial may be due to an imbalance between the transpiration rate and the absorption of this ion, causing a lower leachate volume and an increase in Na⁺ concentration [8,86]. Calcium concentration followed a similar evolution; these two parameters share some of the same pathways in the cell since they are similar ions [88].

4.3. Water and Nutrient UtilizationEfficiency

Water utilization efficiency was unaffected for the fertigation treatments. Although the EC affected the relative water content, there was no effect on TDW. These results were dissimilar to those obtained by Lee and Van Iersel [89], who reported a reduction of WUE in Chrysantemum morifolium under salt stress. The decline in WUE under H₂O₂ supply could be related to the biomass decrease.

As far as nutrient utilization efficiencies were concerned, NUE, PUE and PUE increased in plants fertigated with L_WD. The decrease in NUE, PUE and KUE in L₁₀₀ and L_NSD could be related with an excess of these nutrient requirements by plants, as Ristvey et al. [90] observed in azalea. In contrast, KUE was higher under saline conditions in Atriplex [91], which could occur since it is a halophyte plant, while C. fruticosa is a species semi-tolerant of salt. The decline in NUE under the H₂O₂ supply could be related to the biomass decrease. Nevertheless, the interaction found between factors indicates that there was no effect of H₂O₂ supply under the L₁₀₀ treatment. Lorenzen et al. [63] found different responses in utilizing oxygenated culture solutions: Typha showed a lower NUE than plants in low oxygen culture solutions, but NUE in Cladium was not affected by the oxygen level. With PUE, similar results were found by Lorenzen et al. [63], who observed that oxygen applications in the rhizosphere had only a limited effect in PUE in Typha and Cladium.

5. Conclusions

The dilution (with water or standard nutrient solution) of the leachates obtained from the cultivation of Citrullus lanatus for their reuse as fertigation solutions for the cultivation of Cordyline fruticosa causes important modifications in the nutrient parameters studied. The addition of a commercial oxidant with H₂O₂ causes the acidification of the nutrient solution, while EC is not affected, although it decreases in the leachate obtained in treatment with pure leachates. There is an increase of the NO₃⁻ concentration in the nutrient solutions but a decrease in the leachate when watermelon leachates were diluted with tap water. Phosphate concentration increases significantly in the nutrient solutions and in the leachates. Sulphate concentration decreases in the nutrient solutions but increases in the leachate diluted with tap water. Chloride increases in the nutrient solution constituted by leachates; it seems that there is a positive effect in most saline treatment. Cations follow a similar evolution in all the nutrient solutions tested, with no significant differences in the leachates, except for the Na⁺ concentration, which rises. Hydrogen peroxide causes a reduction of the number of leaves per plant, leaf area, water status and total plant DW, also modifying biomass distribution, with an increase in root weight ratio and a decrease in DW percentage in leaves. Nevertheless, H₂O₂ application enhanced the values of RGB. The concentrations of photosynthetic pigments are not affected. The application of H₂O₂ does not generate stress in the plants, valued by the concentration of proline. Nutrient utilization efficiency is higher under L_WD, and the efficiency in the utilization of water and NO₃⁻ decrease under an H₂O₂ supply. In conclusion, it can be stated that the reuse of leachate diluted with water is recommended in nutritionally undemanding crops such as C. fruticosa. Furthermore, the H₂O₂ supply improves tolerance to salinity and enhances root growth.

Author Contributions

M.T.L.; conceptualization, M.T.L. and P.G.-C.; methodology, M.T.L. and B.M.P.; formal analysis, F.P. and A.L.; investigation, F.P.; resources, M.T.L.; writing—original draft preparation, F.P. and B.M.P.; writing—review and editing, M.T.L., A.L. and P.G.-C.; supervision, M.T.L. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

We thank the company OXI-VIRIN for its collaboration in the supply and in the information received about the product.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Schematic overview of the experimental design.

Figure 2. Evolution of the nutritional parameters in the fertigation treatments of C. fruticosa throughout the trial: (a) pH, (b) EC, (c) NO3−, (d) H2PO4−, (e) SO42−, (f) Cl−, (g) K+, (h) Na+, (i) Ca2+, (j) Mg2+. L100 = raw leachate from C. lanatus, LWD: raw leachate from C. lanatus diluted with tap water until EC of 2.5 dS m−1; LNSD: raw leachate from C. lanatus diluted with standard nutrient solution until EC of 2.5 dS m−1 and H2O2 supply (S0 = without H2O2 and SH2O2 = with 2% H2O2). DAT: days after transplanting. Data are the means of 4 samples per fertigation treatment.

Figure 3. Evolution of the nutritional parameters in leachates from C. fruticosa over the trial. (a) pH, (b) EC, (c) NO3−, (d) H2PO4−, (e) SO42−, (f) Cl−, (g) K+, (h) Na+, (i) Ca2+, (j) Mg2+. L100 = raw leachate from C. lanatus, LWD: raw leachate from C. lanatus diluted with tap water until EC of 2.5 dS m−1; LNSD: raw leachate from C. lanatus diluted with standard nutrient solution until EC of 2.5 dS m−1 and H2O2 supply (S0 = without H2O2 and SH2O2 = with 2% H2O2). DAT: days after transplanting. Data are the means of 4 samples per fertigation treatment.

Figure 4. Interactions between fertigation treatments and H2O2 supply on water and Cl− leachate. L100 = raw leachate from C. lanatus, LWD: raw leachate from C. lanatus diluted with tap water until EC of 2.5 dS m−1; LNSD: raw leachate from C. lanatus diluted with standard nutrient solution until EC of 2.5 dS m−1 and H2O2 supply (S0 = without H2O2 and SH2O2 = with 2% H2O2). Different lowercase letters indicate significant differences between treatments.

Figure 5. Interactions between fertigation treatments and H2O2 supply on Na+ leachate. L100 = raw leachate from C. lanatus, LWD: raw leachate from C. lanatus diluted with tap water until EC of 2.5 dS m−1; LNSD: raw leachate from C. lanatus diluted with standard nutrient solution until EC of 2.5 dS m−1 and H2O2 supply (S0 and = without H2O2 and SH2O2 = with 2% H2O2). Different lowercase letters indicate significant differences between treatments.

Figure 6. Interactions between fertigation treatments and H2O2 supply on NUE. L100 = raw leachate from C. lanatus, LWD: raw leachate from C. lanatus diluted with tap water until EC of 2.5 dS m−1; LNSD: raw leachate from C. lanatus diluted with standard nutrient solution until EC of 2.5 dS m−1 and H2O2 supply (S0 = without H2O2 and SH2O2 = with 2% H2O2). Different lowercase letters indicate significant differences between treatments.

Table 1

Chemical composition of tap water, nutrient solution supplied to watermelon, and the nutrient solution supplied to C. fruticosa. EC is expressed in dS m⁻¹, and the element concentration in mmol/L.

	EC	pH	Cl−	NO3−	H2PO4−	SO42−	K+	Na+	Ca2+	Mg2+
Tap water	0.9	8.1	3.5	0	0	1.1	0	2.6	2	1.4
Nutrient solution supplied to watermelon	2.9–5.0	6.4–8.7	9.7–19.7	9.3–11.9	0.2–1.5	5–9	5.1–9.4	5.4–12.8	3.2–7.9	6.4–13.9
Nutrient solution supplied to C. fruticosa	1.5	5.8	3.5	8.3	1.7	1.8	5.3	2.6	2	1.4

Table 2

Effects of fertigation treatments on height (H) (cm), leaf number (LN), leaf area (LA) (cm²), total fresh weight (TFW) (g), and relative water status (RWS) (g water g⁻¹ DW) in C. fruticosa at the end of the experimental period.

		H	LN	LA	TFW	RWS	R	G	B
Fertigation treatment (L)		ns	ns	ns	ns	*	ns	ns	ns
	L100	24.18 ± 2.02	12.46 ± 2.30	244.54 ± 20.45	12.62 ± 1.88	5.00 ± 0.17 b	92.11 ± 1.98	152.62 ± 1.88	61.62 ± 1.51
	LWD	24.97 ± 1.75	12.29 ± 2.03	235.00 ± 21.20	13.38 ± 1.79	5.37 ± 0.14 a	94.15 ± 1.99	153.38 ± 1.79	63.18 ± 1.42
	LNSD	25.04 ± 2.66	12.33 ± 2.30	231.42 ± 24.49	13.68± 1.47	5.41 ± 0.15 a	93.36 ± 1.97	153.68± 1.47	61.74 ± 1.87
H2O2 supply (S)		ns	*	*	*	ns
	S0	24.62 ± 2.52	13.50 ± 0.80 a	261.22 ± 21.30 a	14.65 ± 1.00 a	4.87 ± 0.21	94.65 ± 1.67 b	134.65 ± 3.00 b	64.05 ± 1.32 b
	SH2O2	24.84 ± 1.81	11.22 ± 0.94 b	212.83 ± 22.39 b	11.80 ± 1.39 b	4.90 ± 0.18	101.80 ± 2.39 a	156.20 ± 3.39 a	70.80 ± 1.47 a
Interactions
L*S		ns	ns	ns	ns	ns	ns	ns	ns

L₁₀₀ = raw leachate from C. lanatus; L_WD: raw leachate from C. lanatus diluted with tap water until EC of 2.5 dS m⁻¹; L_NSD: raw leachate from C. lanatus diluted with standard nutrient solution until EC of 2.5 dS m⁻¹ and H₂O₂ supply (S₀ = without H₂O₂ and S_H2O2 = with 2% H₂O₂). Data are the means ± standard deviation of 4 samples per treatment. Means with different letters within a column are significantly different at p < 0.05 (analysis of variance and least significant difference test). ns: non-statistical differences. *: indicates statistical differences between treatments.

Table 3

Effects of fertigation treatments on total dry weight (TDW) (g), leaf weight ratio (LWR), stem weight ratio (SWR), and root weight ratio (RWR) in C. fruticosa at the end of the experimental period.

		TDW	LWR	SWR	RWR
Fertigation treatment (L)		ns	ns	ns	ns
	L100	2.09 ± 0.19	0.44 ± 0.02	0.29 ± 0.03	0.27 ± 0.02
	LWD	2.09 ± 0.15	0.42 ± 0.02	0.30 ± 0.03	0.27 ± 0.04
	LNSD	2.12 ± 0.13	0.44 ± 0.02	0.29 ± 0.03	0.27 ± 0.03
H2O2 supply (S)		*	*	ns	*
	S0	2.27 ± 0.19 a	0.45 ± 0.02 a	0.30 ± 0.03	0.25 ± 0.01 b
	SH2O2	1.93 ± 0.11 b	0.41 ± 0.01 b	0.29 ± 0.04	0.30 ± 0.02 a
Interactions
L*S		ns	ns	ns	ns

L₁₀₀ = raw leachate from C. lanatus, L_WD: raw leachate from C. lanatus diluted with tap water until EC of 2.5 dS m⁻¹; L_NSD: raw leachate from C. lanatus diluted with standard nutrient solution until EC of 2.5 dS m⁻¹. Data are the means ± standard deviation of 4 samples per treatment. Means with different letters within a column are significantly different at p < 0.05 (analysis of variance and least significant difference test). ns: indicates non-statistical differences. *: indicates statistical differences between treatments.

Table 4

Effects of fertigation treatments on chlorophyll a (Chla), chlorophyll b (Chlb), chlorophyll a + b (Cha + b) and carotenoids (Ct) (mg g⁻¹ FW) and proline (Pro) (µg g⁻¹ FW) in the leaves of C. fruticosa at the end of the experimental period.

		Chla	Chlb	Chla+b	Ct	Pro
Fertigation treatment (L)		ns	ns	ns	ns	*
	L100	1.75 ± 0.23	1.03 ± 0.14	2.79 ± 0.38	1.91 ± 0.23	71.75 ± 3.89 a
	LWD	1.84 ± 0.23	1.08 ± 0.14	2.92 ± 0.37	1.98 ± 0.29	41.81 ± 4.54 c
	LNSD	1.91 ± 0.27	1.12 ± 0.17	3.03 ± 0.43	2.05 ± 0.27	62.00 ± 4.75 b
H2O2 supply (S)		ns	ns	ns	ns	*
	S0	1.85 ± 0.27	1.10 ± 0.17	2.95 ± 0.44	1.99 ± 0.30	70.14 ± 4.33 a
	SH2O2	1.82 ± 0.22	1.06 ± 0.13	2.87 ± 0.35	1.97 ± 0.22	46.90 ± 3.68 b
Interactions
L*S		ns	ns	ns	ns	ns

L₁₀₀ = raw leachate from C. lanatus, L_WD: raw leachate from C. lanatus diluted with tap water until EC of 2.5 dS m⁻¹; L_NSD: raw leachate from C. lanatus diluted with standard nutrient solution until EC of 2.5 dS m⁻¹. Data are the means ± standard deviation of 4 samples per treatment. Means with different letters within a column are significantly different at p < 0.05 (analysis of variance and least significant difference test). ns: non-statistical differences. *: indicates statistical differences between treatments.

Table 5

Effects of fertigation treatments on the total volume of water (W) (cm³ plant⁻¹), NO₃⁻, H₂PO₄⁻, SO₄²⁻, and Cl⁻ leached by the crop (Cl⁻) (mg plant⁻¹) in C. fruticosa at the end of the experimental period.

		W	NO3−	H2PO4−	SO42−	Cl−
Fertigation treatment (L)		*	*	*	*	*
	L100	207.50 ± 2.27 a	270.89 ± 16.66 a	19.64 ± 3.82 b	450.79 ± 24.35 a	326.76 ± 32.80 a
	LWD	211.09 ± 4.95 a	240.22 ± 18.86 b	12.24 ± 5.83 b	387.60 ± 29.85 b	242.25 ± 17.50 b
	LNSD	197.50 ± 3.62 b	243.71 ± 16.68 b	58.20 ± 7.05 a	374.07 ± 18.53 b	186.05 ± 7.66 c
H2O2 supply (S)		*	*	*	*	*
	S0	195.52 ± 3.94 b	238.03 ± 18.99 b	25.12 ± 3.58 b	387.45 ± 41.26	256.57 ± 75.65 a
	SH2O2	215.21 ± 6.05 a	265.18 ± 15.07 a	34.97 ± 4.30 a	420.86 ± 35.94	246.81 ± 49.38 b
Interactions
L*S		*	ns	ns	ns	*

L₁₀₀ = raw leachate from C. lanatus, L_WD: raw leachate from C. lanatus diluted with tap water until EC of 2.5 dS m⁻¹; L_NSD: raw leachate from C. lanatus diluted with standard nutrient solution until EC of 2.5 dS m⁻¹ and H₂O₂ supply (S₀ = without H₂O₂ and S_H2O2 = with 2% H₂O₂). Data are the means ± standard deviation of 4 samples per treatment. Means with different letters within a column are significantly different at p < 0.05 (analysis of variance and least significant difference test). ns: indicates non-statistical differences. *: indicates statistical differences between treatments.

Table 6

Effects of fertigation treatment solution on the K⁺, Ca²⁺, Mg²⁺, and Na⁺ leached by the crop (Na⁺) (expressed in mg plant⁻¹) in C. fruticosa at the end of the experimental period.

		K+	Ca2+	Mg2+	Na+
Fertigation treatment (L)		*	*	ns	*
	L100	112.93 ± 7.89 a	87.98 ± 5.89 a	21.47 ± 2.98	112.38 ± 8.12 a
	LWD	72.32 ± 4.41 c	61.29 ± 4.81 b	18.48 ± 2.48	82.33 ± 7.97 b
	LNSD	94.24 ± 5.66 b	57.25 ± 7.22 b	16.03 ± 2.53	64.80 ± 6.43 c
H2O2 supply (S)		*	*	ns	*
	S0	85.37 ± 3.23 b	64.04 ± 3.20 b	18.25 ± 3.79	71.23 ± 9.64 b
	SH2O2	100.96 ± 4.49 a	73.63 ± 3.77 a	19.06 ± 2.83	101.79 ± 11.54 a
Interactions
L*S		ns	ns	ns	*

L₁₀₀ = raw leachate from C. lanatus, L_WD: raw leachate from C. lanatus diluted with tap water until EC of 2.5 dS m⁻¹; L_NSD: raw leachate from C. lanatus diluted with standard nutrient solution until EC of 2.5 dS m⁻¹ and H₂O₂ supply (S₀ = without H₂O₂ and S_H2O2 = with 2% H₂O₂). Data are the means ± standard deviation of 4 samples per treatment. Means with different letters within a column are significantly different at p < 0.05 (analysis of variance and least significant difference test). ns: indicates non-statistical differences. *: indicates statistical differences between treatments.

Table 7

Effects of fertigation treatments on water utilization efficiency (WUE) (g L⁻¹), nutrient utilization efficiency (NUE) (g g⁻¹), phosphorous utilization efficiency (PUE) (g g⁻¹) and potassium utilization efficiency (KUE) in C. fruticosa at the end of the experimental period.

		WUE	NUE	PUE	KUE
Dilution (L)		ns	*	*	*
	L100	1.84 ± 0.05	1.61 ± 0.13 c	31.40 ± 5.88 b	9.25 ± 1.59 c
	LWD	1.83 ± 0.03	2.91 ± 0.12 a	66.68 ± 7.84 a	14.68 ± 2.11 a
	LNSD	1.86 ± 0.06	2.50 ± 0.11 b	19.57 ± 3.85 c	10.53 ± 2.04 b
H2O2 supply (S)		*	*	ns	ns
	S0	1.99 ± 0.03 a	2.71 ± 0.15 a	42.57 ± 4.32	11.87 ± 3.17
	SH2O2	1.70 ± 0.05 b	2.17 ± 0.11 b	35.80 ± 3.03	11.11 ± 2.83
Interactions
L*S		ns	*	ns	ns

L₁₀₀ = raw leachate from C. lanatus, L_WD: raw leachate from C. lanatus diluted with tap water until EC of 2.5 dS m⁻¹; L_NSD: raw leachate from C. lanatus diluted with standard nutrient solution until EC of 2.5 dS m⁻¹ and H₂O₂ supply (S₀ = without H₂O₂ and S_H2O2 = with 2% H₂O₂). Data are the means ± standard deviation of 4 samples per treatment. Means with different letters within a column are significantly different at p < 0.05 (analysis of variance and least significant difference test). ns: indicates non-statistical differences. *: indicates statistical differences between treatments.

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