Water-Retaining Agent as a Sustainable Agricultural Technique to Enhance Mango (Mangifera indica L.) Productivity in Tropical Soils

Abstract

Reducing fertilizer doses under sustainable agricultural management is possible by increasing nutrient utilization efficiency, which will decrease crop production costs and boost economic return. Soil amendments known as water retention agents (WRAs) are added to the soil to enhance crop growth conditions. We hypothesize that the addition of WRAs may support the soil-retaining nutrients given through fertilization and prevent them from leaching into tropical soils characterized by severe rainfall due to WRAs’ exceptional capacities to absorb and store water. Mango trees (Mangifera indica L. cv Tainong No. 1) aged 18 years were fertilized with 100% or 80% of the recommended doses of nitrogen (N), phosphorus (P), and potassium (K). The experimental design included three treatments, i.e., complete recommended doses of N, P, and K (CRF), 80% of the complete recommended doses (RRF), and water-retaining agent (40 kg ha⁻¹) + 80% of the complete recommended doses (WRARRF). Reducing the fertilization doses by 20% for mango trees in the studied tropical soil significantly (p < 0.05) minimized the nutrient availability in the soil compared to the complete fertilization doses. WRARRF compensated for the nutrient reduction by increasing the availability of N, P, and K. The addition of WRARR increased N, P, and K in mango leaf by 11%, 4%, and 7% in the first year and by 11%, 6%, and 7% in the second year, respectively, compared to CRF. The addition of WRARR increased the partial fertilizer productivity (PFP) value by 36% and 41% in the first and second years, respectively. The highest mango fruit output was achieved by the addition of WRARRF, which resulted in increases in mango fruit yield of 11.9% and 16.5% in the first and second years, respectively, compared to RRF. Fruit quality traits showed the descending order: WRARRF > RRF > CRF. WRARRF produced the maximum economic benefit (USD 7372 per hectare) compared to CRF and RRF. The polyacrylamide/attapulgite water-retaining agent exhibited remarkable improvement in mango fruit yield and economic profit by regulating the release of nutrients in tropical soils. Water-retaining agents are an effective strategy for overcoming the extensive fertilization used in mango orchards, which has resulted in numerous environmental contaminations and the inefficient use of fertilizers.

1. Introduction

The mango (Mangifera indica L.) tree originated in the Himalayan foothills of South Asia around four thousand years ago [1]. It is currently ranked as the second largest tropical fruit and one of the world’s five largest fruits [1]. Mango output is estimated to be over 40 million tonnes worldwide, with India producing the most, followed by China, Thailand, and Mexico [1]. Mango agriculture covers 349,000 hectares in China, and the country produces 3.306 million tons of mangoes, which are valued at about USD 2.87 billion [2]. China’s mango production areas are primarily in the provinces of Yunnan, Guangxi, and Hainan [2,3].

Mango plays an important role in the agricultural and economic development of Hainan Province [4,5,6]. The Hainan mango planting area is subject to long-term tropical marine climate conditions such as high temperature and humidity, which result in intensified runoff, leaching, denitrification, and other effects [6,7]. Mango quality and productivity are greatly influenced by fertilization, which is an essential part of mango orchard management [7,8]. Mango orchards in China are currently facing a number of fertilization challenges, including excessive intensity and inadequate effectiveness [9]. Growers of mangoes usually use chemical fertilizers excessively and carelessly in the hope of maximizing yields and profit [3,7,9]. Excessive amounts of fertilizer are not absorbed by crops, forcing nutrients out of the soil ecosystem and contaminating the air and water [5,9,10].

Water-retaining agents (WRAs) are high-molecular cross-linked polymers that can improve the water-holding capacity of the soil [11,12]. When mixed with soil, WRAs can improve its physical properties by reducing soil bulk density and increasing soil aggregation, permeability, porosity, and water-retention capacity [11,13]. WRAs are soil improvers that can decrease soil strength and aggregate stability, which can enhance soil and water conservation and encourage plant development [11,12,13]. Water-retaining compounds are currently used by many crop growers, and the outcomes are favorable [11,12]. WRAs can improve fertilizer utilization, decrease nutrient loss, and increase soil nutrient absorption [12]. Since crops cannot absorb 80–90% of the phosphorus (P) and 40–70% of the nitrogen (N) in fertilizers, the excess nutrients have a high solubility in water and high diffusivity to the environment [14]. WRAs are utilized for the control of fertilizer releases. The addition of WRAs increased the fertilizer use efficiency and reduced the environmental contamination [15]. Studies such as those by Kong et al. [16] and Zhang et al. [17] have demonstrated the significant impact of WRAs in mitigating environmental pollution levels from by metals toxicity or excessive fertilizer use. WRAs can reduce crop water requirements by 20–40%, increase soil health, and continue operating in the soil for up to 4–5 years [15,18].

Most of the previous investigations have studied the effect of WRAs in increasing soil moisture content and crop yield, but little is known about the economic evaluation and fertilizer productivity in mango orchard. Thus, the aim of the current study was to evaluate the economic value of the addition of WRAs in mango production. The long-term tropical marine climate conditions in the Hainan mango planting area consist of high temperature and humidity, which intensifies the runoff and leaching of nutrients. We hypothesize that due to WRAs’ remarkable ability to absorb and store water, their addition to tropical soils may reinforce the soil-retaining nutrients applied through fertilization practices and keep them from leaching. The current study aims to investigate three factors: (1) the effect of WRAs on nutrients availability and uptake, (2) the response of mango fruit yield and quality to WRAs in tropical soils, and (3) the economic value of WRAs in mango production.

2. Materials and Methods

2.1. Experimental Site of Mango Orchard

The field experiments were conducted for two consecutive seasons from June 2021 to May 2023 at the Hongtai Farm Mango Base (N18°60′36.26″; E108°71′62.26″), Foluo Town, Ledong Li Autonomous County, Hainan Province. The annual average temperature is 20~26 °C, and the annual precipitation is 1653.4 mm. The average temperature in January and June is 16.1 and 32.4 °C, respectively. mango trees (Mangifera indica L. cv Tainong No. 1) aged 18 years with similar shapes were chosen for the experiment. The tested trees had a crown size and moderate yearly trimming level. The trees were cultivated at spacing of 4 m × 5 m with a planting density of 500 plant ha⁻¹. The soil of the experimental site was marine sedimentary dry red soil. The field site had a sandy loam texture with 9.2 g kg⁻¹ organic matter and a soil bulk density of 1.63 g cm³. The soil pH was 6.02 and the available N, P, and K were 38.42, 24.40, and 61.06 mg kg⁻¹, respectively.

The experimental site was irrigated with a micro-sprinkler integrated irrigation and fertilization system. Each planting row was equipped with a 4 mm refractive micro-sprinkler for each tree near the tree head with a flow rate of 85–110 L h⁻¹ and a spray radius of 1.2–1.5 m. All fertilizers were applied through water in the micro-sprinkler irrigation system. A total of 14 irrigations were carried out during the entire growth period supplying 40.0 m³/ha of water each irrigation. The total irrigation amount during the growth period was 560.0 m³/ha. The proportions of irrigation water in the shoot-shooting stage, flowering stage, fruit expansion stage, and fruit maturity stage were 14%, 29%, 43%, and 14%, respectively. Other management measures are consistent with local farmers’ routine field management.

2.2. Experimental Design

The experimental design included three treatments, i.e., complete recommended doses of N, P, and K (CRF), 80% of the complete recommended doses (RRF), and water-retaining agent (40 kg ha⁻¹) plus 80% of the complete recommended doses (WRARRF). The trial was conducted in a randomized complete block design with three replicates and four trees per plot for each treatment. Plots were randomly arranged in the experimental site. The water-retaining agent used in this trial was a long-acting drought-resistant agent produced by Gansu Hairuida Ecological Environment Technology Co., Ltd., Xinhua, Gansu, China. It is a combination of attapulgite and acrylamide particles with a deionized water absorption ratio of 150 to 350 g g⁻¹. The dosage of water-retaining agent (WRA) was based on Yang et al. [19,20]. The complete recommended doses of fertilization (CRF) included the amounts of nutrients, i.e., N, P, and K, that farmers usually use in mango orchards in Hainan, China: 161.8, 79.5, and 184.4 kg ha⁻¹ of N, P₂O₅, and K₂O, respectively.

Table 1. Fertilization plan for field trials during the two years.

Growth Period	Time	CRF (kg ha−1)				RRF and WRARRF (kg ha−1)				Frequency
		Urea	CF-I	CF-II	KS	Urea	CF-I	CF-II	KS
Shooting	25 August–15 October	200	500	-	-	160	400	-	-	5 times
Flowering	5 November–5 December	60	360		90	48	288		72	3 times
Fruit expansion	20 January–10 March	-	-	500	150	-	-	400	120	5 times
Total		260	860	500	240	208	688	400	192	13

Urea, CF-I, and CF-II refer to urea fertilizer contains 46% N, compound fertilizer contains 15-15-15 of N-P₂O₅-K₂O, and compound fertilizer contains 15-6-23 of N-P₂O₅-K₂O. KS = potassium sulfate contains 52% K₂O. Frequency = number of fertilizations in each growth stage.

shows the amount of nutrients and time of fertilization for each treatment. The recommended doses of fertilization and time of application were based on the studies of Zhang et al. [21]. Organic fertilizer in the form of chicken manure (30% organic matter and a total N of 4.0%, based on dry weight) was added to all the treatments at a dose of 5000 kg ha⁻¹. The WRA and organic fertilizer were added to a hole (50 cm long, 30 cm wide, and 30 cm deep) within and below the irrigation line of the fruit tree before shoot promotion in the first of July each year.

2.3. Soil Analysis

A composite soil sample (0–20 cm) was used to determine the basic soil properties before the addition of any fertilizers. The composite soil sample was collected randomly from each plot (3 per plot) and then mixed together to make one sample. The soil samples (0–20 cm) used to study the effect of fertilization treatments on soil nutrient availability were collected after fruit harvest. The soil texture was determined by the pipit method, while the soil organic matter was determined by the dichromate oxidation method [22]. In the pipit method, the soil was first physically and chemically distributed, and then sedimentation was used to quantify each fraction (clay, silt, and sand). A pH meter was used to measure the soil’s pH in a 1:2 soil: water ratio. Using 2 M potassium chloride, the soil’s available nitrogen was extracted and then determined by the Kjeldahl method. The available soil phosphorus was extracted using sodium bicarbonate solution (0.5 M, pH 8.5) according to Olsen method [22]. The ammonium molybdate reaction was used to quantify the amount of extracted P, and a spectrophotometer at 660 nm was used to measure it eventually. Ammonium acetate was used to extract the available potassium, which was subsequently quantified using a flame photometer.

2.4. Leaf Analysis

During the mature stage of autumn shoots, plant leaf samples were collected from the middle layer of the tree’s mature top crown. Leaves were picked from the four directions of east, west, south, and north to form a mixed sample for each treatment. Mango tree plant samples were cleaned with distilled water and dried in an oven at 70 °C, and their dry weights were then noted. N, P, and K concentrations were measured by digesting the oven-dried samples using a mixture of H₂SO₄ and H₂O₂ as per Burt [22] instructions. N, P, and K in the digested plant samples were determined according to the methods in the soil analysis section. The techniques used in the soil analysis section were followed to determine the amounts of N, P, and K in the digested plant samples.

2.5. Determination of Fruit Quality

After the full ripening, the total number of fruit per tree was counted, weighed and expressed as yield per hectare. At the same time, eight fruits were randomly picked from each tree in the four directions of east, west, south, and north and brought back to the laboratory. After the fruit matures naturally, quality indicators such as soluble sugar (SS), vitamin C (VC), titratable acid (TA), and soluble solids (TSS) were measured based on the standard methods in AOAC [23]. Soluble sugar was determined by the 3,5-dinitrosalicylic acid colorimetric method, while VC was determined by the 2,6-dichloroindophenol titration method. TA was determined by the acid–base titration method, while fruit-soluble solids were determined by refraction. Two indices (fruit type index (FTI) and edible rate (ER) were calculated to evaluate the fruit quality. FTI is the ratio of the longitudinal diameter to the transverse diameter of the fruit. ER is the ratio of the mass of the edible part to the total mass of the fruit. In all the above-mentioned measurements, the analysis was performed in duplicates.

2.6. Economic Evaluation

The fruit output value was based on the Hainan mango prices of USD 0.84 and 0.89/kg in the 2021~2022 and 2022~2023 growing seasons, respectively. The organic fertilizer cost was USD 112/t, the water-retaining agent price was USD 4.2/kg, and other production costs were calculated based on the market price of the season. The details of the costs of fertilization, WRA, and other agricultural practices are shown in

Table 2. Cost (USD ha⁻¹) of WRA, fertilization, agricultural process, and total cost.

Growth Season	Treatment	WRA	Fertilizers	Other	Total
2022	CRF	0	1085	8295	9380
	RRF	168	868	8295	9331
	WRARRF	168	868	8295	9331
2023	CRF	0	1193.5	9205	10,399
	RRF	0	954.8	9205	10,160
	WRARRF	0	954.8	9205	10,160

. The calculation formulas related to the partial productivity of chemical fertilizer (PFP), vario-cost ratio (VCR), and economic benefits (EB) were as in Xiao et al. [24] and Zhang et al. [25]:

$$\mathrm{P}\mathrm{F}\mathrm{P}=\mathrm{total}\mathrm{fruit}\mathrm{yield}\mathrm{of}\mathrm{each}\mathrm{treatment}/\mathrm{amount}\mathrm{of}\mathrm{chemical}\mathrm{fertilizer}\mathrm{application}$$

$$\mathrm{V}\mathrm{C}\mathrm{R}=\mathrm{net}\mathrm{income}/\mathrm{fertilization}\mathrm{cost}$$

$$\mathrm{EB}=\mathrm{Gross}\mathrm{return}-\mathrm{Total}\mathrm{cost}$$

$$\mathrm{O}\mathrm{u}\mathrm{t}\mathrm{p}\mathrm{u}\mathrm{t}-\mathrm{i}\mathrm{n}\mathrm{p}\mathrm{u}\mathrm{t} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}=\mathrm{G}\mathrm{r}\mathrm{o}\mathrm{s}\mathrm{s} \mathrm{r}\mathrm{e}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{n}/\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{c}\mathrm{o}\mathrm{s}\mathrm{t}$$

2.7. Data Processing

Two way-ANOVA and Tukey’s test at a 95% confidence level were run by Orginpro 2022b. The principal component analysis and correlation matrix were run by R software version 4.1.1 by ‘factoextra’ and ‘corrplot’ in the R library, respectively.

3. Results

3.1. Soil Nutrient Availability and Leaf Mineral Content

Table 3. Effect of fertilization and WRA treatments on soil nutrients availability.

Growth Season	Treatment	N (g kg−1)	P (g kg−1)	K (g kg−1)
2022	CRF	50.54 ± 1.37 A	38.04 ± 0.36 A	72.82 ± 1.09 A
	RRF	46.32 ± 0.33 B	34.29 ± 0.57 C	67.99 ± 0.66 B
	WRARRF	50.02 ± 0.85 A	36.26 ± 0.37 B	71.74 ± 1.05 A
2023	CRF	54.52 ± 1.11 a	37.59 ± 0.78 a	74.42 ± 1.00 a
	RRF	49.87 ± 0.68 b	34.73 ± 0.61 c	68.80 ± 0.82 b
	WRARRF	54.62 ± 0.89 a	36.23 ± 0.42 b	73.67 ± 1.25 a
	Treatment	*	*	*
	Year	*	ns	ns
	Treatment × Year	*	ns	ns

Means (±standard deviation, n = 3). For each variable, values followed by a different uppercase letter (year 2022) or lowercase letter (year 2023) are statistically different at the p < 0.05 level according to Tukey’s test. “ns” means no significant (p > 0.05); “*” means significant (p ≤ 0.05).

shows the effect of the tested treatments on soil nutrient availability, i.e., nitrogen (N), phosphorus (P), and potassium (K), in 2022 and 2023. The main effect of fertilization treatments significantly (p < 0.05) affected the soil nutrient availability. The season effect and the interaction of treatment x year (T × Y) significantly (p < 0.05) affected just the soil N. Reducing the fertilization doses (RRF) for mango in the studied tropical soil by 20% significantly (p < 0.05) minimized the nutrient availability compared to the complete fertilization doses (CRF). CRF increased N, P, and K availability in the soil by 9, 10, and 7%, respectively, compared to RRF in the first year, while in the second year, these increases were 9, 8, and 8%, respectively. The addition of a water-retaining agent (WRA) to the mango plants fertilized with RRF compensated for the nutrient reduction by increasing the availability of N, P, and K. The nutrient availability in the soil fertilized with CRF or WRARRF was similar. The highest significant values of nutrient availability were found in the soil treated with CRF or WRARRF without significant (p > 0.05) differences between the two treatments.

The leaf mineral content of the mango plant was significantly affected by the addition of water-retaining agent (WRA) and fertilization reduction (

Table 4. Effect of fertilization and WRA treatments on mango leaf mineral content.

Growth Season	Treatment	N (mg kg−1)	P (mg kg−1)	K (mg kg−1)
2022	CRF	16.26 ± 0.18 C	1.13 ± 0.08 B	10.48 ± 0.24 B
	RRF	17.03 ± 0.54 B	1.11 ± 0.03 B	10.37 ± 0.53 B
	WRARRF	18.02 ± 0.19 A	1.18 ± 0.09 A	11.23 ± 0.39 A
2023	CRF	17.00 ± 0.67 b	1.16 ± 0.06 b	10.95 ± 0.38
	RRF	18.07 ± 0.78 ab	1.15 ± 0.06 b	10.94 ± 0.58
	WRARRF	18.84 ± 0.32 a	1.23 ± 0.05 a	11.69 ± 0.68
	Treatment	**	*	*
	Year	ns	ns	ns
	Treatment × Year	*	ns	ns

Means (±standard deviation, n = 3). For each variable, values followed by a different uppercase letter (year 2022) or lowercase letter (year 2023) are statistically different at the p < 0.05 level according to Tukey’s test. “ns” means no significant (p > 0.05); “*” means significant (p ≤ 0.05); “**” means extremely significant (p ≤ 0.01).

). The leaf N of mango ranged between 16.26 and 18.84 mg kg⁻¹, while leaf P ranged between 1.11 and 1.23 mg kg⁻¹. The treatment and the interaction of treatment × year (T × Y) significantly (p < 0.05) affected the leaf-N. The highest non-significant values of K and P in mango leaf were found in WRARRF in both 2022 and 2023. Leaf K ranged between 10.37 and 11.69 mg kg⁻¹ and the highest values were found in WRARRF in 2023 with no significant differences between the years and the interaction of treatment × year. The plants grown on the soil amended with WRARRF, in most cases, exhibited higher leaf nutrient content compared to CRF and RRF. N, P, and K in mango leaf increased by 11, 4, and 7% in the first year and by 11, 6, and 7% in the second year as a result of WRARRF compared to CRF.

3.2. Mango Fruit Yield and Quality

The water-retaining agent (WRA) and fertilization treatments significantly affected the fruit yield per plant and the total yield, but the fruit weight did not respond (

Table 5. Effect of fertilization and WRA treatments on mango fruit yield.

Growth Season	Treatment	Fruit Weight (g)	Fruit Number per Plant	Yield (kg plant−1)	Yield (kg ha−1)
2022	CRF	167.9 ± 3.2	197 ± 9 B	33.3 ± 1.3 B	16,667 ± 633 B
	RRF	165.5 ± 3.1	194 ± 7 B	32.4 ± 1.4 B	16,200 ± 726 B
	WRARRF	162.1 ± 4.0	224 ± 11 A	36.3 ± 1.2 A	18,133 ± 625 A
2023	CRF	169.2 ± 2.5	206 ± 8 b	34.7 ± 1.9 b	17,333 ± 971 b
	RRF	168.8 ± 2.1	200 ± 12 b	33.6 ± 1.3 b	16,800 ± 638 b
	WRARRF	165.9 ± 2.1	233 ± 16 a	39.1 ± 1.2 a	19,567 ± 585 a
	Treatment	ns	**	**	**
	Year	ns	**	**	**
	Treatment × Year	*	**	**	**

Means (±standard deviation, n = 3). For each variable, values followed by a different uppercase letter (year 2022) or lowercase letter (year 2023) are statistically different at the p < 0.05 level according to Tukey’s test. “ns” means no significant (p > 0.05); “*” means significant (p ≤ 0.05); “**” means extremely significant (p ≤ 0.01).

). The highest significant values of fruit yield per plant and the total fruit yield were recorded in the plants grown in the soil amended with WRARRF. Mango fruit yield ranged between 16,200 and 19,567 kg ha⁻¹, and the highest significant values were found in WRARRF in both 2022 and 2023. The values of the fruit yield per plant and the total fruit yield per hectare in CRF and RRF were very similar compared to the WRARRF values. The highest significant value of fruit number per plant was found in WRARRF in the two studied years. The addition of WRARRF to mango plants recompensed the nutrient reduction and increased fruit yield per plant and the total fruit yield per hectare by 12.0% and 11.9% in the first year and by 16.4% and 16.5% in the second year compared to RRF.

The quality traits of mango fruit were significantly affected by CRF, RRF, and WRARRF treatments, while the year and the interaction treatment × year (T × Y) were not significant (p > 0.05) (

Table 6. Effect of fertilization and WRA treatments on mango fruit quality.

Growth Season	Treatment	TSS	SA	VC (mg/100 g)	TA (%)	TSS/TA	SA/TA	FTI	ER
2022	CRF	17.42 ± 0.55 B	13.13 ± 0.47 B	23.26 ± 1.25 B	0.39 ± 0.02 A	44.30 ± 0.62 C	33.39 ± 0.27 C	1.56 ± 0.06 B	67.95 ± 3.73
	RRF	18.63 ± 0.92 A	15.69 ± 0.44 A	24.27 ± 1.02 B	0.37 ± 0.02 B	50.81 ± 0.57 B	42.81 ± 0.66 B	1.63 ± 0.06 A	71.74 ± 4.85
	WRARRF	19.71 ± 0.69 A	16.16 ± 0.28 A	25.69 ± 0.53 A	0.34 ± 0.01 C	57.40 ± 0.76 A	47.10 ± 1.05 A	1.61 ± 0.07 A	70.89 ± 3.27
2023	CRF	17.27 ± 0.76 b	13.27 ± 0.41 c	23.22 ± 1.05 b	0.38 ± 0.02 a	45.46 ± 1.67 c	34.93 ± 1.03 c	1.59 ± 0.08 b	68.13 ± 5.66
	RRF	18.87 ± 0.84 a	15.90 ± 0.31 b	24.44 ± 0.67 b	0.35 ± 0.01 b	53.91 ± 1.04 b	45.43 ± 0.52 b	1.69 ± 0.08 a	72.44 ± 3.39
	WRARRF	19.94 ± 0.63 a	16.73 ± 0.43 a	25.89 ± 1.34 a	0.32 ± 0.02 c	61.70 ± 1.13 a	51.79 ± 1.13 a	1.68 ± 0.08 a	71.12 ± 4.28
	Treatment	*	*	*	*	*	*	**	*
	Year	ns	ns	ns	ns	ns	ns	ns	ns
	Treatment × Year	ns	ns	ns	ns	ns	ns	ns	*

Means (±standard deviation, n = 3). For each variable, values followed by a different uppercase letter (year 2022) or lowercase letter (year 2023) are statistically different at the p < 0.05 level according to Tukey’s test. “ns” means no significant (p > 0.05); “*” means significant (p ≤ 0.05); “**” means extremely significant (p ≤ 0.01).

). The lowest values of mango fruit quality parameters, except titratable acid (TA) were found in the plants treated with CRF. Mango fruit quality parameters can be arranged according to the effect of the treatments in descending order: Although the difference was not significant, in general, WRARRF registered the highest values, thus showing that reducing the fertilization and adding WRA can increase the fruit quality.

3.3. Economic Evaluation

The partial fertilizer productivity (PFP) of the added chemical fertilizers is shown in Figure 1. The tested treatments significantly affected PFP values in the two studied years. The lowest significant values were found in CRF, while the maximum values were found in WRARRF in 2022 and 2023. WRARRF increased the PFP value by 36% and 41% compared to CRF, respectively, in the first and second years. The treatments can be arranged according to their effects in PFP in the descending order: WRARRF > RRF > CRF.

The fruit value of mango plants ranged between USD 14,000 and 17,532 ha⁻¹ in the two studied years (

Table 7. Economic evaluation (USA dollar ha⁻¹) of the fertilization and WRA treatments.

Growth Season	Treatment	Gross Return (USA Dollar ha−1)	Output–Input Ratio	VCR of WRA
2022	CRF	14000 ± 532 B	1.49 ± 0.06 B	-
	RRF	13608 ± 610 B	1.46 ± 0.07 B	-
	WRARRF	15232 ± 525 A	1.63 ±0.06 A	9.7
2023	CRF	15531 ± 870 b	1.49 ± 0.08 b	-
	RRF	15053 ± 572 b	1.48 ± 0.06 b	-
	WRARRF	17532 ± 525 a	1.73 ± 0.05 a	24.4
	Treatment	**	**
	Year	**	**
	Treatment × Year	**	**

Means (±standard deviation, n = 3). For each variable, values followed by a different uppercase letter (year 2022) or lowercase letter (year 2023) are statistically different at the p < 0.05 level according to Tukey’s test. “**” means extremely significant (p ≤ 0.01).

). The fruit value in the second season was higher than the first one in all the treatments. Reducing the fertilization doses by 20% (RRF) did not cause any significant reduction in the fruit value in the two seasons compared to the complete recommended doses (CRF). The addition of WRAs to mango plants fertilized with RRF increased fruit value by 8.8% and 12.9% in the first and second years, respectively. The highest significant fruit value in the two years was found in the plants grown in soil amended with WRARRF. However, WRARRF exhibited superiority in the second year over all the other treatments.

The vario-cost ratio (VCR) of WRA in the current study was 9.7 and 24.4 in the first and second years, respectively (Table 7). The value of VCR in the second season was much higher compared to the first season. The input–output ratio was significantly affected by the tested treatments in the two seasons. The input–output ratio ranged between 1.46 and 1.73, and the lowest values were found in CRF and RRF, while the highest significant value was found in WRARRF. WRARRF increased the input–output ratio by 8.5% and 11.6% compared to CRF and RRF, respectively, in the first year and by 16.1% and 16.9% in the second year.

Figure 2 illustrates the effect of tested treatments on the economic benefit. The economic benefit ranged from USD 4277 to 7372 ha⁻¹ in the two studied years. The economic benefit in the second season was higher than the first season in all the treatments. Reducing the fertilization by 20% (RRF) did not cause a significant reduction in the economic benefit in the two seasons compared to the complete recommended doses (CRF). The economic benefit of the soil fertilized with CRF and RRF was similar in the first and second seasons. The highest significant economic benefit in the two years was found in the plants grown in soil amended with WRARRF. WRARRF increased the economic benefit by 28% and 38% compared to CRF and RRF, receptively, in the first year and by 44% and 51% in the second year.

3.4. Multivariate Analysis

We used the principal component analysis (PCA) and the correlation matrix to investigate the complex interactions between mango fruit yield and nutrients. Figure 3 shows the correlation matrix between the available soil nitrogen (Ns), phosphorus (Ps), potassium (Ks), plant content of nitrogen (Np), phosphorus (Pp), potassium (Kp), yield, partial fertilizer productivity (PFP), total soluble solid (TSS), soluble sugar (SA), vitamin C (VC), titratable acid (TA), solidity–acid ratio (TSS/TA), sugar–acid ratio (SA/TA), fruit type index (FTI), and edible rate (ER). EB was positively correlated with Ns, Ps, Ks, yield, and PFP. Moreover, yield, PFP, TSS, SS, and VC were positively correlated with Ns, Ps, and Ks. According to the PCA in Figure 4, the economic benefit, fruit value, yield, and fruit quality were significantly related with soil nutrient availability. The loading evaluation also showed that the economic benefit, fruit value, yield, and fruit quality were found in the soil treated with WRARRF.

4. Discussion

The findings demonstrate that increased nutrient availability in the soil caused an increase in nutrient absorption, which in turn increased mango tree production. The N, P, and K in mango leaf increased by 11%, 4%, 7% in the first year and by 11%, 6%, and 7% in the second year as a result of WRAs addition. Moreover, the highest significant values of nutrients availability were found in the soil treated with WRAs or with the complete recommended doses of N, P, and K. The addition of WRAs to mango plants fertilized with low doses of fertilizers compensated for the nutrient reduction by increasing the N, P, and K availability and uptake. WRAs have the ability to improve plant nutrient accumulation and regulate soil nutrient content by controlling the soil water content and soil microbial abundance composition [12]. The principal component analysis confirmed that the mango fruit yield was associated with the addition of WRAs and significantly correlated with nutrients availability and uptake. Increasing nutrient availability and uptake by plants as a result of WRAs has been also reported in other studies, e.g., Xu et al. [12] and Baak et al. [26]. The addition of WRA not only increases nutrients availability but also enhances their absorption and regulates their release into the soil according to the plant’s needs, which preserves the added fertilizers from leaching [12].

The addition of water-retaining agents (WRAs) led to a significant increase in mango yield and fruit quality under conditions of low fertilization rates. Although fertilization doses were reduced by 20%, the addition of WRAs led to an increase in mango fruit yield by 12–16%. Water-retaining compounds were used successfully in improving the productivity of many crops [11,12,14]. WRA’s high capacity for both water absorption and retention means that it improves plant growth by keeping soil moisture levels high [12,15,27]. WRAs are soil amendments that can promote plant growth by improving soil structure, aggregation, and water availability [27,28,29]. The incorporating of WRAs into the soil can enhance its permeability, porosity, and water-retention capacity, hence improving soil conditions that facilitate plant growth [13,29]. The results of the principal component analysis revealed that most of fruit quality indexes were associated with the addition of WRAs. The composition of biological compounds that control fruit quality characteristics in mango is linked to the regulation of water relations [8]. The addition of WARs contributed to keeping soil moisture and preventing the plants from being exposed to stress, which caused increases in the quality of the fruits [11,15]. The availability of water and nutrients led to an improvement in the process of photosynthesis, which contributed to increased growth and quality [3,8]. The application of a water retention compound has been shown in other experiments to increase soil water content, which in turn increases the photosynthetic rate and yield of spring millet [28].

WRAs gave the maximum fruit value and of economic benefit, which was USD 17532 and 7372 ha⁻¹, respectively. The main reason explaining the increasing of fruit value and economic benefit is the highest increase in mango fruit yield as a result of WRAs addition. A global meta-analysis conducted by Zheng et al. [29] confirmed that WRAs can increase crop yield by 12.8–17.2%. In our study, the addition of 40 kg ha⁻¹ of WRAs caused 38% and 51% increases in the economic benefit in the first and second year, respectively. The addition of 38–65 kg ha⁻¹ of WRAs to some crops, e.g., cotton, tomato, and cucumber, caused a significant increase in the economic benefits [29]. Loading low fertilization doses (20% reduction) with WRAs reduced costs and at the same time increased mango fruit production, which increased the fruit value and the economic benefit. WRAs are added to soil once and can continue working for 4–5 years [12,15,18]. The Food and Agriculture Organization of the United Nations considers a VCR > 2 to be economically reasonable [24]. The addition of WRAs resulted in a vario-cost ratio (VCR) of 9.7 and 24.4 in the first and second years, respectively, indicating that its use is economically reasonable and with interesting prospects for mango production in tropical soils. The findings clearly confirm the superiority of WRAs in the second season compared to the first one, which confirms that WRAs are materials with an extended effect and are an effective tool for sustainable agriculture management. WRAs increased the partial productivity of the chemical fertilizer (PFP) value by 36% and 41%, respectively, in the first and second year. The result can be due to its positive effect on the availability and uptake of nutrients that compensated for the lower supply of fertilizers [12,14,15].

5. Conclusions

The findings of the current study presented a practical solution that guarantees mango farmers high incomes while preserving the environment by reducing fertilization rates. The findings of the current study prove, based on two-year field studies, that adding polyacrylamide/attapulgite as a water-retaining agent increases economic benefit up to 51% and reduces fertilization doses by 20%. The water-retaining agent increased the partial fertilizer productivity up to 41% and caused 16.5% increases in the mango fruit yield. The water-retaining agent serves as a soil amendment, supporting the soil-retaining nutrients provided through fertilization and preventing them from leaching. Studying the ability of the water-retaining agent to absorb nutrients and the rates of release of the elements is necessary in different typses of soil.