Natural Organic Residues Enriched with Ca and Mg: Application in Lettuce Grown with Water Blade

Abstract

Natural organic residues (NOR), a residue from extracting organic matter from soils with NaOH, has shown to be an interesting and promising environmental adsorbent, due to its high surface area and porosity, in addition to the diversity of functional groups. In this context, this work developed a material from NOR enriched with Ca^II and Mg^II and evaluated its application in lettuce crops irrigated over water in sandy and clay soil. NOR was extracted from a soil collected in Alagoas, Brazil, with 25% organic matter content. The nutrients were inserted into NOR by adsorption in a multielement solution of Ca^II and Mg^II (10 mg L^-1) at pH 8.0. The highest adsorption rate of NOR (78.92 and 55% for Ca^II and Mg^II, respectively) occurred at pH 8.0. The characterization of NOR showed a content of organic matter (OM) = 15.40, C/H ratio = 0.185 and E₄/E₆ = 2.18 and characteristic bands of OH, C=C and Si-O stretching were observed in the samples. Lettuce cultivation with NOR was conducted in a protected environment for sandy and clayey soils. The results obtained demonstrate the influence of nutrients adsorbed in NOR and water blade on lettuce cultivation, especially in its variables of commercial interest.

Keywords:
humic substances; irrigation; nutrients; Lactuca sativa L.

Introduction

Population growth and demand for food have driven the expansion of agriculture worldwide.¹ The rapid development of the agricultural sector has intensified the use of chemical inputs to improve soil fertility and increase crop productivity.¹ On the other hand, excessive use of chemical fertilizers can cause significant environmental impacts, such as soil acidification, eutrophication of water bodies, and alteration of the carbon and nitrogen cycle.¹ Therefore, several agricultural activities have sought alternatives that enable the reduction in the use of chemical fertilizers, maintaining high crop productivity and in some cases the use of materials originating from biomass.²

The use of inputs of natural origin has been highlighted in recent years,² in particular, the use of humic substances (HS).³-⁵ Humic substances (HS) are the main components of natural organic matter (NOM) present in soil and water and are formed by biochemical and chemical reactions in the decomposition process of plant and microbial residues.⁶ As their main characteristic, HS allow greater water retention, better aeration and, consequently, greater resistance to erosion, and can act as complexing agents for metal ions in the soil solution, reducing the toxicity of these elements.⁷-⁹

Natural organic residues (NOR) are the poorly soluble fraction of HS under all pH conditions.⁶,¹⁰ It is known to be an active solid phase of soils and represents approximately 50-70% of HS of terrestrial origin.⁷,¹⁰,¹¹ NOR has a large surface area with various functional groups, such as esters, methoxyalkanes, carboxylates, and polar aromatic groups.⁷,¹²,¹³ These characteristics enable a wide range of applications in environmental remediation studies of organic and inorganic compounds.¹³,¹⁴ Furthermore, studies¹³,¹⁵,¹⁶ have shown that NOR interacts with organic pollutants, reducing their availability and consequently the potential toxic effects that these pollutants can cause in the soil.

NOR also plays a role in soil fertility systems in conjunction with mineral fertilization. Studies by Skowronska et al.,¹ demonstrate that interactions between NOR and mineral fertilizers enhance nutrient availability in the soil, increase carbon and nitrogen content, and improve fertility. Goveia et al.¹⁷ demonstrated NOR adsorption capacity for essential plant micronutrients (Zn, Co, Fe, Ni, Cu, Mn, and Mo) and evaluated their release in water.

Despite being the predominant component of HS, NOR is still underexplored in research regarding its efficiency in the adsorption/desorption process of micronutrients and their release into the soil.¹⁷ Few studies⁶ assess the influence of NOR on the availability of metals such as Ca and Mg in the soil or evaluate its capacity to supply these micronutrients to different agricultural crops.

In addition to the need for natural inputs associated with nutrients for agricultural purposes, water consumption is also a concern due to climate change and water shortages in several regions of the planet, particularly in semi-arid areas and regions prone to desertification in Brazil.¹⁸ Proper water management can help prevent nutrient leaching, reduce costs, and preserve aquatic ecosystems.¹⁸,¹⁹

Lettuce cultivation requires proper irrigation management, as its composition is 95% water. Maintaining a continuous water layer in the crop ensures that soil moisture remains above 80% throughout the growth cycle, allowing water replacement in the leaf area, which experiences intense evapotranspiration.²⁰,²¹ Thus, this study aimed to develop a low-cost material derived from NOR, a residue of NOM, enriched with essential plant nutrients (Ca^II and Mg^II) and to evaluate its application in lettuce cultivation irrigated over a water blade in both sandy and clay soils. The goal was to explore strategies to enhance plant development, reduce environmental impacts, and conserve water resources.

Experimental

Soil collection and NOR extraction

Soil samples were collected in the municipality of Floriano Peixoto, in the state of Alagoas (Northeastern Brazil), near the Pratagi River. The composite samples were transferred to wooden trays and air-dried until reaching a constant mass. They were then sieved using plastic sieves with 2 mm openings.

HS were extracted through alkaline extraction using NaOH, the most widely used methodology in this field, as recommended by the International Humic Substances Society (IHSS).⁶ Specifically, HS were extracted using 0.1 mol L^-1 NaOH for 4 h, with a soil-to-extractor ratio of 1:10, under a nitrogen atmosphere. The low-solubility fraction, known as NOR, was separated by centrifugation and thoroughly washed with deionized water.¹³,¹⁹,²²

Characterization of NOR

The determination of the organic matter (OM) content in the NOR samples was performed in quintuplicate by gravimetric analysis using a muffle furnace (7LAB), heating 10.0 g of the sample for 4 h at a temperature of 650 °C.¹⁷ The Fourier transform infrared spectroscopy (FTIR) spectra of the NOR samples were obtained using KBr pellets (sample-to-KBr ratio of 1:100) on a Varian 640 IR spectrometer.²³ The ratio between the absorbances at 465 and 665 nm (E₄/E₆) ratio was determined by measuring approximately 2.0 mg of NOR in 10 mL of a 0.05 mol L^-1 NaHCO₃ solution, followed by absorbance readings at 465 and 665 nm on a DR 3900 spectrometer.²³,²⁴ The samples were characterized by ¹³C nuclear magnetic resonance (NMR) with cross polarization (CP) and magic angle spinning (MAS) on a Bruker Avance III 400 MHz spectrometer, with a rotation of 5 kHz, contact time of 2 ms, relaxation waiting time of 5 s, and 11,000 scans.¹⁷,¹⁹

Enrichment of NOR with Ca^II and Mg^II

NOR enriched with Ca^II and Mg^II was prepared in adsorption tests using a multielement solution. 50.0 g of NOR were mixed with 150 mL of a 10 mg L^-1 metal solution. The enrichment was carried out at pH 4.0, 6.0, and 8.0. The solution was kept under constant stirring at 150 rpm for 120 min at 25 ± 0.2 °C. Afterward, the enriched NOR was separated. The adsorption efficiency of NOR for Ca^II and Mg^II was determined by calculating the difference between the initial concentration of the adsorbent (C₀, in mg L^-1) and the equilibrium concentration of the adsorbent (C_e, in mg L^-1), which was expressed relative to the initial concentration, according to equation 1. The equilibrium Ca and Mg concentration were determined by microwave-induced plasma atomic emission spectrometer (MP-AES).

(1) $$\text{Adsorption}(\mathrm{\%})=\frac{{\mathrm{C}}_0-{\mathrm{C}}_{\epsilon }}{{\mathrm{C}}_0}\times 100$$

Application of Ca^II and Mg^II enriched NOR in irrigated lettuce cultivation

Two soils, classified as sandy and clayey, were used, and were collected at a depth of 0.0-0.2 m. Their characteristics are presented in Table 1. The lettuce used was curly lettuce (Lactuca sativa L.).

Table 1
Chemical characteristics for sandy and clayey soils collected in semi-arid region in Alagoas State, Brazil, used in the cultivation of curly lettuce

pH (H2O)	OM / %	Concentration / (mg L-1)
		P	K	Na	Ca	Mg	Al	H
Sandy soil
5.0	0.8	9.0	62	31	280.6	109.4	6.3	28.3
Clay soil
5.2	1.0	9.0	101	33	160.3	72.9	16.19	24.2

The crops were grown in plastic containers with a volume of 4 L and 4 kg of properly sieved soil. The irrigation system was a localized drip irrigation system with drip tapes that had a flow rate of 2 L h^-1, with each container containing a dripper. Four water depths were used for irrigation (L1 = 50; L2 = 75; L3 = 100; and L4 = 125% of crop evapotranspiration). NOR enriched with Ca^II and Mg^II was added to the soil surface according to the defined levels: H0 = 0.0; H2 = 2.0; H4 = 4.0; and H6 = 6.0 g.

The design adopted for both soils was a randomized block design (RBD), with four replications, in a 4 × 4 factorial scheme, resulting in 16 treatments and 64 experimental units (Figure 1). The treatments were represented by: four levels of NOR enriched with Ca^II and Mg^II (H0 = 0.0; H2 = 2.0; H4 = 4.0; and H6 = 6.0 g) and four water depths (L1 = 50; L2 = 75; L3 = 100; and L4 = 125% of crop evapotranspiration) (Table 2). The treatments were the same for both soils used. The common fertilization for all treatments was carried out based on the recommendations of the 5^th approach for the use of correctives and fertilizers in soils (Table 2).¹⁹,²⁵

Table 2
Calculation of fertilization for lettuce crops based on the recommendations of the 5^th approach for the use of correctives and fertilizers in Minas Gerais (Ribeiro et al.²⁵)

Mineral fertilization / (g plant-1)
Urea (CH4N2O)	0.175
Phosphor (P)	3.50
Potassium (K)	0.43

Figure 1
Diagram demonstrating the randomized block design (RBD) of the experiment carried out in a greenhouse using curly lettuce. One plant per bucket represents a portion of the experiment.

Variables analyzed after the application of NOR enriched with calcium and magnesium in lettuce crops

Thirty days after transplanting the seedlings in the greenhouse, the plants were harvested and taken to the laboratory for analysis. Twelve lettuce variables were analyzed: chlorophyll index, canopy diameter, plant height, fresh mass of the aerial part, dry mass of the aerial part, number of leaves, leaf area, fresh mass of the root, dry mass of the root, root size, water content in the leaves, and water use efficiency.

Results and Discussion

Characterization of NOR

NOR is the most stable fraction of NOM, as it is bound to inorganic colloids. In this study, we chose not to use aggressive extractants to isolate NOR to avoid altering the humic structure of the material.²² Organic matter content in the NOR samples are lower than that reported by Jacundino et al.,²² for NOR extracted from soils in the same region but from different periods (Table 3). However, as expected, NOR has a high ash content (84.60%), confirming the results described in the literature.²⁶

Table 3
Organic matter content, C/H atomic ratio and E₄/E₆ ratio of the NOR sample

Sample	OM / %	C/H	E4/E6
NOR	15.4	0.185	2.18

The C/H ratio refers to the degree of aromaticity, and the higher the ratio, the greater the presence of aromatic structures.²⁷ The C/H values obtained were lower compared to the results reported in the literature.¹⁴,¹⁸

The E₄/E₆ ratio is an indicator of the level of aromaticity of the humic material. High values of this ratio suggest a structure with low aromaticity, while low values indicate high aromaticity. According to the literature,¹⁷ ratios lower than 4 are associated with a greater presence of condensed aromatic structures, while values higher than 4 indicate a smaller quantity of these structures. A reduction in the E₄/E₆ ratio is associated with an increase in molar mass, condensation of aromatic carbons, intensified humification of aromatic structures, and greater conjugation of double and single bonds, as aromaticity is inversely related to the number of aliphatic groups. The NOR in this study presented E₄/E₆ ratio values lower than 4, indicating a greater number of condensed aromatic structures, corroborating the data from the C/H ratio. Valle²⁶ determined E₄/E₆ ratio values for NOR samples, which ranged from 2.28 to 6.33. The highest values were attributed to the influence of the d-d transition in Fe^III ions of the hematite structure.

From FTIR, it was possible to identify the main functional groups present in NOR (Figure 2). In the spectrum, a broad band in the region of 3450 cm^-1 is attributed to the stretching of OH groups that may originate from alcohols, phenols, and carboxylic acids.²⁸ The band observed at 1600 cm^-1 is attributed to the C=C stretching of aromatic groups. In the absorption region at 1000 and 1200 cm^-1, absorption peaks are evident, which can be attributed to the C-O stretching of alcohols, phenols, and carbohydrates, as well as Si-O stretching.²⁹

Figure 2
FTIR spectrum (KBr) obtained for NOR samples.

By analyzing the ¹³C NMR spectra, it was possible to assign the percentages of the different functional groups present in NOR (Table 4).

Table 4
Levels of functional groups present in NOR determined by ¹³C NMR (D₂O)

Sample	C-aliphatic 0-65 ppm / %	Esters-OH 65-110 ppm / %	C aromatic 110-140 ppm / %	Phenols 140-160 ppm / %	-COOH 160-190 ppm / %	-CO 190-220 ppm / %
NOR	27.86	18.48	27.90	8.93	13.34	3.49

The ¹³C NMR results (Table 4) indicate that NOR has equivalent aliphatic and aromatic group contents, which corroborates the E₄/E₆ results. Aliphatic carbon was also evident in the infrared spectra, with C=O stretching further supporting these findings.

The characterization of NOR is important to understand the functional groups present in the structure of the material, as well as its affinity with nutrients. The results suggest affinity between the functional groups present in the NOR and Ca and Mg ions as described by Jacundino et al.²² and Souza et al.²⁷

Enrichment of NOR with Ca^II and Mg^II

The enrichment of NOR was studied by adsorbing Mg^II and Ca^II at pH 4.0, 6.0 and 8.0. The highest adsorption percentages were 78.92% for Ca^II and 55% for Mg^II at pH 8.0. The adsorption of metal ions onto NOR may be attributed to its functional groups, which interact with the metals, facilitating their adsorption.²⁸

An increase in solution pH reduces the H⁺ concentration in the medium, leading to the attraction of positively charged metal ions by the hydroxyl groups of NOR. This interaction enables ion exchange in equivalent amounts, thereby promoting adsorption.²⁸,³⁰

According to Goveia et al.,¹⁷ the adsorption of micronutrients onto NOR occurs through various mechanisms, including ion exchange, surface adsorption, chemical adsorption, and complexation, which may take place simultaneously. Studies in the literature highlight the efficiency of NOR in metal adsorption, including Cu, (Nguyen-Phuong et al.²⁸), Ni (Zhang et al.⁸), and Zn (Zhong et al.⁹), demonstrating its potential as an adsorbent for metal cations in contaminated environments.

The formation of NOR with the adsorption of micronutrients will enable the incorporation of Ca and Mg into the soil, allowing a gradual desorption process to occur. This process ensures that these micronutrients remain available in the soil to meet the nutritional needs of cultivated lettuce, contributing to its growth and development, as these nutrients are essential for plant growth.

Application of NOR enriched with Ca^II and Mg^II in lettuce cultivation

After lettuce cultivation with the application of NOR enriched with Ca^II and Mg^II, the data related to the variables number of leaves (NL), chlorophyll index (SPAD), fresh matter of the aerial part (FMAP), fresh matter of the root (FMR), dry matter of the aerial part (DMAP), dry matter of the root (DMR), water content in the leaves (WCL), and water use efficiency (WUE) were subjected to variance analysis using the Shapiro-Wilk test (p < 0.05). The data are presented in Tables 5 and 6. Significant results were subjected to regression analysis.

Table 5
Analysis of variance of lettuce crop in response to NOR levels and water blade (WB) grown in sandy soil, variables: number of leaves (NL), chlorophyll index (SPAD), fresh matter of the aerial part (FMAP), fresh matter of the root (FMR), dry matter of the aerial part (DMAP), dry matter of the root (DMR), water content in the leaves (WCL), and water use efficiency (WUE)

Variation factor	NL	Mean square: biometric variables
		FMAP	FMR	DMAP	DMR	WCL	WUE	SPAD
NOR	3	1924.58c	38.26a	33.32c	13.29c	18.52a	1203.28c	39.10b
Water blade (WB)	3	10894.14c	785.82c	116.04c	5.93c	36.50c	1941.89c	45.79c
H × WD	9	867.07c	247.52c	34.66c	11.60c	45.99c	766.63c	22.09c
Blocks	3	50.15	9.19	5.35	0.66	3.40	33.59	3.64
Residue	45	35.69	9.95	4.03	0.86	5.58	16.92	6.50
CV / %		6.79	9.06	19.56	16.30	23.41	8.25	6.70

• Significant at
• ^a
  0.05,
• ^b
  0.01,
• ^c
  0.001 by the F-test. CV: coefficient of variation.

Table 6
Analysis of variance of lettuce crop in response to NOR levels and water blade (WB) grown in sandy soil, variables: number of leaves (NL), chlorophyll index (SPAD), fresh matter of the aerial part (FMAP), fresh matter of the root (FMR), dry matter of the aerial part (DMAP), dry matter of the root (DMR), water content in the leaves (WCL), and water use efficiency (WUE)

Variation factor	NL	Mean square: biometric variables
		FMAP	FMR	DMAP	DMR	WCL	WUE	SPAD
NOR	3	211.62ns	34.79b	1.82 ns	9.52c	3.92c	156.50b	37.05c
Water blade (WB)	3	5979.7c	291.03c	23.07c	45.52c	2.19b	6916.74c	25.33b
H × WD	9	292.36b	111.97c	3.40c	13.92c	2.37c	129.65c	6.95ns
Blocks	3	89.79	11.74	1.32	1.23	1.06	24.68	6.63
Residue	45	87.09	7.44	0.83	0.38	0.49	27.77	4.90
CV / %		8.37	10.13	17.85	16.20	1.09	7.23	8.61

• Significant at
• ^a
  0.05,
• ^b
  0.01,
• ^c
  0.001 by the F-test. ns: not significant; CV: coefficient of variation.

Chlorophyll index

It is well known that Ca^II and Mg^II play a crucial role in plant growth, and their availability in the soil is essential for crop development. As shown in Tables 5 and 6, for lettuce cultivation in sandy and clayey soils, respectively, the chlorophyll index (SPAD) variable was statistically significant at a 5% probability level. In sandy soil, SPAD was significant for the factors NOR and water blade (WB), whereas in clayey soil, it was significant for the interaction between these factors (NOR × WB).

For the NOR factor, variations in SPAD can be explained by a quadratic regression (R² = 0.92). The quadratic response indicates a decrease in the chlorophyll index as NOR levels increase, reaching a minimum point at a dose of 2.64 g of NOR. Beyond this point, the index increases with higher NOR levels. These results are presented in Figure 3.

Figure 3
Chlorophyll index (SPAD) for the NOR and water blade factors for the cultivation of lettuce in sandy soil. (a) For NOR a quadratic effect with concavity downwards was observed (minimum point) while for (b) the water blade the linear model was the one that best adjusted the data.

This increase may be related to a higher concentration of Ca and/or Mg present in NOR levels, which promotes a greater quantity and, consequently, greater availability of nutrients in the environment. These results may be attributed to the effects of Ca and Mg on plant growth and, consequently, on chlorophyll. Mg is a central constituent of chlorophyll; therefore, its availability directly influences the chlorophyll content in plants. Calcium, on the other hand, plays a crucial role in crop development by contributing to cell wall formation, and its presence is as important as that of major nutrients such as nitrogen and phosphorus. Studies in the literature³¹,³² have reported similar findings, showing an increase in chlorophyll content in leaves with magnesium application.

For water blade in sandy soil, SPAD was negatively affected, as shown by the regression analysis (Figure 3b), with increasing irrigation levels. The increase in irrigation water can lead to nitrogen losses through leaching from the zone of highest root system concentration, consequently reducing the availability of this element.¹² A decrease in nitrogen results in lower greenness intensity and reduced chlorophyll content. In sandy soil, which has a higher sand content, a greater number of macropores, and is generally poor in NOM, the nitrate ion retention capacity is low, making it more susceptible to leaching.³³ Sangoi et al.³⁴ observed that nitrogen losses were numerically greater and occurred more rapidly in sandy soil with low cation exchange capacity (CEC).

For the study in clayey soil, a significant interaction was found between NOR levels and water depths, with the results following a polynomial regression model (Figure 4), where both factors exhibited a quadratic model with upward concavity (equation 2). In clayey soils, characteristics such as pH, cation exchange capacity, and NOM content influence nutrient adsorption/desorption reactions in the soil-plant relationship.³⁵ Clayey soils allow greater micronutrient mobility, facilitating plant adsorption processes, as demonstrated in the studies by Scheren et al.³⁶

Figure 4
Chlorophyll index (SPAD) for interaction between NOR × water blade, for the cultivation of lettuce in clayey soil.

Fresh matter of aerial part, fresh matter of the root, dry matter of aerial part, dry mass of the root

NOR levels combined with irrigation levels significantly affected FMAP, FMR, DMAP, DMR in both sandy and clayey soils (Tables 5 and 6). FMAP production was 195.79 g plant^-1 in sandy soil (Figure 5a; equation 3) and 121.89 g plant^-1 in clayey soil (Figure 6a and equation 7) when the NOR level and water blade were 2.06 g and 234.25 mm (92% of evapotranspiration (ETc), respectively. For clayey soil, the highest FMAP was obtained at NOR levels of 6.0 g and 125% of ETc water blade.

Figure 5
(a) Fresh matter of the aerial part (FMAP), (b) dry matter of the aerial part (DMAP), (c) fresh matter of the root (FMR) and (d) dry matter of the root (DMR) analyzed in lettuce grown in sandy soil.

Figure 6
(a) Fresh matter of the aerial part (FMAP), (b) dry matter of the aerial part (DMAP), (c) fresh matter of the root (FMR) and (d) dry matter of the root (DMR) analyzed in lettuce grown in clay soil.

ETc is characterized as crop evapotranspiration

The response surface, with quadratic behavior, of FMR in clayey soil showed the highest weight, corresponding to 39.29 g plant^-1, at H = 5.09 g and WB = 279.30 mm (109% of ETc) (Figure 6c; equation 9). In sandy soil, the response of FMR in lettuce showed a strong interaction between the factors (NOR and water blade), as demonstrated by the isoquant curves, which were classified as hyperbolas (Figure 6c; equation 9). The highest value of DMR for sandy soil was 7.69 g plant^-1, using H = 2.74 g and WB = 238.61 mm (98% of ETc). For clayey soil, the DMR yield response showed the same pattern as FMR, with a strong interaction between the factors NOR and water depth (Figure 6d; equation 10).

Lettuce is a vegetable that requires nutrients, especially during the final phase of its vegetative development, when it demands a significant increase in nutrition and a good supply of water throughout its growth cycle. This supports the high influence of water blade on the variables analyzed, as well as the impact of NOR on the supply of Ca and Mg (Figures 5a-5d). Additionally, it can be observed that cultivation in sandy soil yielded better results in aerial dry mass, which may be related to the availability of nutrients in the soil. When a greater amount of water is applied to sandy soil, it enhances the mobility of nutrients, which, however, are more easily leached and, consequently, less available for absorption by the plants.

Water content in the leaves and water use efficiency

The WCL and WUE variables showed significant interactions between the two factors studied for both soils (Tables 5 and 6). Lettuce plants grown in sandy soil exhibited their maximum water content in the leaves at 94.38% (Figure 7a; equation 11). For clayey soil, it was not possible to determine the maximum or minimum levels of the factors within the range, as seen in Figure 7c (equation 13). However, for the combination of not NOR application (0.0 g) and a 125% ETc water depthblade (274.95 mm), the highest water content observed was 93%. It was observed that WCL increases linearly as water blade increase, positively affecting water content in the leaves. On average, the green parts of most plants have a water content between 80 and 90%, which varies according to environmental water conditions.³⁷

Figure 7
(a) Water content in the leaves (WCL), (b) water use efficiency (WUE) in the cultivation of lettuce in sandy soil, (c) water content in the leaves (WCL) and (d) water use efficiency (WUE) in the cultivation of lettuce in clay soil.

The highest water use efficiency (WUE) was observed for lettuce grown in sandy soil (90.68 kg ha^-1 mm^-1), which occurred with the application of a water blade of 110.22 mm (43% of ETc) and 1.28 g of NOR (Figure 7b; equation 12). For clayey soil, the maximum WUE was approximately 90 kg ha^-1 mm^-1, with the maximum NOR level applied and a water blade of 113 mm (51% of ETc) (Figure 7d; equation 14). Based on the graph showing the variation of WUE in clayey soil (Figure 7d), a further increase in the NOR level could result in a higher WUE, since, within the range of levels, it is not possible to determine the maximum point for this factor.

The highest WUE was observed for lettuce grown in sandy soil (90.68 kg ha^-1 mm^-1), which occurred with the application of a water blade of 110.22 mm (43% of ETc) and 1.28 g of NOR (Figure 7b; equation 12). For clayey soil, the maximum WUE was approximately 90 kg ha^-1 mm^-1, with the highest application of NOR and a water blade of 113 mm (51% of ETc) (Figure 7d; equation 14). Based on the graph showing the variation of WUE in clayey soil (Figure 7d), a further increase in the NOR level could result in a higher WUE, as the maximum point for this factor cannot be determined within the given range.

Conclusions

The application of NOR enriched with calcium and magnesium has shown agronomically significant effects on lettuce cultivation, both independently and in combination with water management. The presence of Ca and Mg in the organic matrix enabled more efficient use of these elements, enhancing soil fertility and supporting crop development. This effect is particularly relevant for agricultural systems in semiarid regions or areas prone to desertification, where sustainable soil management strategies are crucial.

Studies have shown that higher doses of NOR (6 g) had the greatest impact on lettuce growth parameters. Furthermore, studies involving water blades have proven effective in supporting water management in semiarid or desertification-prone areas. It is also noteworthy that the interaction of Ca and Mg via NOR promotes beneficial effects, such as improved soil fertility, which supports agricultural processes in cultivar planting. The use of natural residues as source of nutrients can be expanded depending on their interaction with other essential nutrients.

In addition to direct productivity benefits, the use of NOR represents an environmentally responsible approach, as it adds value to natural residues and reduces dependence on synthetic inputs. This not only mitigates the environmental impact of agriculture but also contributes to the conservation of natural resources and the closure of nutrient cycles.

In this context, the findings represent a valuable contribution to global scientific knowledge, demonstrating how organic residues can be strategically transformed into sustainable agricultural inputs. This research supports international efforts toward resilient, circular agricultural practices aligned with the Sustainable Development Goals (SDGs), particularly in combating soil degradation and enhancing food security.

Data Availability Statement

The authors declare that all data is included in the text and any additional information can be requested from the corresponding author.

Acknowledgments

The authors gratefully acknowledge the financial support provided by Alagoas Research Support Foundation (FAPEAL) under grant number (APQ2022021000006). We also thank Federal University of Alagoas for providing research infrastructure.

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Edited by

• Editor handled this article:
  Andrea R. Chaves (Executive)

Publication Dates

• Publication in this collection
  06 Oct 2025
• Date of issue
  2025

History

• Received
  19 May 2025
• Published
  27 Aug 2025