Introduction

Egypt is facing significant water scarcity challenges. The country is located in a mostly arid region and is characterized by hot dry climate. The average annual precipitation in Egypt ranges from less than 25 mm in some parts of the Western Desert to around 200 mm in the northern coastal regions. Due to the limited rainfall, Egypt depends mainly on the Nile River for its water resources1,2,3. The Nile provides approximately 90% of Egypt’s freshwater supply, and the agricultural sector consumes about 85% of the country’s total water consumption for irrigation purpose. The rapid increase in population and the increasing demand for water have necessitated the adoption of efficient water-saving strategies in irrigated agriculture4,5.

The concept of water use efficiency (WUE) involves the challenge of how to increase crop productivity without increasing water consumption. Enhancing WUE ensures maximizing yield per unit of water, rather than yield per unit of land. Therefore, improving WUE will be reflected in sustainable use of limited water resources6,7. Deficit irrigation (DI) and the incorporation of soil amendments, including hydrogels, have been proposed to enhance WUE and mitigate negative impacts of water stress in arid and semi-arid regions8,9.

Deficit irrigation (DI) is defined as an irrigation management strategy, in which plants are intentionally exposed to a certain level of water stress throughout the entire growing season or at certain growth stages. DI implies that water is supplied at levels below crop water requirements or full evapotranspiration (ETc)10,11. Although yields will be reduced under deficit irrigation, the reduction in irrigation costs and water conservation may compensate for yield reductions. When the amount of land under irrigation is constrained by limited water availability, the economic returns to water will be maximized by reducing the depth of water applied and increasing the area of land under irrigation12. In order to ensure the success of DI irrigation, it is necessary to consider the ability of the soil to retain water. Sandy soil is characterized by excessive infiltration rate, low fertility and low water and nutrient holding capacity, therefore DI in sandy soils may expose plants to severe water stress. Therefore, the success of DI in these soils is more likely with modification in soil hydro-physical characteristics13,14.

Super absorbent hydrogels may offer a promising solution for improving water retention and hydro-physical characteristics of sandy soils. Super absorbent hydrogels are substances that can absorb and retain large amounts of water, about 400–500 times its own weight15,16. They have been used in agriculture as a soil amendment to improve water management and increase crop yields. According to17,18, the addition of hydrogel increased the water holding capacity of sandy soils and reduced deep percolation losses.

One of the main benefits of hydrogels in agriculture is their ability to retain water and release it slowly over time, which can help plants survive drought conditions. Hydrogels can also reduce water consumption and increase WUE by reducing both runoff and deep percolation. In addition to water management, hydrogels can also improve soil structure and nutrient availability. They can help loosen compacted soil, increase aeration, and improve the soil’s ability to hold nutrients. This can lead to healthier plants with better root development and increased yields19.

Vegetable crops production such as lettuce, is considered a major challenge in terms of efficient irrigation management, due to the crop’s sensitivity to water stress. Several studies have indicated that vegetable crops are very sensitive to water deficits. Water stress in vegetable crops is often accompanied by significant yield reductions and poor marketing quality12,20. The sensitivity of vegetable crops, especially fast-growing ones, to water stress may be attributed to two main reasons. First, the shallow root system of most vegetable crops restricts their ability to absorb water from deeper soil layers under water deficit conditions21,22. Secondly, marketing of vegetable crops depends on their signs of freshness, which is a direct result of their water content and deteriorates significantly when exposed to water stress23,24. From this perspective, incorporation of hydrogels with DI as drought-resistant strategy can sustain vegetable productivity and quality and save water in water-limited arid and semi-arid regions25,26.

Previous studies have concluded that hydrogels improve soil physical properties while also increasing WUE and growth parameters in arid and semi-arid areas. However, information concerning the effectiveness of hydrogel in reducing the adverse effect of water stress on leafy vegetables is still limited. Moreover, more research is needed to determine the optimal application rates of hydrogels for different crops and soil types. The aim of this study is to study the effect of super absorbent hydrogel application rate on soil hydro-physical properties and lettuce plant growth parameters, under different soils and irrigation levels.

Materials and methods

Experimental site

The study was carried out in pot experiment during two consecutive seasons 2022 and 2023 in Nagrig, Basyoun city, El-Gharbia governorate, Egypt (30° 58′ N, 30° 52′ E). Soils in the pots were collected from the surrounding farmlands in El-Gharbia governorate, from the top 20 cm. The collected soils were air-dried and sieved through a 2 mm sieve to remove any organic residues or visible roots and then homogenized by mixing thoroughly before use. The chemical and physical properties of the soils are shown in Tables 1 and 2. The study area is characterized by an arid climate, the mean annual precipitation and reference evapotranspiration are 50.3 mm and 1670 mm, respectively. The average monthly values of weather conditions in the experimental site are shown in (Table 3).

Table 1 Chemical properties of the experimental soils.
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Table 2 Physical and hydro-physical soil properties of the tested soils.
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Table 3 Average of monthly meteorological data during 2022 and 2023 in the experimental site.
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Experimental design

To study the effect of different levels of super-absorbent hydrogel on soil hydro-physical properties under deficit irrigation, a factorial experiment was carried out. The experimental treatments included (1) irrigation amount factor at four levels: 100 (I1), 85 (I2), 70 (I3) and 60% (I4) of ETc, (2) soil type factor at three levels: clay (S1), loamy sand (S2), and sandy-clay soil (S3) and (3) four hydrogel concentrations: 0 (H0), 0.1(H1), 0.2 (H2) and 0.3% (H3) (w/w). Treatments were arranged in split plot experimental layout in a Randomized Complete Blocks Design (RCBD) with three replicates for each treatment. Soil type was arranged in the main plots and hydrogel levels, irrigation amounts and their combinations, were considered as sub-plots. The experiment is composed of three replications for each treatment and each replication is made of one pot; in total 144 pots were used.

Transplantation and irrigation management

Lettuce seedlings were obtained from commercial nursery and transplanted in plastic pots (35 cm inner diameter, and 30 cm height). Each pot was filled with 12 kg of air-dried soil and placed outdoors in open field conditions. Two seedlings were transplanted in each pot on the 1st of March of each growing season. Pots were irrigated with tap water to field capacity, chemical analysis of irrigation water is shown in Table 4. Plants were thinned to one plant per pot 25 days after transplanting. Nitrogen (N), phosphate (P), potassium (K) and microelements were uniformly applied to all pots according to the recommendations of Egyptian ministry of Agriculture.

Table 4 Chemical analysis of irrigation water.
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The irrigation volumes were determined by measuring volumetric soil water content using the VH400 dielectric soil moisture sensor probe (Vegetronix, Inc., Riverton, Utah, USA). The output of the VH400 sensors was displayed using the VG-METER-200 handheld moisture meter (Vegetronix, Inc., Riverton, Utah, USA). Irrigation events were scheduled when soil moisture was depleted below field capacity in the top 10 cm of soil due to shallow root depth of lettuce (5–10 cm). The amount of moisture depletion was calculated before each irrigation and summed to obtain the seasonal water consumption.

Plant measured parameters

The harvest took place when the plants reached the marketable size (67–70 days after transplanting in pots). Chlorophyll content was measured immediately before harvest using SPAD-502 portable nondestructive chlorophyll meter (Konica Minolta Inc., Tokyo, Japan). Chlorophyll measurement was performed on a single fully developed leaf in the middle of lettuce head. After harvesting, plant vegetative growth parameters (e. g., fresh and dry weight of aerial part, root wet and dry weight, and number of leaves) were measured and recorded. Dry weights were determined after oven drying at 70 °C until constant weight is reached.

Water use efficiency (WUE) (kg m−3) indicates the biomass produced per unit of water used by the crop. It was evaluated as the ratio between the marketable yield (kg ha−1) and seasonal crop evapotranspiration (mm).

Soil hydro-physical properties

The impact of different concentrations of hydrogel on hydro-physical characteristics of different soils was evaluated. The evaluation was based on studying the change in both the hydraulic conductivity coefficient and the bulk density of the soil before and after the experiment using undisturbed soil samples27.

Saturated hydraulic conductivity Ks (cm h−1) was calculated using Darcy’s law as follow:

$$\frac{Q}{A} = K_{s} \frac{dh}{{dl}}$$

where Q: water flow rate passing through soil column (cm3 s−1), A: area of soil column (cm2), dh/dl: hydraulic gradient. Soil bulk density \(\rho\) (g cm−3) was determined using the following equation:

$$\rho = \frac{{M_{d} }}{{V_{t} }}$$

where \({M}_{d}\): mass of dry soil (g) , \({V}_{t}\): total or bulk volume of soil (cm3).

Statistical analysis

Analysis of variance of the obtained data in 2022 and 2023 seasons was carried out using Statistix 10 statistical software. For means comparisons, least significant difference (LSD) was applied (P < 0.05 significance level).

Ethical approval

We confirm that all methods were carried out in accordance with relevant guidelines in the method section.

Results

Effect of different treatments on lettuce growth parameters

Data in Fig. 1 and Table 5 shows the effect of different treatments (irrigation levels, hydrogel application rates and soil types) on head fresh weight (g/plant). The average values for both growing seasons revealed that irrigation levels significantly affected lettuce fresh weight (P ˂ 0.05%). The highest values were achieved by full irrigation I1 (100% ETC), compared to I4 (DI with 60% ETC), which gave the lowest fresh weight values.

Figure 1
figure 1

Head fresh weight as affected by irrigation level, hydrogel rate and soil type.

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Table 5 Effect of irrigation level, hydrogel rate and soil type on head fresh weight of lettuce (g/plant).
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Concerning the effect of hydrogel, data in Table 5 indicated that hydrogel application positively affected crop fresh weight in both seasons. Fresh weight significantly increased with increasing hydrogel application rate. The highest value of fresh weight (g/plant) was 361.74 g followed by 325.69, 282.92 g for H3, H2 and H1, respectively. While the lowest value was 153.06 g under the control treatment H0. Regarding the effect of soil type on fresh weight of lettuce, the data showed that clay soil had significantly higher fresh weight than loamy sand and sand clay soil.

Moreover, with regard to the effect of interactions, the data revealed that the highest values of fresh weight were obtained by I1H3S1 (irrigation at 100% of ETc, hydrogel application rate of 0.3 g/kg soil and clay soil), followed by I2H3S1 and I2H2S1, respectively. The difference in fresh weight between I1H3S1 and I2H3S1 was not statistically significant. Same trend was observed in sandy soil, where the decrease in fresh weight was not statistically significant between I1H3S3 and I2H3S3. On the other hand, the lowest fresh weight was obtained under I4H0S2 (Deficit irrigation at 60% ETc, without hydrogel application in sandy soil).

Similarly, dry weight and number of leaves increased with increasing hydrogel application rates for all soil types, Table 6. These parameters were higher with higher irrigation levels and in plants grown in clay soil, followed sandy clay and sandy soil, respectively.

Table 6 Effect of hydrogel application rate on lettuce growth parameters in different soil types.
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Effect of different treatments on water use efficiency

Data in Fig. 2 reveal that irrigation levels significantly affected water use efficiency (WUE). The highest WUE value was 10.01 (kg m−3) and was achieved under I2 (85% ETc), while the lowest WUE (8.61 kg m−3) was observed in fully irrigated plants (100% ETc). Concerning the effect of hydrogel application rate on WUE, data also showed that WUE was significantly affected by the hydrogel addition rate. The higher the hydrogel application rate, the greater WUE. Moreover, water use efficiency increased under H3 compared to H0 (untreated soil), from 5.91 to 13.52 kg m−3, from 4.65 to 10.85 kg m−3 and from 4.97 to 11.84 kg m−3 for clay, loamy sand and sandy clay soil respectively.

Figure 2
figure 2

Effect of irrigation level, hydrogel rate and soil type on water use efficiency (kg/m3).

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The interaction effect of different levels of irrigation, hydrogel application and soil type showed statistically significant variation for WUE. The highest WUE was recorded from I2H3S1 (15.23 kg m−3) followed by I2H2S1 (13.7 kg m−3) which was statistically similar to I3H3S1 (13.5 kg m−3). The lowest WUE was recorded from I1H0S2 (4.03 kg m−3). Given that I2H3S1 treatment resulted in the highest WUE and did not result in significant reduction in marketable yield (fresh weights), it can be adopted as irrigation water saving strategy. These results are consistent with who stated that hydrogels can enhance plant growth, plant productivity, and water use efficiency in arid and semiarid regions28,29.

Effect of different treatments on soil hydro-physical properties

The effect of hydrogel rates on bulk density ρ (g cm−3) and hydraulic conductivity k (cm h−1) of different soil types is presented in Table 7. The data showed that there was no significant interaction between irrigation level and hydrogel addition rate on soil bulk density and hydraulic conductivity. A variable response to hydrogel rates was detected depending on soil type.

Table 7 Effect of hydrogel on soil properties.
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As for clay soil (S1), The lowest bulk density and hydraulic conductivity values were 1.08 g cm−3 and 0.38 cm h−1 and were achieved under H3, while the highest were 1.19 g cm−3 and 0.42 cm h−1 and were achieved under H0. There was no significant difference between H1 and H2 in both ρ and Ks values.

For loamy sand soil (S2), there was a significant decrease in density and hydraulic conductivity values at all hydrogel levels. The decrease of both ρ and Ks was proportional to the increase in hydrogel application rate. The lowest bulk density and hydraulic conductivity values were 1.47 g cm−3 and 2.56 cm h−1 and were achieved under H3, while the highest were 1.68 g cm−3 and 2.67 cm h−1 and were achieved under H0. For sandy clay soil (S3), Ks values decreased with increasing hydrogel, but the decrease was only significant at H3 application rate. Similar tendency was observed for ρ, where the lowest values were achieved under high rates of hydrogel (Table 7).

Discussion

The results of this study showed that lettuce subjected to water stress had significantly lower yield and growth attributes than those with full irrigation requirements. These results are consistent with Kurunc30 and Ibrahim et al.31 who stated that water stress in lettuce is often accompanied by significant yield reductions and poor marketing quality. The sensitivity of lettuce to water stress may be attributed to its shallow root system that limits its ability to absorb water from deeper soil layers under water deficit conditions21,32,33. However, the application of super absorbent hydrogels mitigated these negative effects in water-stressed plants. The results revealed that hydrogel application enhanced fresh and dry weights, chlorophyll content and number of leaves. Despite the fact that lettuce plants are sensitive to water stress, hydrogel can be used to reduce the impact of water shortage (85% of ETc) without causing negative effects on production.

With respect to chlorophyll content, several studies have reported that chlorophyll can be used as a stress indicator32,34. Under water stress conditions, plants adapt to drought by stomatal closure to prevent water loss through transpiration. Stomatal closure results in degradation of photosynthetic pigments and the damage of chlorophyll structure under water deficits35,36. For all soil types, chlorophyl content significantly increased at higher hydrogel application rates, Table 6. Hence, it can be concluded that hydrogel can positively reduce plant water stress, enhances water availability and mitigates the negative effect of water deficiency on chlorophyll synthesis. These results are consistent with32,37, who reported significant decrease in chlorophyll content under water stress conditions and untreated soil.

Results also revealed that clay soil had significantly higher crop productivity than loamy sand and sand clay soil. This may be attributed to small and negatively charged particles of clay soils compared to sandy and sandy loam soils. In addition, clay soil often has a higher organic matter content leading to improved soil structure, water retention, and nutrient availability, all of which contribute to better crop growth and productivity38,39. In contrast, sandy soils are characterized by large pore spaces and large particles with less surface area for water absorption, leading to lower water retention and reduced plant available water27,40. Results revealed that hydrogel incorporation into sandy soil has improved the productivity and growth characteristics of the lettuce crop under DI conditions. The same trend of hydrogel for enhancing lettuce production under DI was also observed in clay soil as well. No significant difference in lettuce productivity under 85%ETc (I2) at hydrogel application rate of 0.3% for sandy and clay soils, Table 5. These findings are in line with Albalasmeh et al.41 and Sepehri et al.42, who reported that the crop productivity increased with increasing hydrogel concentration. Higher concentration of hydrogel can improve soil physical properties, enhance WUE and crop growth under DI conditions.

Based on lettuce yield and irrigation water amount, DI with 85% ETc (I2) was determined to be the best irrigation level in terms of WUE. According to Wallace43 and Howell44, WUE is the ratio of yield to plant water consumption and can be increased by maintaining the same yield while reducing irrigation water consumption. WUE was highest under I2 due to reduced amount of irrigation (the numerator in WUE equation), without significant yield reductions. Full irrigation may not achieve the highest WUE. Maximizing yield was the main objective of research studies in the twentieth century; recently the emphasis has changed to sustainable use of limited water resources. In areas experiencing water scarcity, the economic returns of water will be maximized by reducing the crop water consumption and increasing the irrigated area7,45,46.

Saturated hydraulic conductivity describes soil’s ability to transmit water when all pores are fully saturated. It provides information on the rate of water movement within the soil system, indicating possible water and nutrient leaching rates, which may affect plant growth and development and may lead to groundwater contamination27,47. The decrease in hydraulic conductivity values can be attributed to the hydrophilic properties of hydrogels. They swell by absorbing large volumes of water and decrease the drainage pore space between soil particles. As a result, more water is stored in the hydrogel structure and less water percolates from one soil layer to another. The results are also in agreement with48,49 who stated that Ks decreased with the increase in hydrogel application rate. When hydrogel concentration increased, the swelling of the hydrogel reduced the available paths for downward water movement. Ks values decrease considerably as soil becomes unsaturated since less pore space is filled with water49,50.

Regarding the effect of hydrogel on soil bulk density, the reduction in ρ values may be attributed to the displacement and rearrangement of soil particles around the swollen hydrogel particles. The hydrogel particles within the soil matrix absorb water and become larger in size. The soil volume increases and therefore the ratio of the dry soil mass to its volume decreases49,51. The obtained results are also in line with49 who reported that ρ values of fine textured soil was less affected by hydrogel application than the coarse sandy soil. They also stated that the minimum ρ value was found at the highest hydrogel rate (1.45 g cm−3), whereas the largest measured bulk density was found under the control (1.56 g cm−3). Soil bulk density (ρ) is a good indicator of soil compaction, the higher the bulk density the more compact it is. High ρ values tend to negatively influence soil aeration, and root growth. Hence, it can be concluded that hydrogels can decrease soil bulk density, improve soil porosity and enhance soil aeration.

Conclusion

For different soil textures, the incorporation of hydrogel under deficit irrigation has significantly enhanced plant fresh and dry weight, chlorophyll content, and number of leaves. These results indicate sustainable lettuce performance under DI when using hydrogels. The difference in fresh weight in clay soil between I1H3S1 and I2H3S1 was not statistically significant. The same trend was observed in sandy soil, where the decrease in fresh weight was not statistically significant between I1H3S3 and I2H3S3. This means that despite the fact that lettuce plants are sensitive to water stress, hydrogel can be used to reduce the impact of water shortage (85% of ETc) without causing negative effects on production. On the other hand, the lowest fresh weight was obtained under I4H0S2 (Deficit irrigation at 60% ETc, without hydrogel application in sandy soil). Reduction in saturated hydraulic conductivity of soils confirms that this hydrophilic hydrogel can swell by absorbing large volumes of water and consequently improve water retention in sandy soils.