Agricultural water-retaining agents represent adaptive chemical water-saving materials for farmland conservation in China. Functionally, they are super absorbent polymers (SAPs), also termed water absorbents or water-holding agents. These moderately cross-linked, water-swellable polymeric materials carry abundant hydrophilic groups such as carboxyl and hydroxyl groups. Widely adopted in modern agriculture, SAPs modify soil bulk density, increase total porosity and optimize aeration, thereby improving water, soil and fertilizer use efficiencies. Nevertheless, their field performance is constrained by physicochemical properties, soil conditions and climate. Accordingly, exploring the coupling among SAPs, soil moisture and soil physical properties, as well as their long-term action mechanisms, is practically valuable for water-efficient agriculture and green sustainable development.

1. Characteristics and Mechanisms of Water-Retaining Agents

1.1 Core Characteristics

Agricultural SAPs possess key traits that underpin their field performance.

First, strong water retention-release capacity. Upon water uptake, SAPs form dispersed gel systems that immobilize soil water, suppress free-water movement and mitigate evaporation and leakage, thus elevating soil moisture. During dry periods, stored water sustains root-zone moisture for continuous crop water supply. Most adsorbed water is bioavailable to crop roots with minimal backward flow or loss, stabilizing water supply for plant growth.

Second, long service life and stable drought resistance. Polyacrylamide-based SAPs, the dominant agricultural type, maintain field efficacy for several years through repeated water-absorption-desorption-drying cycles. Although water-absorbing capacity gradually declines over cycles until functional failure, their service lifespan satisfies multi-season drought mitigation for crops. Persistence depends on polymer type, soil texture and application rate.

Third, alleviation of crop drought stress. Soil-incorporated SAPs increase root-zone available water and reduce water loss, relieving drought-induced growth inhibition and supporting normal reproductive development. Under water scarcity, SAPs raise leaf water and chlorophyll contents, boost root vitality and improve crop physiological status; such mitigation strengthens with reasonable rate increases.

Fourth, regulation of crop transpiration and photosynthetic physiology. Soil-mixed SAPs lower leaf transpiration rate, elevate chlorophyll level and photosynthetic efficiency, modulate enzyme activity and optimize plant metabolism. Crops inherently vulnerable to drought gain more pronounced stress tolerance from SAPs amendment.

1.2 Mechanisms of Action

As polyelectrolytes, SAPs feature complex three-dimensional cross-linked networks. Their unique physicochemical structure governs water absorption-retention via combined physical and chemical adsorption.

Abundant hydrophilic groups distributed along polymer networks ionize upon water contact and bind water molecules through hydrogen bonding for physical water trapping. Ionized electrolytes generate osmotic pressure gradients against external solutions, driving continuous water ingress into polymer chains to achieve chemical water absorption.

Linear segments enable molecular-chain stretching and swelling to determine water-absorbing capacity, while cross-linked networks preserve structural integrity and prevent collapse. Synergy of both structural features enables SAPs to swell into water-insoluble hydrogels. After water release, the polymer framework recovers rapidly, enabling cyclic water uptake and reuse.

2. Effects of Water-Retaining Agents on Soil Properties

2.1 Effects on Soil Physical Properties

Soil moisture is critical for crop development. SAPs application slows topsoil moisture depletion and sustains long-term soil wetness for drought mitigation. Performance varies markedly with application methods. Conventional furrow, hole and soil-mixing applications all raise topsoil water content, with furrow application delivering the best overall outcomes.

Soil mixing readily forms atmosphere-connected macropores in topsoil, accelerating water loss, surface crusting and restricting rainwater infiltration. Hole application yields concentrated distribution with limited soil contact area, lowering water-absorption efficiency; local swelling may also generate pores that accelerate evaporation. By contrast, furrow-placed SAPs are covered by topsoil, cutting off atmospheric pore connections and minimizing water loss for prolonged water retention.

SAPs-mediated soil-structure improvement is texture-dependent. In loose sandy soils and sandy loams, hydrated SAPs gels bind discrete particles, facilitate aggregate formation and ameliorate soil architecture. In medium-to-heavy loams, well-developed aggregates, high clay and organic-matter contents suppress SAPs water absorption, leading to limited improvement. SAPs therefore work better in light-textured loose soils.

2.2 Effects on Soil Hydraulic Properties

SAPs modulate soil hydraulic behavior, modifying infiltration, evaporation and water-holding capacity. Such effects are governed by soil texture, pH and application approaches.

In textural terms, SAPs perform optimally in sandy soils: they enhance water retention and reduce evaporation and deep percolation, outperforming outcomes in medium-to-heavy loams. For pH response, neutral soils maximize SAPs water-absorbing performance; acidic or alkaline conditions partly suppress capacity, while SAPs can mildly buffer soil pH.

Application modes directly shape hydraulic modification. Both mixing and furrow placement reduce infiltration rate and deep percolation, with effects intensifying at higher rates. Soil mixing more effectively curtails leakage and evaporation owing to uniform polymer distribution across soil profiles that blocks water-loss pathways. Furrow application produces concentrated placement with limited spatial coverage and weaker intervention on soil water movement.

Numerous studies have documented SAPs influences on soil hydraulics. Still, inconsistencies across temperature-humidity regimes, observation durations and testing standards leave dynamic patterns and water-soil coupling mechanisms between amended and non-amended layers insufficiently understood, requiring further research.

3. Application Methods and Effects of Water-Retaining Agents in Agricultural Production

3.1 Seed Coating

Seed coating represents a lightweight, efficient SAPs-application technique. Seeds coated with SAPs-fertilizer-pesticide mixtures acquire a moisturizing protective film, supplying water and nutrients for germination, boosting biotic-abiotic stress tolerance, lowering seedling diseases and pests, and improving germination uniformity and rates. For wheat and rice, SAPs coating shortens emergence cycles, enhances seedling quality and stabilizes or raises grain yields.

3.2 Substrate Seedling Cultivation

SAPs blended into growing substrates suit industrial seedling raising and potted cultivation for vegetables, trees and ornamentals, supporting robust seedling production. Appropriate rates optimize substrate water-fertilizer conditions, increase chlorophyll content, photosynthetic rate and root activity, elevate physiological vigor and stress resistance, and improve seedling-quality index and biomass. Excessive dosage causes over-wet substrates with poor aeration, inhibiting root respiration and growth; hence precise rate control is essential.

3.3 Root Dipping

Root dipping is widely practiced for sweet potato, vegetable and sapling transplanting. Dipping root systems in SAPs solutions forms a moisturizing surface layer that prevents rapid root dehydration, shortens seedling recovery periods and increases survival rates. Post-transplant plants exhibit improved plant height, leaf area and biomass, benefiting economic-crop and sapling transplant production.

3.4 Direct Soil Application

Broadcasting, furrow and hole placement are dominant large-scale field-application strategies for food crops, cash crops and revegetation. Surface broadcasting is labor-suitable for large-scale planting of lawns and forages; furrow and hole placement serve targeted cultivation for field crops. Soil-incorporated SAPs elevate topsoil moisture, mitigate water-fertilizer deep leaching, improve soil fertility and nutrient-use efficiency, and optimize rhizosphere water-fertilizer environments. For wheat, maize, cotton and oats grown on sandy land, rational SAPs input improves photosynthesis and nutrient accumulation, upgrades grain quality and stabilizes yields, showing high value for water-saving agriculture and degraded-soil amelioration.

4.Conclusion

Possessing favorable water-absorption-release, drought-mitigation and soil-amelioration properties, agricultural SAPs constitute key chemical water-saving materials for China’s water-saving agriculture. Relying on three-dimensional cross-linked polymer networks, SAPs realize water storage via combined physical-chemical adsorption. Via seed coating, substrate mixing, root dipping and soil incorporation, they optimize soil physical-hydraulic status, improve water-fertilizer conditions, regulate crop physiology and strengthen stress resistance to realize quality-yield promotion.

SAPs have been widely deployed across agricultural scenarios with proven water-saving, yield-boosting and soil-improving benefits. Nevertheless, field efficacy remains constrained by soil texture, application approaches and ambient conditions. Long-term functional mechanisms and soil-water coupling dynamics demand deeper investigation. Future research should prioritize precise application technologies, site-specific schemes and long-term efficiency mechanisms. Optimized field-application systems will unlock SAPs potential in water conservation, soil improvement and yield stabilization, providing technical support for water-efficient farming, cultivated-land quality improvement and green agricultural sustainability in China.