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The utilization of super absorbent polymer has revolutionized agricultural practices, particularly in terms of soil management. This innovative material, with its unique water-absorbing properties, holds immense potential in enhancing soil health and crop production.
One of the most significant impacts of super absorbent polymer on soil is its ability to affect soil physical properties. Among these properties, soil moisture content stands out as a critical factor that directly influences crop growth. The application of super absorbent polymer effectively suppresses the decline in soil moisture content within the application layer, thus exhibiting remarkable drought resistance with a duration that can span 15 to 20 days.
However, it is noteworthy that the soil moisture content beneath the application layer and in untreated areas decreases more significantly. This decrease is even more pronounced with increased polymer application rates. The mode of application also plays a role in determining the effect on soil moisture. Studies have shown that in agricultural fields, the use of furrow, hole, or mixed application methods with a rate of 45 kg/hm2 and a depth of 0–20 cm can significantly increase soil moisture content in the 0–40 cm layer by 7.5% to 12.7%. Notably, furrow application is more effective than hole or mixed application.
The reasons for these differences are multifaceted. Mixed application tends to distribute the polymer primarily in shallow soil layers, leading to expansion and contraction upon water absorption. This process can create soil pores connected to the atmosphere, resulting in soil water loss and surface crusting that reduces rainwater infiltration. On the other hand, hole application concentrates the polymer in smaller areas, reducing its contact with soil and water absorption capacity. Excessive concentration can create large expansion forces, leading to the formation of pores connected to the atmosphere and increased soil water evaporation. In contrast, furrow application, while also creating similar pores, is less likely to connect them to the atmosphere due to the overlying soil, resulting in slower soil water loss.
Furthermore, the effect of super absorbent polymer on soil physical properties is influenced by soil texture. As the polymer dosage increases, the aggregation of sandy soil becomes more prominent, while the effect on medium and heavy loam soils is less significant due to their inherent aggregation properties. The mechanism behind this increase in soil aggregation is the gel-like form of the polymer after water absorption, which effectively binds loose soil particles into clumps. Therefore, the application of super absorbent polymer is particularly effective in sandy loam and clay loam soils, where the higher content of clay and organic matter enhances the ion exchange process, thus inhibiting water absorption by the polymer.
The influence of super absorbent polymer on soil hydraulic properties is intricately linked to soil texture and pH levels. It is generally agreed that the application of super absorbent polymer significantly improves the water-holding capacity, reduces water evaporation, and decreases permeability in sandy soils compared to medium and heavy loam soils. Additionally, the polymer exhibits optimal water absorption in neutral soils, with a slight decrease in alkaline and acidic soils. Notably, the polymer can also alter soil pH levels; experiments have shown that it can reduce the pH of sandy clay and loam soils from 6.03 to 5.6.
The mode of application also plays a crucial role in determining the effect on soil hydraulic properties. Analysis of experimental data reveals that the water infiltration rate decreases significantly with furrow and mixed application methods compared to the control. Notably, the maximum decrease in infiltration rate can reach 45.1% for mixed application and 11% for furrow application. This decrease becomes more pronounced as the polymer application rate increases, leading to reduced deep soil water leakage. In this context, mixed application appears to be superior to furrow application. Furthermore, soil water evaporation is reduced by 15.37% to 50.42% with the application of super absorbent polymer compared to the control. Mixed application results in a further reduction of 6% to 25% compared to furrow application, with the lowest evaporation rates observed at a polymer application rate of 90 kg/hm2. This is attributed to the more uniform distribution of the polymer in the soil with mixed application, enabling it to effectively absorb water and reduce deep soil water leakage and evaporation. In contrast, furrow application results in a more concentrated distribution of the polymer, leaving unaffected leakage and evaporation channels.
