This study empirically evaluated the effects of a superabsorbent polymer (SAP)-based soil moisturizing agent on plant-available water (PAW) in soils with low water-holding capacity. Granitic weathered soil (Masato) and sandy soil were treated with two SAP types, TerraCottem and polyacrylamide (PAM), applied at varying rates, and the resulting changes in PAW were measured. The results indicated that SAP-treated samples showed significantly increased PAW compared to the control group. The most pronounced improvements were observed with the TerraCottem treatment at 1.0 g and the PAM treatment at 0.2 g. PAW increased by up to 110.1% in sandy soil. However, excessive SAP application was found to potentially cause adverse physical effects, such as soil structural instability and repeated swelling-shrinking cycles. Additionally, some SAPs showed the potential to cause a discrepancy between total water retention and actual plant-available water by retaining moisture within water potential ranges inaccessible to plants (pF > 4.2). These findings suggest that the effectiveness of the SAP application depends on complex interactions among soil texture, polymer application rate, soil water potential, and properties. In conclusion, SAPs represent a promising approach to alleviating water stress and stabilizing the growth of urban trees. However, tailored application strategies that account for site-specific soil properties are essential for optimizing their beneficial effects.