Soil salinization severely threatens global crop production. To address this, a novel pH-sensitive, biodegradable semi-interpenetrating polymer network (semi-IPN) hydrogel, CMC-g-poly(AM-AMPS)/Pal/UF, was synthesized based on carboxymethyl cellulose (CMC), acrylamide (AM), 2-acrylamido-2-methylpropane sulfonic acid (AMPS), palygorskite (Pal), and urea-formaldehyde (UF). Engineered for arid environments, the hydrogel's architecture provides a massive water reservoir (272.01 g/g) while ensuring its eventual, complete integration into the soil ecosystem via biodegradation (>50 % mass loss in 90 days). Its NPK nutrient release was pH-dependent, with significantly slower release under acidic conditions (pH 4) compared to neutral (pH 7) and alkaline (pH 10) conditions, attributed to the protonation/deprotonation of functional groups and pH-influenced UF hydrolysis. The semi-IPN architecture, with UF chains physically intertwined throughout the network, acts as a structural brake on nitrogen diffusion, prolonging its release to similar to 71 % over 30 days even under alkaline conditions. Release kinetics were well-described by the Korsmeyer-Peppas model, indicating non-Fickian diffusion for nitrogen and Fickian diffusion for phosphorus and potassium. Application to saline-alkali soil significantly enhanced soil water retention, increased available NPK content and enzyme activities (sucrase, urease, phosphatase, etc.), while reducing soil pH, Na+, CO32-, HCO3-, ESP, and SAR. Consequently, the hydrogel conferred remarkable tolerance to saline-alkali conditions in wheat seedlings. It achieved this by correcting ion imbalances (lower leaf Na+/K+ ratio) and suppressing oxidative burst, which manifested as a comprehensive enhancement of the plant's antioxidant machinery and a restoration of normal growth. This work establishes a design principle for multifunctional hydrogels that simultaneously remediate soil and augment plant stress tolerance, presenting a paradigm shift from separate soil amendments and fertilizers to an integrated, biodegradable system for agriculture in salinized regions.