Water scarcity, nutrient loss, and heavy metal contamination pose intertwined challenges to sustainable agriculture in arid and semi-arid regions. Despite advances in superabsorbent polymers (SAPs), few studies have integrated water retention, nutrient conservation, and pollutant immobilization into a single material platform using regionally abundant waste biomass and raw clay-both unmodified. Herein, we report a low-cost, green multifunctional superabsorbent composite (CFPAA/ATPs) synthesized via room-temperature, one-pot free-radical polymerization without inert gas protection, using Caragana fiber (CF), natural attapulgite clay (ATP), and acrylic acid (AA). The optimized composite (CFPAA/ATP-4) exhibits an equilibrium water absorption of 831.1 g/g in deionized water and 65.1 g/g in 0.9 wt% NaCl solution. It also shows rapid swelling kinetics across various media (different pH values, fertilizer solutions, salt solutions, and surfactant solutions) and maintains good cyclic stability over six cycles. Soil column experiments demonstrate that CFPAA/ATP-4 significantly enhances water and nutrient retention in sandy soil while effectively immobilized Pb2+ and Cd2+. Pot trials with soybean seedlings further confirm improved survival rates, plant height, and biomass accumulation in both shoots and roots. In summary, CFPAA/ATPs represent an eco-friendly, multifunctional material that integrates efficient water absorption, fertilizer retention, and pollution control into a single system, showing great potential for alleviating soil drought, enhancing fertilizer utilization, and promoting sustainable agricultural development in arid regions.