Saline-alkaline water represents a major environmental challenge in arid and semi-arid regions, threatening agricultural productivity and ecological sustainability. While membrane processes such as nanofiltration and reverse osmosis offer viable desalination pathways, their performance is often limited by scaling caused by hardness ions. Effective pretreatment to remove hardness is therefore essential for stable and efficient membrane operation. This study developed an integrated sodium phosphate chemical softening and microfiltration (SPCS-MF) process for the pretreatment of agricultural saline-alkaline water. The platform achieved simultaneous effective hardness control and the recovery of valuable phosphorus fertilizer. Guided by geochemical modeling (Visual MINTEQ 3.1), we established a softening benchmark of 82% Ca2+ removal to prevent gypsum scaling in subsequent nanofiltration at 90% recovery. The optimized Na3PO4 softening achieved 84.5% Ca2+ and 44.8% Mg2+ removal within 1 min, demonstrating kinetics 30 times faster than conventional Na2CO3 softening. Microfiltration ensured robust solid-liquid separation, producing a high-quality effluent (turbidity <0.1 NTU, SDI15 < 5) suitable for NF feed. Notably, the SPCS-MF process generated amorphous calcium magnesium phosphate (ACMP) precipitates, which can be served as slow-release phosphate fertilizers. Rice cultivation trials demonstrated ACMP's agronomic performance comparable to commercial slow-release phosphate fertilizers, promoting leaf development and biomass yield. By transforming hardness ions into soil-beneficial fertilizers, our strategy presents a circular paradigm for agricultural water management, simultaneously addressing scaling control, water recovery and nutrient recycling.