Waste-derived calcined crab shell coupled with ferric sulfate for rapid amelioration of soda saline-alkali soil and immediate maize sowing.

Rapid amelioration of soda saline-alkali soil is important where planting windows are narrow and pre-incubation is impractical. We evaluated calcined crab shell and anhydrous Fe2(SO4)3 as a simplified Ca-Fe amendment. After 25 d, the combined treatment (PM) lowered soil pH from 10.54 to 8.94, reduced carbonate by 93.0%, decreased exchangeable Na and exchangeable sodium percentage (ESP) by 39.2% and 29.9%, respectively, and increased water-soluble Ca to 2.21 mg kg-1, approximately three times the CK value. Relative to ferric sulfate alone, PM had higher water-soluble Ca at day 25 and lower exchangeable Na and ESP. XRD, FTIR, SEM, and HR-TEM indicated secondary mineral transformation after PM addition. Two new low-angle reflections were consistent with layered periodicity and resembled basal reflections reported for CaFe-LDH. Considered together with FTIR evidence of mineral transformation and microscopy evidence of layered morphology, these diffraction features support an LDH-like structural interpretation; CaFe-LDH represents one plausible but non-unique structural analogue among candidate layered phases, and a unique phase assignment requires phase-specific confirmation. In a separate immediate-sowing pot assay, PM achieved 100% seedling establishment and the best early growth among treatments, with chlorophyll content, shoot length, and root length reaching 35.02 SPAD, 15.38 cm, and 3.92 cm, respectively. Fe-driven acidification was the main driver of alkalinity reduction. The concurrent increase in water-soluble Ca and decreases in exchangeable Na and ESP were consistent with Ca-for-Na exchange. Overall, this strategy links shell-waste valorization with rapid in situ geochemical adjustment and supports immediate maize sowing under the tested pot conditions.