This study targets key challenges in ameliorating the plow-layer soil of coastal saline soils. A field experiment under a wheat–maize rotation was established with six treatments: CK, control with no organic inputs; A1, 45 t ha⁻¹ organic manure; A2, 45 t ha⁻¹ organic manure + microbial inoculant; A3, 45 t ha⁻¹ organic manure + microbial inoculant + plastic-film mulching; A4, 90 t ha⁻¹ organic manure; and A5, 135 t ha⁻¹ organic manure. By applying high rates of organic manure alone or in combination with microbial inoculation and mulching, we aimed to strengthen soil water–salt regulation, improve plow-layer soil quality, and ultimately promote crop growth and yield formation. We further quantified treatment-induced shifts in soil physicochemical properties and linked them to crop growth and yield responses. The results indicated that, compared with CK, plow-layer soil organic carbon increased by 45.56% and 107.91% under A3 and A4, respectively, while soil salinity decreased by 70.57% and 67.42%. All manure-based treatments increased yield relative to CK, with the highest yields achieved under A3 and A4: wheat yield reached 7628.16 and 7888.01 kg ha⁻¹, and maize yield reached 8828.29 and 8716.01 kg ha⁻¹, respectively. Overall, high-rate organic manure—especially when integrated with microbial inoculation and plastic mulching—substantially enhanced soil fertility while alleviating salinity stress, resulting in an integrated “fertility build-up–salinity reduction–yield enhancement” amelioration effect. This technology package offers a feasible pathway for improving coastal saline farmland and stabilizing productivity under rotation systems, with strong potential for further on-farm demonstration and wider adoption.