Impact of Maize Straw, Compost and Green Manure on Soil Physical Capacity of a Calcareous Clay Soil

The incorporation of organic carbon sources is a key strategy to improve soil physical quality and water availability in arid and semi-arid regions. However, the long-term effects of different maize biomass forms on soil water retention and pore structure in calcareous clay soils are still poorly understood. This three-year field experiment evaluated the impact of maize straw (MS), maize compost (MC) and maize green manure (GMM) applied at 10, 20 and 40 Mg ha−1, compared with an unamended control (C), on soil physical capacity in the 0–20 cm layer of a semi-arid calcareous clay. Undisturbed soil cores were collected annually, and soil water retention curves were determined to calculate total porosity (ΘS), field capacity (ΘFC), permanent wilting point (ΘPWP), available water capacity (ΘAWC), macroporosity (ΘMA), microporosity (ΘMI), air capacity (ΘAC), the S index and related hydraulic parameters. Maize compost was the most effective amendment. In particular, compost at 40 Mg ha−1 (MC4) significantly (p < 0.05) increased ΘAWC and improved the pore-size distribution compared with the unamended C. Across the three years, MC4 produced the strongest improvement in soil water retention, increasing ΘAWC by 41%–70% relative to the C, while high-dose compost and green manure markedly reduced macroporosity (up to 51.4%). GMM also significantly (p < 0.05) enhanced ΘMI and aeration compared with C, contributing to improved water retention and plant water use. Overall, the different maize biomass forms altered ΘS, ΘMA, ΘMI and the S index, with compost producing the largest and most consistent improvements in soil physical quality. These findings demonstrate that high-rate maize compost and green manure are promising management options to enhance soil water storage, hydraulic properties and resilience in arid and semi-arid calcareous clay soils.