Synergistic Effects of Organic Materials and Clay on Maize Yield and Water Use Efficiency in Sandy Soil

Most physicochemical properties of soil organic matter (SOM) enhancement are related to soil structure. In poorly structured sandy soil, modifying the mechanical composition may be a prerequisite for effective SOM management and crop yield improvement. This study investigated the effects of organic materials and introduced clay (Quaternary red clay soil with 38.5% clay) on soil water dynamics, maize growth, root morphology, and water use efficiency (WUE) in a sandy field soil (4.3% clay). The treatments included: control (CK), biochar addition (BC), lignite organic fertilizer addition (LG), and biochar and lignite organic fertilizer combined with introduced clay (BCR, LGR). Compared with CK, organic amendments alone (BC, LG) increased the silt + clay content (SC) and SOM content by 4.3%–5.8% and 5.3–9.2 g kg−1 in the 0–20 cm soil layer (p < 0.05), and improved grain yield (GY) and WUE by 19%–37% (1428–3312 kg ha−1, p < 0.05) and 19%–41% (3.3–6.8 kg ha−1 mm−1, p < 0.05). The introduced clay (BCR, LGR) showed larger improvements: SC and SOM increased by 19.4%–21.2% and 7.0–11.0 g kg−1, and GY and WUE increased by 2385–5016 kg ha−1 and 4.1–9.5 kg ha−1 mm−1. Additionally, volumetric water content (VWC) was significantly linearly correlated with SC and SOM content (p < 0.05). Sandy soil with an SC of around 16% exhibited reduced SOM decomposition and stable GY. PLS‐PM analysis indicated that WUE was mediated by VWC as an intermediate variable, whereas organic amendments alone had a limited direct impact on VWC. The synergistic organic‐clay co‐amendment provides a feasible long‐term strategy for the sustainable development of sandy farmland.