Organic–Inorganic Composite Amendments Improve Aggregate Stability and Soybean Yield Components in Yellow River Alluvial Sandy Soil

Yellow River alluvial sandy soil suffers from poor structural stability and low fertility, constraining soybean productivity. We hypothesised that co-applying straw return with an organic–inorganic composite would modify Fe–Al oxide fractions, promote water-stable macroaggregate formation, and thereby improve soybean physiological performance and yield components, with the ternary combination outperforming single or binary regimes. A soil-column experiment was conducted at Shandong Agricultural University Experimental Station, Tai’an, China, using a completely randomised design (n = 3) with six treatments: straw-only control (CK), organic fertiliser (JO), biochar (JB), organic fertiliser+attapulgite (JOA), biochar+attapulgite (JBA), and organic fertiliser+biochar+attapulgite (JOBA). Compared with CK, JOBA increased water-stable macroaggregates (>0.25 mm) by 268.82%, mean weight diameter by 89.88%, and decreased soil bulk density by 19.97%; it also increased catalase activity by 35.74%, pods per plant by 110.34%, seeds per plant by 128.57%, and 100-seed weight by 60.82%. The JOBA showed the greatest overall improvement, reflecting complementary functions among amendments. However, these findings were obtained in a controlled short-term column experiment; field validation and even long-term evaluation are required.