Combined Application of Humic Acid and Attapulgite Improves Physical Structure and Nutrients in Coastal Saline-Alkali Soils

Coastal soils are subject to salinization, a process that degrades soil structure, exacerbates nutrient leaching, and depletes soil organic matter. Both organic fertilizers and clay amendments have been shown to play a key role in soil structure, nutrient availability, and soil health. The different dosages (0%, 3%, and 6%) of humic acid (HA3 and HA6) and attapulgite (AT3 and AT6) alone or in combined application (HAT3 and HAT6) on the quality of coastal saline-alkali soil were studied. The results showed that in terms of soil physiochemical properties, the combination of HA and AT (HAT) significantly (p < 0.05) improved soil moisture content and soil CEC by 4.01%–5.25% and 53.45%–401%, and significantly (p < 0.05) reduced soil salinity and soil Na⁺ content by 25.38%–45.18% and 27.57%–38.16%, compared to the control. In terms of soil nutrient content, HAT treatments significantly (p < 0.05) improved the contents of soil TN, AP, and SOC by 15.25%–26.88%, 19.67%–32.11%, and 73.71%–99.25%, compared to the control, respectively. Furthermore, the soil quality indexes (SQI) constructed based on PCA analysis indicated that the improvement effect of each treatment on soil quality ranked as HAT6 > HA6 > HAT3 > HA3 > AT6 > AT3 > Control. In short, HAT treatment can produce larger surface area and more elements (e.g., N, P, and Ca) content through the combination of HA and AT, accelerate salt leaching, and increase soil nutrient content by promoting soil aggregate formation and ion exchange capacity, thereby improving soil quality more than the single application of HA and AT. These results offer valuable insights for the formulation of ecological restoration strategies targeting large-scale coastal saline-alkali terrains.