Background and aims Alkaline dispersive subsoils often contain a range of physicochemical constraints that restrict root growth and limit accessibility to subsoil resources. Co-application of inorganic and organic matter (OM) demonstrated a synergistic effect on improving structural stability of alkaline dispersive subsoils. This study aims to quantify this synergistic effect on soil physicochemical and biological properties, water use and wheat grain yield.

Methods An alkaline dispersive subsoil was amended with different amendments or un-amended control (CON) in a factorial combination with OM (+ OM) or without OM (-OM). Poultry litter (as a source of OM) was applied at 20 t ha−1, and its equivalent rate of major macronutrients (N, P, K and S) were matched in the inorganic fertilisers (NPKS) treatment. Elemental sulphur (ES), gypsum (GYP) and anionic polymer (PAM) were applied at 1, 5 and 5 t ha−1, respectively. Amendments were applied as a vertical band at 20 – 40 cm depth in a PVC core of 15 cm diameter and 65 cm height and incubated under controlled conditions for 13 months before sowing wheat. At harvest, biomass, grain yield, root growth of wheat and soil properties were recorded.

Results Ameliorating alkaline subsoil significantly improved root growth, water use and grain yield. The synergistic effect of the co-application of OM on grain yield was only evident for the nutrient-poor amendments (ES, GYP and PAM) but not for the nutrient-rich amendments (NPKS and PAM + NPKS). Observed grain yield improvement was associated with improved soil physicochemical and biological properties impacted by the co-application of different amendments.

Conclusions The synergistic effects of the co-application of OM and inorganic amendments on soil physicochemical and biological properties and root proliferation demonstrated as an effective strategy to ameliorate alkaline dispersive subsoils and improve crop productivity in the long term.