Enhancing Root Proliferation in an Alkaline Dispersive Subsoil: a Comparative Study of Organic and Inorganic Amendments with Different Amelioration Mechanisms

Abstract Purpose: Alkaline dispersive subsoils contain a range of physicochemical constraints that restrict root proliferation and limit water and nutrient extraction, leading to yield penalties. We investigated the effectiveness of organic, inorganic and a combination of organic and inorganic (combined) amendments with contrasting chemical compositions in mitigating constraints on crops grown in these subsoils. Methods: An alkaline dispersive subsoil (20 – 40 cm depth) with pH w 8.9 and an exchangeable sodium percentage (ESP) of 12.9% was incubated for 14.5 months with 19 different organic (crop residues, animal manures and composted materials), inorganic (gypsum, polyacrylamide (PAM) and their combinations (PAM + Gypsum)) and combined (combination of wheat stubble and chemical fertilisers) amendments or control. The amendments were applied as a band within a soil core. Following incubation, the incubated subsoil was mounted on a custom-built sand core and wheat ( Triticum aestivum cv. Lancer) was grown. Water use was monitored weekly. Plant biomass, root biomass, root length density, and soil physicochemical properties were determined at harvest. Results: Organic amendments with a low C:N ratio (i.e., high N content) enhanced root proliferation (up to 63%) through the nutrient patches from the mineralising organic matter and improved porosity by macro-aggregate formation (84%), and promoting fungal (500%) and bacterial (47%) abundance. In contrast, inorganic amendments such as gypsum, alone or in combination with PAM, improved micro-aggregate formation (14%) by reducing soil pH (11%) and ESP (14%) and increasing electrical conductivity (EC; 97%). Wheat stubble, alone or combined with chemical fertilisers, significantly increased macro-aggregate formation (67%), root proliferation (61%) and water use (21%). Plant biomass and water use were associated with increased root proliferation through the amended soil layer. Conclusions: The co-application of organic and inorganic amendments with contrasting modes of action might have additive effects on ameliorating alkaline dispersive subsoils with multiple physicochemical constraints.