Context Soil acidity, high soil strength, and poor subsoil structure are major constraints to crop productivity in coarse-textured and texture-contrast soils of southern Australia. These interacting limitations restrict root growth, reduce access to subsoil water and nutrients, and constrain yield potential in water-limited environments. While deep tillage and surface liming have been used to address individual constraints, their benefits are often short-lived and insufficient to overcome multiple subsoil limitations simultaneously. Aims This study aimed to evaluate the short- and long-term effects of soil profile re-engineering on soil physical, chemical, and hydrological properties, and to determine the persistence of these changes over four cropping seasons at two contrasting sites in Western Australia. Methods In May 2021, four soil re-engineering treatments were established at Bolgart (deep sand) and Meenar (loamy duplex): untreated control; deep loosening with lime; deep loosening with lime and clay; and deep loosening with lime, clay, and compost – all applied between 0 and 80 cm depth. Soil properties including soil strength, bulk density, volumetric water content, pHCa, soil organic carbon (SOC), and cation exchange capacity (CEC) were measured at establishment, three months post-treatment, and 4 years later (2024). Key results At both sites and sampling times, untreated soils had subsoil strength exceeding the critical 2.5 MPa threshold for root growth below 10-cm depth. All soil re-engineering treatments significantly decreased soil strength to well below this threshold and maintained these improvements over 4 years, despite partial recompaction. Soil strength increases between 2021 and 2024 were substantially smaller than typically reported following strategic tillage alone. At Bolgart, treatments incorporating clay and compost increased soil water storage in the 0–80 cm profile by up to 25 mm relative to the control, whereas at Meenar, greater water retention in the untreated subsoil reflected limited root access rather than improved water availability. Lime incorporation increased subsoil pHCa by 1.5–1.7 units – an order of magnitude greater than surface liming – raising pHCa above critical thresholds at 10–70 cm depth and maintaining these improvements over 4 years. Incorporation of compost and clay resulted in marked increases in SOC and CEC, improving soil buffering capacity, stabilising soil physical condition, and reducing the likelihood of re-acidification. Conclusions Soil profile re-engineering produced rapid, substantial, and persistent improvements in subsoil physical, chemical, and hydrological properties, with benefits maintained for at least four cropping seasons across contrasting soil types. Implications These findings demonstrate that soil re-engineering can overcome multiple interacting subsoil constraints simultaneously and provide a mechanistic basis for the large yield and water-use efficiency gains reported in Part II of this series. With the development of cost-effective machinery, soil re-engineering offers a promising pathway to sustainably increase productivity in water-limited, constraint-prone cropping systems.
Azam et al. (Fri,) studied this question.