Overcoming Constraints with Deep Spading on Previously Ameliorated Soils: Enhanced Root Growth and Productivity and the Distribution of Soil Organic Carbon on 13-Year-Old Clay-Ameliorated Sands

Deep sandy soils in Western Australia’s Mediterranean climate face compaction, low fertility, poor water-holding capacity and limited soil organic carbon (SOC) storage; constraints likely to intensify with climate change. Subsoil (below 10 cm) SOC levels are estimated at 29–50% of attainable capacity. Clay amelioration and deep tillage have been widely adopted, but deep spading beyond 600 mm on previously clayed soils remains a new frontier, though subsoil dynamics, root–carbon interactions and economic viability remain unclear. A non-randomised, farmer-led blocked strip trial at Scaddan, WA (2025) evaluated deep spading to 600 mm and 900 mm on a sandy soil clay ameliorated with 1000 t ha−1 of subsoil in 2012. Deep spading lowered penetrometer resistance (all four reference strips exceeded 2500 kPa, no spaded strip did) and, less clearly, bulk density below 40 cm (by 0.05–0.10 g cm−3), and subsoil clay at 30–60 cm tended to be higher (4.4% at 900 mm, 2.9% in the reference; not significant), while CEC remained low (below 1 cmolc kg−1 below 30 cm). Mean root abundance below 30 cm was 0.46 on a 0–4 scale in the reference and 0.73 and 1.06 at 600 and 900 mm (significant for 900 mm only), respectively, while spading approximately doubled biomass and grain yield, with 55% and 75% higher tiller densities at 600 and 900 mm (significant at 900 mm), respectively. Total profile organic carbon stocks did not differ significantly among treatments (at fixed depth or equivalent soil mass), and the pattern with depth (a treatment × layer interaction) was consistent with redistribution of carbon towards depth, although no measurements were made before spading. First-season gross margins were higher with spading ($106 and $170 ha−1) but not significantly so after the spading costs. These single-season results provide preliminary evidence that deep spading on previously ameliorated deep sands can relieve subsoil constraints and increase productivity; whether this improves crop resilience under a drying, more variable climate, or gives lasting economic and carbon benefits, needs multi-season trials.