A comparative study of methods to quantify wheat yield penalty attributable to soil sodicity across north-eastern Australia

Soil sodicity is a major limitation to wheat production in Australia, yet estimating its yield impact remains challenging due to interactions with climate, management, co-occurring soil constraints, and heterogeneous data support. This study compares three approaches for quantifying and mapping yield penalties attributable to sodicity across the Northern Grains Region (NGR) during 2000–2016. Two empirical approaches were evaluated alongside simulations based on a process-based cropping system model, the Agricultural Production Systems sIMulator. Climatic conditions were stratified into wet, moderate, and dry years based on August vapour pressure deficit, representing atmospheric demand during a critical growth period around anthesis in the NGR.

The approaches produced systematically different estimates in both magnitude and spatial extent. Empirical Approach 1 and APSIM, both anchored to spatially distributed soil-profile data, generated coherent regional patterns and higher mean penalties, with APSIM consistently producing the largest values. In contrast, Empirical Approach 2, trained on spatially clustered yield-monitor data, produced lower regional averages but greater within-field variability and more pronounced high-end penalties, particularly under dry conditions.

These differences reflect contrasts in data support, model structure, and representation of sodicity effects. The empirical approaches estimate realised sodicity-related yield penalties within the observed data domain. In contrast, APSIM estimates the yield gain from removing sodicity relative to sodicity-constrained water-limited yield potential, resulting in larger absolute penalties in environments with higher attainable yield. The strongly scale- and method-dependent estimates of yield penalties therefore provides complementary insights for regional to paddock-scale decision-support for managing soil constraints in dryland cropping systems.