Identifying Soil and Climate Drivers of Soil Water Conditions Favourable for Deep Phosphorus Placement for Wheat in Australia Using Spatial Modelling

Context Phosphorus (P) fertiliser is often placed shallow, near the seed, and plant-available P tends to be concentrated in the topsoil (0–5 cm), because of its relatively immobile nature and the dominance of no-till cropping systems. Concern about stratification of P in layers prone to surface drying has prompted research into deep (20–30 cm) placement of P to take advantage of the generally wetter conditions of the subsoil. However, field experiments testing deep P strategies across Australia have recorded variable benefits of deep placed P. Favourable soil water conditions for responses to deep P are assumed to be a dry surface soil, which limits access to the shallow P, in combination with a wet subsoil at the depth of deep P placement. However, it has not been studied when, where and how often these conditions occur. Objective The spatial modelling analysis presented here aimed to identify how soil and climate factors interact in a cropping systems context to create these favourable soil water conditions for responses to deep P placement. Methods We used the APSIM model to simulate soil water dynamics for a dryland wheat (Triticum aestivum L.) cropping system at 568 locations across the Australian wheat belt. Unsupervised machine learning clustering analyses were applied to the simulation results to identify regions with similar surface and subsoil soil water dynamics across three crop growth stages and seven soil textures. Results Favourable soil water conditions were predicted to be more likely on coarser-textured soils than finer-textured soils, and during the seedling growth crop stage. Four regions were identified which presented different opportunities for deep P placement. Regions were separated by seasonal timing, frequency and amount of rainfall and the aridity index. Soil water conditions were more likely to be favourable when infrequent rainfall promoted surface soil drying but pre-season rainfall ensured wet subsoils. Conclusions From a soil water perspective, the opportunities for wheat to benefit from deep P placement are determined by sufficient opportunity for surface soil drying (infrequent growing season rainfall events, high level of evaporative demand) and whether the subsoil has had an opportunity to wet up. Significance Understanding the factors of influence and their interactions allows prediction of when and where conditions conducive to deep P placement being effective are more likely to occur. This approach of analysing favourable soil water conditions also has applicability for other management strategies such as early deep sowing and soil amelioration.