Response of wheat to phosphorus enriched ironstone gravel

David Weaver; David Rogers; Ronald Master; Peta Richards; Robert Summers; Simon Clarendon

Just Accepted

This article has been peer reviewed and accepted for publication. It is in production and has not been edited, so may differ from the final published form.

Response of wheat to phosphorus enriched ironstone gravel

David Weaver , David Rogers, Ronald Master, Peta Richards, Robert Summers , Simon Clarendon

Abstract

Context: Gravel fractions (>2 mm) in soil are almost always excluded from laboratory analysis and glasshouse experiments as they are considered to be inert, however the >2 mm fraction is always present in field experiments. Aims: To determine whether the >2 mm fraction of ironstone gravel (IG) enriched with phosphorus (P) can supply P to wheat (Triticum aestivum L.). Methods: An IG soil was separated into different size fractions (<2, 2-4, 4-6, 6-8 and 8-10 mm), and adsorption and desorption experiments, volumetric moisture measurements and glasshouse experiments were conducted. Each of the >2 mm fractions were enriched with P to different levels and added to a sand culture, or to the enriched <2 mm fraction in different amounts (25%, 50%, 75% IG). Wheat was grown in pots and growth correlated to P added from enriched soil fractions, weighted Colwell P, soil solution P concentrations and volumetric water content. Key results: The <2 mm fraction of the IG soil adsorbed more P than the >2 mm fraction of the IG soil likely due to its greater specific surface area. Volumetric water content decreased as gravel amount increased. Wheat was more responsive to P for larger gravel sizes than smaller gravel sizes. Phosphorus enriched IG was able to support the growth of wheat in the absence of any other P source. For the same level of P enrichment, dry matter (DM) decreased as gravel amount increased. Conclusions: IG influences wheat growth through P retention and release and soil moisture. Volumetric water content can be reduced significantly by high gravel contents, leading to reduced wheat growth despite sufficient P fertility. Implications: Depending on the nature of the soil matrix, soils with high amounts (~50%) of larger IG are likely to require lower P applications to optimize crop yield. Soil sampling strategies and laboratory testing need to consider how to practically include the >2 mm fraction during sample collection and analysis.

SR24151 Accepted 05 June 2025