Response of wheat to phosphorus-enriched ironstone gravel

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) soil 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% and 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 compared to smaller gravel sizes. The P-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 decreased as gravel amount increased. Conclusions The 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 optimise crop yield. Soil sampling strategies and laboratory testing need to consider how to practically include the >2 mm fraction during sample collection and analysis.