Development of a standard pot trial for the agronomic evaluation of recycled phosphorus fertilizers using struvite and ryegrass as a model

As phosphorus (P) recycling fertilizer become increasingly important, standardized evaluation methods are needed, yet suitable pot trial protocols remain poorly standardized. Therefore, this study refined our previous standardization approach using black peat, quartz sand and bentonite by testing sustainable peat substitutes (bark humus, wood fiber, coconut pulp) and by optimizing basic fertilization. Additionally, the fertilizer performance of struvite (MgNH4PO4·6H2O, STR) was evaluated. During preselection, bark humus proved the most suitable peat substitute. Thus, a ryegrass pot trial was conducted using our standard substrate containing either black peat (S1) or bark humus (S2). Substrates were adjusted to pH 5.5, 6.5 and 7.0 (S1) or 6.5 and 7.0 (S2). P fertilization treatments included soft rock phosphate (sRP), STR, triple superphosphate (TSP), and a control (P0). Two basic fertilization strategies (bfs A and B) were compared. Following their respective initial fertilization, bfs A only resupplied N after each cut, whereas bfs B resupplied all nutrients except P based on plant offtake by the previous yield. Both P-deficient substrates enabled clear differentiation between fertilized and unfertilized treatments. In the first cut, bfs B enhanced yield and P uptake in STR and TSP treatments due to better N supply, but subsequent P depletion masked these effects. Basic fertilization based solely on yield forecasts (bfs A) was less reliable, while integrating actual yields (bfs B) improved demand orientation, though further refinement remains necessary. No significant differences in dry matter yield and P uptake were detected between ground STR and TSP under the tested conditions.