Testing phosphorus availability for maize with DGT in weathered soils amended with organic materials.

Aims and background: The phosphorus (P) fertilizer value of organic materials (OM) such as plant residues or manure can be larger than that of mineral fertilizers in weathered soils. Recently, the diffusive gradients in thin films (DGT) technique was shown to better predict crop response to applied mineral P fertilizer among soils than established tests. The goal of this study is to evaluate the potential of the DGT technique to predict plant-available P in different soils amended with various qualities of OM. Methods: In this study, maize was grown in a greenhouse on two Kenyan soils characterized by strong P sorption. Phosphorus was applied at various rates (deficient to adequate) as triple superphosphate (TSP) only, or at one intermediate P dose in a P-substitution trial with four different OM: farmyard manure (FYM) or residues of Tithonia diversifolia each at low or high P content. All soils were amended with adequate amounts of other nutrients. Next to the DGT technique, two established soil P tests (anion exchange membranes (AEM), and Olsen) were performed on incubated soils for all P treatments. Results: Results show that the relative yield (% of maximum yield) to soil P test values correlated most strongly for DGT measured P concentrations, R = 0.74, compared to Olsen (R = 0.60) and AEM (R = 0.62). Conclusions: Results of this study demonstrate that DGT relates best with DM yields when soils are amended with a combination of TSP and OM compared to the established soil P tests Olsen and AEM. [ABSTRACT FROM AUTHOR]