Optimising phosphorus supply from nutrient enriched sludge granules using diffuse gradient in thin films (DGT)

Sakrabani, R., Pawlett, M., Read, R. and Deeks, L., Cranfield University, UK



Sewage sludge recycling can be an important source of phosphorus in agricultural systems. The purpose of this study was to investigate the role of organo-mineral fertilisers (OMF) derived from sewage sludge in reducing agricultural demand for phosphorus. A field trial was established to investigate OMF’s influence on phosphorus dynamics, optimal application rates for ryegrass yield, and the potential for phosphorus accumulation. The results were compared to application of urea and biosolids as a fertiliser. Diffusive gradient in thin films (DGT) and the more conventional Olsen P method were used to observe phosphorous. It is thought that the DGT method better reflects the bioavailable P fraction. Comparison of the two methods showed a significant positive correlation (R2 = 0.63). Soil analysis of pre-fertiliser application showed that both the OMF and urea fertilisers plots had lower P levels than control and biosolid plots. After post-harvest there was reduction of P as determined by the Olsen method where urea had been applied. This reduction was not observed in the DGT results. Differences between Olsen-P and DGT methods observed after urea application may be the result of different P pools being observed. Results indicate that DGT data may be a better indicator of plant available P than Olsen-P and thereby its use may prevent unnecessary application of phosphorus fertiliser.


Biosolids, Diffusive gradient in thin films, Organo-mineral fertiliser, Phosphorus, Rye-grass, Sewage sludge


Phosphorus (P) is vital to sustain crop production. Due to population growth, and thereby increase crop production, mining of rock phosphate has become became an essential component of modern agriculture (Ashley et al. 2011). However phosphorus use has also led to eutrophication causing enrichment of water bodies. Phosphorus is a non-renewable resource, and as such global phosphorus scarcity has implications for food security. Remaining rock phosphate is lower in phosphorus concentration, higher in contaminants (such as cadmium) and more difficult to access (Cordell et al. 2009). The future availability of the world’s main sources of phosphorus is uncertain (Cordell et al. 2009). Consequently the farming community need to rely on renewable sources of P such as sewage sludge, livestock manure, digestates, food and organic waste (Ashley et al. 2011). There are challenges in relying on organic sources of P in terms of the availability of P when applied to soils and also the homogeneity and traceability of these sources. Nevertheless this will still provide a solution to sustainable P management.

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