Phos4You project : phosphorus availability of phosphorus fertilizers recovered from municipal wastewater

The current European Fertilizer Regulation1, next to the national legislation2, implies the use of several different chemical methods to assess phosphorus (P) availability in fertilizers. The variety of those methods is large and comparison between them is difficult. With the emerging production of recovered P fertilizers, adequate measurements of P availability are needed. The Interreg project Phos4You recognizes this demand for standardized quality assessment methodology, in particular P availability. Ten different chemical P extraction and two P determination methods are being evaluated on struvites, P-slag, Fe-P pellets and ash recovered from municipal waste water and compared to triple superphosphate. Also, plant experiments are being carried out in pot and field trials. Multi-nutrient fertilizer (NIST SRM) is used as a reference. Pot trials are conducted with 3.5 g/m3 seeds of ryegrass and P-poor substrate soil, with incremental addition of fertilizer (30, 60 and 90 kg P2O5/ha). Essential nutrients other than P are being supplied with continuous addition of Hoagland solution to avoid deficiencies for other nutrients. Pot trials are run in parallel in 3 countries (BE, IR, FR). The first pot trial mimics plant uptake measurements with passive sampling techniques (i.e. Diffusive Gradient in Thin films (DGT) (iron-oxide binding layer), Rhizon and Plant Root Simulator (PRS) probes) on a weekly basis. Plant height and dry matter is being determined along with chemical P plant extraction measurements. In the second trial, P lipid index is examined as an early P availability indicator whereas the third trial focuses on field testing of struvite on different soil types (i.e. commonly found in Ireland and P-poor substrate soil). Moreover, field trials with struvite are run for scale validation where the product is being tested on two different soil types in Ireland. Chemical extraction methods, passive sampling techniques and biomarker experiments are being compared to find the most efficient way of measuring the P availability. It is hypothesized that biomarker and passive sampling methods can give fast results (compared to the pot experiment) with better correlation to real plant uptake than chemical extraction. The resulting standardized methodology will define the most effective P availability method for the various products from different processes and locations, while ensuring the applicability of the recycled material on the market