Improving the prediction of fertilizer phosphorus availability to plants with simple, but non-standardized extraction techniques

In the framework of the circular economy, new P fertilizers produced from diverse secondary raw materials are being developed using various technologies. Standard extraction methods (neutral ammonium citrate (NAC) and H₂O) provide limited information about the agronomic efficiency of these often heterogenous new products.

Here, we compared these extractions with two alternative methods: 0.5 mol L^-1 NaHCO₃ and a sink extraction driven by phosphate adsorption onto ferrihydrite (“Iron Bag”) on 79 recycled and mineral reference fertilizers. We compared their capacity to predict shoot biomass and P content of rye (S. cereale L.) grown in a greenhouse on three soils of contrasting pH with a subset of 42 fertilizers.

The median extracted P (% of total P) was H₂O (1%) < NaHCO₃ (25%) < Iron Bag (67%) < NAC (85%). The NaHCO₃ extraction stood out as a cost-effective and reliable method to predict plant shoot biomass and P content (R² ranging between 0.65 and 0.86 in the slightly acidic and alkaline soil). Notwithstanding, the other methods provide complementary information for a more detailed characterization of how P solubility may be impacted by e.g. soil pH, granulation, or time. The implications of this work are therefore significant for fertilizer production, regulation, and use.