The phosphate mineral reserves required for fertiliser production could be exhausted on a global scale in just a few decades. This study presents a method for recycling a Swiss industrial by-product into a phosphate fertiliser.
Phosphorus (P) is a crucial element for crops. The necessary global reserves for producing phosphate mineral fertiliser could be exhausted in several decades, whilst all of the phosphorus consumed in Switzerland is imported. Consequently, recycling P into fertiliser is a priority for the Swiss Confederation.
Recycling calcium phosphite into phosphate fertiliser
The aim of this study was to find a solution for upgrading calcium phosphite (Ca-Phi), an industrial by-product generated in Switzerland. The quantity of P contained in Ca-Phi represents 5 to 10% of the P imported via mineral phosphate fertilisers. Nevertheless, plants can only metabolise P in the form of phosphate and not in the form of phosphite (present in Ca-Phi), which can be phytotoxic.
This study therefore concerned the possibility of oxidising phosphite (from the Ca-Phi) into phosphate using soil microorganisms. Since soil microbial flora is strongly influenced by soil type and the crops grown, we studied the effect of the microbial flora of an acidic sandy soil and an alkaline clay soil sown with four green manure species (oats, mustard, pea, lupin) with contrasted root morphologies. Since the Ca-Phi was not water-soluble, we assumed that the microbial flora would allow the phosphite to be mobilised and then oxidised during a green-manure crop such that phosphate would be available for a subsequent maize crop without phosphite traces being detectable in the maize. To quantify the efficiency of this method, Ca-Phi fertilisation was compared with a reference phosphate fertiliser (triple superphosphate).
- Fertilisation with Ca-Phi was as effective as with the reference phosphate fertiliser; moreover, the Ca-Phi did not negatively impact the services provided by the different green manure species for the following maize crop.
- This method would allow the Ca-Phi produced industrially to be upgraded as a phosphate fertiliser. However, bearing in mind the ban on the use of phosphite as a biostimulant in Switzerland with effect from 1 January 2024 (FOAG, personal communication), phosphite should be oxidised during the manufacture of a phosphate fertiliser rather than in the soil.
- The fact that the phosphite from the Ca-Phi was oxidised by contrasted microbial flora in under 8 weeks suggests that a number of organic substrates could be used (e.g. compost, solid and liquid manure).
- The purity of Ca-Phi would represent a significant environmental benefit, as using it to formulate a phosphate fertiliser would eliminate problems associated with the trace metals often present in the phosphate fertilisers distributed in Switzerland.