A new circular fertiliser framework for greenhouse hydroponic systems using phosphorus as a case study

Recirculating hydroponic fertigation as used in high-tech greenhouses is over 95% nutrient-efficient, but relies on nutrients from finite mineral reserves. Moving away from conventional ‘linear’ fertilisers to ‘circular’ alternatives brings various questions. Whilst this has been investigated for open-field agriculture, in recirculating soilless greenhouse horticulture, many of these questions remain open. We propose a framework specific to recirculating hydroponic systems, focusing on (1) side-streams as a source of nutrients, (2) the applicability of recovered fertilisers and (3) the risk of contaminants. The framework is applied to phosphorus (P), using parameters from Dutch tomato greenhouses. P side-streams available in the Netherlands are compared. Many possibilities exist, and recovered P may be more economically feasible for greenhouses since products are over twice as costly as those used in arable farming. We then investigate whether insoluble compounds (e.g. struvite) can be applied if dissolved in acid on-site, using simulated chemistry analysis. This shows an H⁺ to P ratio ranging between 0.1 and 2, depending on the product. The difference with a conventional nutrient recipe becomes at most 1 mmol l⁻¹. Lastly, we propose an approach to quantify the risk of contaminants. With it, we calculate limits for inorganic contaminants for soilless P fertilisers, finding them to be stricter than current legislation, in many cases by several orders of magnitude. By quantifying the unique requirements of soilless systems in a way that can be directly linked to the fertiliser products themselves, the presented framework can help include soilless systems worldwide in a closed nutrient loop.