Phosphorus (P) is a critical raw material in Europe due to rock phosphate reserves being finite, depleted, and in geographical regions which may limit accessibility. Recently, the war in Ukraine has exacerbated the P shortage, because of the decline in Russia’s phosphate production and limited exports due to sanctions1, causing rock mineral fertiliser prices to increase significantly.
As phosphorus is a critical element, and rock phosphate reserves are not a long-term sustainable solution, research into maximising the efficacy of alternatives – such as biowaste materials – is a priority. Biomaterial waste including sewage sludge (SS), sewage sludge ash (SAS), biogas digestate solid fraction (BGF) and meat/bone meal (MBM) from the meat industry, could all potentially be used as P fertiliser. However, the P solubility and release to the soil from these sources varies greatly, and is often lower than that of mineral P fertilisers derived from rock phosphate.
Localised application of mineral P fertilisers – close to seeds – is a strategy that boosts plant and root growth by attracting root proliferation to the P-rich ‘hot-spot’, providing greater amounts of P to the plant. In contrast to this, previous studies have shown that biowastes may have sufficient soluble P to attract root growth to the ‘hot-spot’, but not enough to sustain plant needs. Therefore, in this study, acid and alkaline pre-treatments were proposed, aiming to enhance the P solubility of biowastes to levels similar to mineral fertilisers – ensuring adequate P supply to the plant in the early stages of growth.
This study investigated pre-treating different, more sustainable biowaste sources prior to soil application, to assess the impact on P release to the soil. In addition, the researchers noted the amount of P that was water soluble, as it can diffuse further into the soil penetrating beyond the ‘hot spot’ zone, thus benefitting overall plant growth by increasing P availability in a more distributed manner. This research advances understanding in technological fields towards creating viable P fertiliser products from biowastes – lowering Europe’s dependency on rock phosphate fertiliser.
The researchers used discs of soil with a low P content, taken from a farm plot in Copenhagen, Denmark. These soil discs were covered with a nylon mesh and then a layer of one of four biomaterials: sewage sludge, sewage sludge ash, meat and bone meal and the solid waste from biogas feedstock. Prior to application, each of the biomaterials were treated with either sulphuric acid, sodium hydroxide or calcium hydroxide – the latter two chemical treatments being alkaline.
The discs were then placed into a reaction system to measure the effects of the different treatments on the P dynamics of the soil samples. Six replicates were made of each of the disc treatment types, with three discs incubated for two days and three incubated for 12 days.
The experiment showed that acidification increased the P solubility of all the biomaterials, whereas the alkalinisation of sewage sludge and sewage sludge ash with sodium hydroxide increased the apparent recovery of P from the soil (the amount of available phosphorus present in the soil after the application of biomaterial). The researchers suggest that more P can be recovered from soil after the application of sodium hydroxide pre-treated biomaterial because it alkalinises the soil – thereby lowering its ability to bind with P.
These experiments also examined concerns regarding organic contaminants when applying biomaterial waste (e.g. sewage sludge) as fertiliser to crops. The researchers found that the pH changes of pre-treated biomaterials had the added advantage of reducing microbial growth in this layer when applied to the soil – also showing promise as a sanitation treatment.
The researchers suggest that applying pre-treated biomaterials onto soil should enable a larger P-rich zone in the soil, with more P available to plants. Existing research has shown that applying acidified meat and bone meal and biogas digestate to crops, increased fertiliser efficiency and P uptake by plants. However, the authors observed that other elements, i.e. aluminium and ammonium, are also released into the soil – which might have toxicity implications for roots close to where the fertiliser is applied.
Thus, further studies are needed to assess the effects on plant growth and P uptake under field conditions, with a focus on acidification for all the biomaterials, as well as alkalinisation with sodium hydroxide for sewage sludge and sewage sludge-ash. In addition, research on the wider impact of these practices on soil health, soil biodiversity and the environment is required.
Bio-based thinking and circular economy principles are central within a number of European policies, including the Circular Economy Action Plan and Waste Framework Directive. The integration of the sustainable and efficient use of natural resources is also central to the .
This research contributes to the development of innovative bio-based technology, in the form of a more useable P fertiliser from biowaste. Preventing these biowaste products going to landfill, and reducing European dependence on finite, expensive sources of rock-phosphate based mineral fertiliser is a priority within the European Union and worldwide.
Acknowledgments: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 860127 in the FertiCycle project.
- Before the war with Ukraine, Russia was the world’s fourth largest producer of phosphate.
Sica, P., Kopp, C., Müller-Stöver, D. S. and Magid, J. (2023) Acidification and alkalinization pretreatments of biowastes and their effect on P solubility and dynamics when placed in soil. Journal of Environmental Management, 333: 117447. Available from:
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The contents and views included in Science for Environment Policy are based on independent, peer reviewed research and do not necessarily reflect the position of the European Commission. Please note that this article is a summary of only one study. Other studies may come to other conclusions.
- Publication date
- 4 October 2023
- Directorate-General for Environment