Guest Blog: Phosphorus in the food system,

Shane Rothwell, Kirsty Forber, Paul Withers, Lancaster Environment Centre, Lancaster University

RePhoKUs (The Role of Phosphorus in the Resilience and Sustainability of the UK Food System) is an interdisciplinary project led by Lancaster University funded under the Global Food Security Programme that has explored how to manage phosphorus (P) in our food system more efficiently and sustainably. Phosphorus leakage from the food system is polluting our rivers, lakes and coastal waters impacting on biodiversity. P is also a finite global commodity that is mined in only a handful of countries around the world. As the UK has no P mine reserves of its own, we are entirely dependent on imported P as fertilisers, or P imbedded in food and feed, to meet our food system demand. The current situation in Ukraine and our reliance on Russian mined P has starkly highlighted the geopolitical challenge of maintaining affordable, adequate P supplies for our food system. Building resilience to these challenges through better use and stewardship of imported P is therefore critical to our food and water security.

The RePhoKUs project has examined how we currently manage phosphorus use at different spatial scales in the UK, from the entire national food system to regional and catchment scales. This analysis has demonstrated that the UK food system is only 43% efficient in its P use (converting P imported in fertiliser, feed and food into food consumed and exported commodities). The rest of the P either accumulates unused in our agricultural soils through surplus application, or is lost to landfills or our aquatic environment where it is currently the most common reason for failing to meet water quality targets for good ecological status in England.

The greatest point of inefficiency in the UK food system is the overapplication of P to our agricultural soils above what is actually needed by the crops and grass we grow. In 2018 we estimated that 90,000 tonnes of surplus P accumulated unused in soils, which is actually greater than the 82,000 tonnes of fertiliser P imported for that year. The key driver of this agricultural P surplus is livestock manures, and areas of high livestock density in the West of the country have the highest regional P surplus. In contrast, some arable dominated regions in the East of the country currently have a negative P balance, suggesting P offtake is currently greater than P input. The RePhoKUs project has also clearly demonstrated that agricultural P surplus is one of the key causes of diffuse P losses to water – i.e. the regions or catchments that have the highest agricultural P surplus also have the largest P losses and the highest riverine P concentrations. Tackling the agricultural surplus is therefore a critical challenge for not only improving efficiency of a finite and increasingly valuable resource, but also reducing the environmental impact of P use in agriculture.

The project examined two particular catchment areas in more detail – the Wye and Upper Welland. P use in the Wye catchment is a dominated by livestock farming, with most of the P coming into the catchment in imported feed. The manure produced by the catchment livestock population contains 1.5 times the P needed for grass and crop production in the catchment suggesting a P surplus of up to 3000 tonnes/year. In contrast, the Upper Welland catchment, a more arable dominated area with a lower livestock population, is currently in P deficit by just over 200 tonnes/year.

Arable areas such as the Welland catchment that are currently in P deficit are partly relying on existing soil P reserves to meet crop demand. These reserves are a result of the historic surplus application of fertiliser and manure that has been occurring for decades, this reserve is known as Legacy P. While being a potential source of crop-available P, and offering some resilience to disruptions to P fertiliser supply, in many areas these reserves are unnecessarily high and pose an additional threat to water quality.

Our investigations suggest that the poorly-buffered soil types present in the Wye catchment in particular could pose a greater threat of diffuse P pollution at soil indices above the agronomic optimum. Recent analysis of over 13000 soil samples from the Wye catchment in the last five years shows that 55 % of agricultural soils in the catchment are currently above the recommended agronomic optimum (i.e. Index 3 and above). Our trial work suggests that these soils could be safely drawn down to the agronomic optimum Index 2, possibly even Index 1 in some cases, without negative impacts on crop yield.

We theoretically already have enough P in our food system to meet all our crop and grass P demand from manures and other secondary P resources such as biosolids, however, logistics and material handling challenges just mean that this recyclable P is poorly distributed. The challenge, therefore, is how best to handle those materials to optimize the use of all the valuable nutrients and carbon they contain.

The concept of a circular economy is not a new one, though with current fertiliser price hikes and future uncertainty over fertiliser supplies as demonstrated with the situation in Ukraine, addressing this issue is timelier than ever. Investment in new innovation and technology to process and handle manures is needed more than ever to help livestock farmers reduce regional P surpluses and create a circular nutrient economy that produces fertiliser grade materials that can meet our crop demand in other parts of the country that need it. Our analysis suggests that sufficient manure processing could theoretically meet all our P fertiliser demand, bringing our food system efficiency up to 76%, and removing our reliance on imported P fertilisers.

These current issues we are facing with P management are representative of wider food system level failures and are not the sole responsibility of any particular sector or individual to sort out. What is needed is collective responsibility across all stakeholders and appropriately scaled regional solutions to ensure we have a resilient supply of nutrients to support our food production systems while minimizing the impact on our environment.