Phosphorus is an essential nutrient for plant growth and global food production, but reserves are not unlimited, and overuse can lead to leaks and environmental degradation of waterbodies. A new study from Lincoln University in New Zealand, together with Lancaster University suggests a large portion of the world’s phosphorus supply is stored in agricultural soils, something that could extend the lifespan of global phosphorus reserves.

Phosphorus fertilisers come from geological phosphate rock deposits, with extraction, processing, and distribution an energy-intensive and costly process. Many farmers apply phosphorus as ‘insurance’ against poor phosphorus utilisation in soil, leading to an accumulation of phosphorus stocks in cropland and pasture soils.

The research estimated farmland worldwide contains up to 4.26 gigatons of total phosphorus, with up to 90% in forms not immediately accessible to plants. These figures represent between 32% and 41% of the estimated global phosphorus rock reserves in 2020, meaning if they were efficiently utilised, current fertiliser application rates could be sustained for another 150 years.

By selecting crops with higher phosphorus-use efficiency and employing soil additives such as phosphorus-solubilising microbes, farmers could reduce their dependence on mined phosphorus. Adjusting crop rotation strategies and land-use planning could also help maximise the use of existing soil reserves.

A key impact of this research is its potential to enhance water quality. Phosphorus in runoff from agricultural fields, entering rivers and lakes is a leading cause of nutrient enrichment leading to excessive growth of unwanted aquatic plants – a process responsible for producing algal blooms. By carefully managing soil phosphorus levels and reducing unnecessary fertiliser inputs, water quality could be significantly improved without any detrimental effects on food production, the researchers say.

The study found levels of phosphorus accumulation vary by global geographical region. Farmlands in Europe, North America and China have high levels of phosphorus due to a long history of fertiliser use, while parts of Africa (where fertilisers have not been used until recently) show deficiencies, and could benefit from strategic redistribution or alternative fertiliser approaches.

The study also calculated phosphorus stocks in non-productive areas like forests. While potential reserves there are high, they are not easily accessible and any conversion to agricultural use would lead to increased carbon emissions, soil erosion, and biodiversity loss.

Phil Haygarth, Professor of Soil and Water Science at Lancaster University and co-author of the study, said: “The world’s soils contain significant and yet unused resources of soil phosphorus. We need to focus our innovation into how we can become more efficient in accessing this and this will have long term benefits for food production and for environmental quality.”

The research underlines the need for a reassessment of phosphorus management strategies. The researchers say that rather than mining new phosphorus sources, the agricultural sector should prioritise unlocking soil-bound phosphorus. This would extend the availability of phosphorus-based fertilisers into the future and improve sustainability by reducing nutrient pollution in waterways, they add.

The paper has been published in Nature Food and can be accessed here