Biochar from human waste could solve global fertiliser shortages, study finds

Excrement contains nutrients needed for crop growth and a new source of them could cut farming’s huge CO2 output.

The Guardian | Aug 11, 2025
By Andy Deng

About a quarter of global greenhouse gas emissions come from agriculture, the IPCC says. Photograph: Alex McGregor/Alamy

Charcoal made from human waste could help solve fertiliser shortages as well as reduce pollution and energy use, a study has found.

Biochar is a form of charcoal made from organic matter treated at high heat, which is often used on farming soil as a fertiliser. The process also removes carbon from the atmosphere, making it a useful carbon sink.

The study estimated that biochar made from solid human excrement could provide up to 7% of the phosphorus used around the world each year. Although the biochar process converts only solids, nutrients taken from urine could be added to it, and the researchers found this could provide for 15% of annual phosphorus application, 17% of nitrogen, and up to 25% of potassium.

Treated sewage sludge is already spread on farmland, but its use is controversial as it often contains microplastics, heavy metals, Pfas forever chemicals, pathogens, and pharmaceuticals.The researchers say biochar can avoid this problem by separating the waste at source.

See also:

Biochar in the circular bionutrient economy

Proceedings of the National Academy of Sciences | Aug 11, 2025 
By Johannes Lehmann, Edmundo Barrios, Mariana Devault, and John Trimmer   

The spatiotemporal disconnect between nutrients derived from excreta and food waste on the one hand and nutrients required for crop production on the other has led to global environmental pollution, disproportionate energy demands, loss of nutrients, and therefore dwindling supplies of finite resources. By strategically managing organic residues, a circular bionutrient economy reduces health and environmental impacts [e.g., greenhouse gas (GHG) emissions and eutrophication] while sustaining crop production. Here, we examine what role biochar can play in transforming nutrients from residues, specifically excreta, that pose an environmental burden into fertilizer products to grow crops.