A new study suggests that biochar made from human excrement could help address global fertiliser shortages while reducing pollution and energy use. Biochar, a charcoal-like substance produced by heating organic matter at high temperatures, is already used as a soil fertiliser. Researchers estimate that biochar from solid human waste could supply up to 7% of the world's annual phosphorus demand, and with added urine nutrients, up to 15% of phosphorus, 17% of nitrogen, and 25% of potassium.
While treated sewage sludge is currently used on farmland, concerns over microplastics, heavy metals, PFAS, pathogens, and pharmaceuticals limit its acceptance. Biochar avoids these issues by processing waste at source. The process also reduces the weight and volume of solid excrement by up to 90%, offering efficiency gains over transporting water-heavy sewage sludge.
The study, published in PNAS and led by Dr Johannes Lehmann of Cornell University, highlights that biochar production allows nutrient proportions to be tailored to specific crops, reducing problems like weed growth and eutrophication. Agriculture accounts for 25% of global greenhouse gas emissions, and synthetic fertiliser production is energy-intensive and environmentally damaging. Nitrogen fertiliser production alone emits 2.6 billion tonnes of CO2 annually, more than global aviation and shipping combined.
Phosphate rock mining scars landscapes and produces radioactive waste, while potash mining contributes to soil salinisation and water contamination. Dr Lehmann noted that recycling nutrients through a circular economy could reduce dependence on finite mineral reserves, such as Morocco's 70% share of global phosphates. This approach could empower countries to produce food without relying on imported fertilisers, potentially mitigating climate migration driven by agricultural failure.
