For thousands of years, human urine has had real value.

After our bodies absorb nutrients from our food, they excrete the rest – mainly via urine. An adult produces about 500 litres a year. This liquid gold is very rich in nitrogen, phosphorus and potassium, three compounds vital for plant growth.

Ancient civilisations from Greece to China to Rome used it as fertiliser for crops, alongside faeces.

Synthetic fertilisers have largely replaced natural fertilisers such as urine. Half the world’s population now relies on food grown using synthetic fertilisers. Urine is still used as fertiliser by smallholder farmers who cannot afford synthetic fertilisers or want a sustainable alternative.

This year, the Iran war has made synthetic fertilisers more expensive. If the era of cheap fertilisers is ending, urine-based fertilisers could make a comeback. Call it “peecycling”.

Of chamber pots and nightsoil

Before modern toilets, humans were a lot more intimately acquainted with the waste they produced. Peasants and smallholders would store their collective waste in cesspits. In cities, people used privies or chamberpots.

As cities grew, so did the volume of waste. While people slept, nightsoil workers would go door-to-door collecting human waste.........

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