Human urine is superior to urea, a common nitrogen-rich mineral fertiliser, according to the results of a study carried out in a farmer's field outside Nepal's capital city.

Researchers who tested the effects of applying different combinations of urine, compost and urea on sweet pepper, Capsicum annuum, found that urine synergises best with compost. Urine for the study was sourced from mobile public toilets in the city and compost prepared from cattle manure.

Results of the study, published in Scientia Horticulturae, showed that urine-compost mixtures produced the tallest plants and bore the most fruit.

A "synergistic effect" was attributed to several factors including reduced nitrogen loss and enhanced availability of organic carbon in the soil. "Human urine could be a viable alternative to chemical fertilisers for sustained crop production," the study suggests.

Blending urine with compost minimises the risk of salt accumulation, said Debendra Shrestha, lead author of the study and researcher at Tribhuvan University's institute of agriculture and animal science.

In Nepal, where collection and use of farmyard manure is common, farmers apply urine directly to the soil. "We need to start moving towards the application of urine in combination with compost," Shrestha told SciDev.Net.

Surendra Pradhan, post-doctoral fellow at the west Africa office of the International Water Management Institute, Ghana, has also tested combinations of urine with poultry manure and with human faeces to address the problem of disposing of sanitary waste.

"Urine alone is not a long-term solution," said Pradhan, who has published several papers on urine as fertiliser.

Urine lacks sufficient organic matter to sustain plant growth for more than a few years, but provides faster-releasing nutrients that complement slow-release nutrients from compost, which has a higher content of organic matter and beneficial microbes.

Pradhan, who is developing a business model to make urine more competitive, said marketing urine and compost mixtures may be difficult because of cultural factors. "I hardly think compost and urine can compete with mineral fertiliser."

Apart from farmers being reluctant to handle their own urine, commercially available mineral fertilisers often have the advantage of being subsidised by the government, Pradhan told SciDev.Net.