Could you fertilize Martian crops with human poop?

If humans ever settle on Mars, getting reliable food will be one of the major challenges they face.

“If you’re ever going to have a tomato on Mars, you’re going to have to grow it there,” says Bruce Bugbee, a Utah State ­University professor who helps NASA develop life-support systems for ­astronauts in space.

Setting aside light and water issues, plants still need nitrogen to grow. Human feces contains nitrogen as well as bacteria that break it down into nitrate, which plants prefer for growth. But lingering alongside those useful bacteria will be any bad bacteria the crew brought with them. If they dumped the waste directly onto the plants, the harmful bacteria could thrive and proliferate and make people sick. To avoid this, says Bugbee, Martians will have to compost the feces over several months to weed out the bad microbes.

So it’s possible to use human waste for fertilizer, but they’d better have a contingency plan for the first few months.

This article was originally published in the January/February 2017 issue of Popular Science.

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