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.