[{"data":1,"prerenderedAt":176},["ShallowReactive",2],{"\u002Frabbit-hole\u002Fthe-martians-potato-problem-is-actually-a-soil-problem":3},{"id":4,"title":5,"body":6,"createdAt":167,"description":168,"extension":169,"meta":170,"navigation":171,"path":172,"seo":173,"stem":174,"__hash__":175},"rabbitHoles\u002Frabbit-hole\u002Fthe-martians-potato-problem-is-actually-a-soil-problem.md","Could you really grow potatoes on Mars with human feces as fertilizer?",{"type":7,"value":8,"toc":160},"minimark",[9,18,21,24,29,48,51,60,75,79,82,135,138,153,157],[10,11,12,13,17],"p",{},"I was watching ",[14,15,16],"em",{},"The Martian"," and got stuck on a small detail. When Mark Watney sets up his potato farm, he starts opening little foil packets of human waste, and I wanted to know why the waste was in packets.",[10,19,20],{},"That part turned out to be simple. Astronauts can't flush a toilet into a sewer, so solid waste gets collected, sealed, and stored, and the packets are just bags of astronaut poop rather than some special NASA fertilizer product. But the question underneath it was more interesting: could you actually grow potatoes in human waste, the way the movie shows? The short answer is that you sort of can, but not like that.",[10,22,23],{},"TL;DR: It turns out you can't just poop on potatoes and get more potatoes.",[25,26,28],"h2",{"id":27},"why-the-movie-version-wouldnt-really-work","Why the movie version wouldn't really work",[10,30,31,32,35,36,39,40,47],{},"Start with what Watney literally does: he mixes the crew's waste straight into the ground and plants potatoes in it. The waste part isn't the crazy part. Human feces really are rich in nitrogen and phosphorus, the nutrients plants want most. The catch is that raw waste also carries ",[14,33,34],{},"E. coli",", ",[14,37,38],{},"Salmonella",", viruses, and parasites, which is the whole reason ",[41,42,46],"a",{"href":43,"rel":44},"https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FReuse_of_human_excreta",[45],"nofollow","\"human waste can be fertilizer\""," does not mean \"spread it on your dinner today.\"",[10,49,50],{},"On Earth you close that gap by composting, which takes sustained heat and months of time, and Watney has neither, so he uses it raw. On its own that's a real food-safety gamble.",[10,52,53,54,59],{},"But the waste isn't even the hard part. The dirt is. ",[41,55,58],{"href":56,"rel":57},"https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FSoil_biology",[45],"Earth soil is alive",": a single teaspoon of healthy soil holds hundreds of millions of bacteria and yards of fungal threads, a whole food web that plants don't sit on top of so much as plug into, with fungi trading nutrients through roots and bacteria converting nitrogen into forms plants can use. Martian regolith has none of that. It's mineral dust that has been sterile rock for billions of years, so Watney isn't fertilizing poor soil. It's more like he's trying to conjure living soil out of dead rock, and a few packets of waste don't get you there.",[10,61,62,63,68,69,74],{},"And the movie ignores one more thing: the Martian regolith is mildly poisonous. It's full of ",[41,64,67],{"href":65,"rel":66},"https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FPerchlorate",[45],"perchlorates",", chlorine-based salts that NASA's Phoenix lander found in 2008 and that have turned up just about everywhere we've looked since. At real concentrations they're toxic to people and hard on plants, so before you grew anything you would have to get them out: wash the regolith, treat it chemically, or use ",[41,70,73],{"href":71,"rel":72},"https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FBioremediation",[45],"microbes that break perchlorates down",".",[25,76,78],{"id":77},"what-would-it-actually-take-to-farm-potatoes-on-mars","What would it actually take to farm potatoes on Mars?",[10,80,81],{},"The process of turning waste into something safe to grow food in is called composting. Done properly, hot composting is roughly five steps:",[83,84,85,99,105,111,123],"ol",{},[86,87,88,92,93,98],"li",{},[89,90,91],"strong",{},"Balance the carbon and nitrogen."," Waste is heavy on nitrogen, so you mix it with carbon-rich \"browns\" like sawdust, straw, dried leaves, or wood chips, aiming for a ",[41,94,97],{"href":95,"rel":96},"https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FCarbon-to-nitrogen_ratio",[45],"carbon-to-nitrogen ratio around 30 to 1",". Too much nitrogen and the pile goes anaerobic and reeks of ammonia; too much carbon and it never heats up.",[86,100,101,104],{},[89,102,103],{},"Make the pile big enough."," A heap needs enough mass, on the order of a cubic yard, to insulate its own core, or the heat leaks out faster than the microbes can make it.",[86,106,107,110],{},[89,108,109],{},"Keep it damp and aerated."," The microbes doing the work are aerobic, so the pile needs oxygen and moisture somewhere around 50 to 60 percent, about as wet as a wrung-out sponge. In practice that means turning it, which also drags the cooler outer material into the hot middle.",[86,112,113,116,117,122],{},[89,114,115],{},"Get it hot and hold it there."," This is the step that actually sanitizes. The core has to reach roughly ",[41,118,121],{"href":119,"rel":120},"https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FCompost",[45],"55°C, about 131°F, and stay there long enough to kill the pathogens",". The U.S. standard for treated biosolids is several days at that temperature in an enclosed system, or a couple of weeks with repeated turnings in an open pile.",[86,124,125,128,129,134],{},[89,126,127],{},"Then let it cure."," After the hot phase, the pile has to sit and mature for months while slower organisms finish the breakdown and the last pathogens die off. That's why doing this safely around food crops is measured in ",[41,130,133],{"href":131,"rel":132},"https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FComposting_toilet",[45],"months to a year",", an amount of time that would have left poor Watney to die of starvation.",[10,136,137],{},"On Mars you'd have more work to do than that: deal with the perchlorates first, either washing the salts out of the regolith or leaning on perchlorate-reducing bacteria to break them down; build real soil structure and organic matter over several growing cycles instead of one; and hold water, warmth, light, and air pressure steady inside a sealed space the entire time.",[10,139,140,141,146,147,152],{},"Which is roughly what the people actually studying this keep finding. In one ",[41,142,145],{"href":143,"rel":144},"https:\u002F\u002Fwww.ncbi.nlm.nih.gov\u002Fpmc\u002Farticles\u002FPMC10933922\u002F",[45],"2024 experiment",", potatoes grown in Mars regolith simulant completed their life cycle in the raw material but produced only scarce tubers, and it was mixing in compost that made them viable. There's even a running ",[41,148,151],{"href":149,"rel":150},"https:\u002F\u002Fcipotato.org\u002Fpotatoes-mars-media-tools\u002F",[45],"Potatoes on Mars project"," that's been testing this kind of thing since 2016. The realistic version of Watney's farm involves many slow cycles of turning dead rock into living soil.",[25,154,156],{"id":155},"would-it-actually-work","Would it actually work?",[10,158,159],{},"Maybe. The movie makes it look cleaner and safer than it would be, and no real astronaut would mix untreated waste straight into food soil if there were any other option. A real Mars farm would need careful management of pathogens, nutrients, salts, water, microbial populations, and probably the chemistry of the regolith itself. But none of the underlying pieces are made up. Human waste really can become fertilizer, Martian regolith really does hold many of the minerals plants need, microbes really could help turn dead rock into something more like soil, and potatoes really are an efficient way to grow a lot of calories in a small space.",{"title":161,"searchDepth":162,"depth":162,"links":163},"",2,[164,165,166],{"id":27,"depth":162,"text":28},{"id":77,"depth":162,"text":78},{"id":155,"depth":162,"text":156},"2026-08-09T15:00:00.000Z","A rabbit hole that started with a small question about The Martian and ended with me reading about composting, gut bacteria, and what Martian dirt is actually made of.","md",{},true,"\u002Frabbit-hole\u002Fthe-martians-potato-problem-is-actually-a-soil-problem",{"title":5,"description":168},"rabbit-hole\u002Fthe-martians-potato-problem-is-actually-a-soil-problem","F5LNrsMzhqEo-Xw5YZ7JNvBl4YBh5ULHIPM7uf_H8ww",1786998375520]