Matt Damon grew potatoes on Mars using poop. Is that realistic?

Matt Damon planting potatoes

Andy Weir’s The Martian — both the novel and Ridley Scott’s film adaptation — is one of our favourite stories here at The Prepared. Astronaut and botanist Mark Watney (played by Matt Damon in the film) finds himself stranded on Mars fighting to survive without much more than his brain.

One of the keys to Watney’s survival is twelve fresh potatoes sent with the astronauts to Mars for a Thanksgiving meal during their trip.

He uses those 12 potatoes to grow more potatoes, which, when combined with vitamin pills and rations, keep him alive for a year and a half (549 sols to be precise).

We recently published beginner guides to gardening, composting, and what crops to grow in a survival garden (which includes potatoes), so we got to wondering just how realistic the theories and skills shown in the movie are.

We already have space gardeners. Astrobotany is a field in and of itself, and astronauts have grown bok choy, lettuce, zucchini, and other vegetables in space, typically with hydroponics, in which plants are grown in water instead of soil.

ISS garden
Despite NASA being a shell of its former glory, the International Space Station has an impressive garden, even by Earth’s standards.

Summary:

  • Potatoes are a potent survival crop, packed with calories and nutrients, so Watney made the right choice among his few options. They’re even credited as the force behind the European conquest of much of the world.
  • There’s a lot of good science behind this story and it does come close to realistic, but in the end, it probably would not have worked as shown because it’s unlikely a random potato would take to the salty Martian soil and he wasn’t able to grow enough of them to meet his calorie needs.
  • Using poop (“humanure”) is valid, generally safe, and in this case, Watney did have just enough raw materials to create the amount of fertilizer needed.
  • Watneys techniques were mostly solid, such as the way he cut and cured the starter Thanksgiving potatoes, how he created his crop rows, how he planted, and so on.
  • Unbeknownst to Weir when he wrote the novel, Martian soil is full of toxic perchlorates that would have made things even more difficult for Watney.
  • You can survive on potatoes alone, but only for so long. Those who have tried have lost a great deal of weight.
  • Recent experiments have demonstrated that potatoes can grow on Mars, but likely only varieties that can tolerate the salty Martian soil.

Can you live on potatoes alone?

Potatoes weren’t Watney’s only option. He had other seeds as well, like peas and beans. He also had enough vitamins and protein (in the form of astronaut rations) to last for years. From the book:

There’s enough multivitamins there to last years.

I searched through the food supplies and found all sorts of things that I can plant. Peas, for instance. Plenty of beans, too. I also found several potatoes. If any of them can still germinate after their ordeal, that’ll be great. With a nearly infinite supply of vitamins, all I need are calories of any kind to survive.

And there’s five times the minimum protein in each food pack, so careful rationing of portions takes care of my protein needs for at least four years. My general nutrition is taken care of. I just need calories.

What Watney needed was calories:

My best bet for making calories is potatoes. They grow prolifically and have a reasonable caloric content (770 calories per kilogram). I’m pretty sure the ones I have will germinate. Problem is I can’t grow enough of them. In 62 square meters, I could grow maybe 150 kilograms of potatoes in 400 days (the time I have before running out of food). That’s a grand total of 115,500 calories, a sustainable average of 288 calories per day. With my height and weight, if I’m willing to starve a little, I need 1500 calories per day. Not even close. So I can’t just live off the land forever. But I can extend my life. The potatoes will last me 76 days.

I just need calories. I need 1500 calories every day. I have 400 days of food to start off with. So how many calories do I need to generate per day along the entire time period to stay alive for around 1425 days? I’ll spare you the math. The answer is about 1100. I need to create 1100 calories per day with my farming efforts to survive until Ares 4 gets here.

Weir was spot on with the choice of potatoes, which might be the most important crop in human history, often credited with fueling the European conquest of the world.

When you think of potatoes, you probably think of starch and empty calories. But potatoes can be packed with nutrition. Potatoes are a good source of many vitamins, like vitamin C, magnesium, and niacin. And of course, they’re high in calories, about 168 in a medium Russet, and carbohydrates, about 38 grams per medium Russet. In the book, Watney figures 150 calories per potato, so we’ll go with that. But while they have more protein than you might think, about 5 grams in a medium Russet, a potato-only diet does not provide enough. Potatoes are also low in zinc, chloride, selenium, and iodine.

However, in Watney’s case, he supplemented with vitamin pills and protein-heavy rations. Given that he’s a sci-fi astronaut, we’ll assume those provisions are of the highest quality and he was able to get well-rounded nutrition.

But it still may not have been enough. Terrestrial experiments with potato-only diets have resulted in major weight loss. In 2010, Chris Voigt, then head of Washington State’s Potato Commission, ate nothing but potatoes for 60 days to protest anti-potato federal proposals, and he lost 21 pounds in that time. Australian Andrew Taylor lost 115 pounds eating nothing but potatoes for a year.

Many of us could stand to lose that much weight, but not someone trying to survive on Mars for four years. Astronauts have to be pretty fit, so it’s doubtful Watney had that much extra fat to burn.

In the end, Watney only lived on Mars for a year and a half, so combined with supplementation his potato-heavy diet is plausible, but it would have made the grueling work of survival, including a long trip across the planet’s surface, difficult.

How to grow potatoes on Earth

Before we look at growing potatoes on Mars, we need to understand how to grow them on Earth, which I cover in our guide to the best survival crops. Here’s a quick review:

  • Potatoes are usually grown from other potatoes, rather than from a seed. The “eyes” that you see growing on potatoes will sprout and grow into more potato plants.
  • Potato farmers usually slice seed potatoes into several chunks, each with one or two eyes.
  • Potatoes are usually planted in trenches and covered in soil.
  • Potatoes need more phosphorus and potassium than nitrogen, but all three are important for growth.
  • More soil is added to the potatoes as the plant grows to encourage tuber formation and keep potatoes protected from the sun.
  • Potato tubers grow underground and are typically harvested at the end of the season.

How Mark Watney grows potatoes on Mars

Let’s review the methods Watney used to grow his potatoes. He started out the exact same way I tell you to start gardening — by building a compost pile:

When they made compost heaps and tried to conserve every little ounce of living matter, I laughed at them. “Look at the silly hippies! Look at their pathetic attempts to simulate a complex global ecosystem in their backyard.” Of course, now I’m doing exactly that. I’m saving every scrap of biomatter I can find. Every time I finish a meal, the leftovers go to the compost bucket.

Compost has innumerable benefits for soil, but in this case, Watney was using it primarily as a fertilizer. But scraps and leftovers weren’t enough, so he turned to humanure.

Historically, manure was the primary means of fertilizing soil. Human manure has an NPK rating of approximately 6-4-2. But the problem with using poop is the potential for pathogens.

Author Andy Weir came up with a clever solution for this problem. The space toilet used by the Martian astronauts freeze-dried their poop, so it was effectively pathogen-free while preserving those nutrients. Watney collected those freeze-dried poop bags to fertilize his potatoes.

The problem is that the freeze-drying process also killed bacteria crucial for plant life. So Watney pooped into bags to mix with the other crews’ droppings to kick-start the bacteria, with the assumption that he was already infected with whatever pathogens would be in his poop.

To cut a long story short, Watney filled a section of his habitat with a mix of Martian soil, soil brought from Earth, and his humanure mix.

Watney also used extremely intensive methods to maximize the number of potatoes he could grow:

For starters, I can give attention to each individual plant. I can trim them and keep them healthy and not interfering with each other. Also, as their flowering bodies breach the surface, I can replant them deeper, then plant younger plants above them. For normal potato farmers, it’s not worth doing because they’re working with literally millions of potato plants. Also, this sort of farming annihilates the soil. Any farmer doing it would turn their land into a dust bowl within twelve years. It’s not sustainable. But who cares? I just need to survive for four years.

At the end of his Martian farming career, Watney estimates that he grew about 2,000 potatoes.

Flaws in the Watney plan

Before evaluating the unique challenges of growing plants on Mars, let’s explore a few problems with Weir’s account. Overall, The Martian is a great, well-researched yarn, and I have no interest in trashing it, but a few things stood out to me.

One thing you’re probably wondering about: can you get sick by fertilizing crops with your own poop? Gawker took on this question years ago, asking several medical professionals and the resounding answer was no, you won’t get sick from consuming your own poop.

But another problem with poop is that unless properly composted, fresh manure can damage or even kill plants thanks to its high nitrogen content. Watney didn’t have months to spend waiting for his poop to break down, so he applied it more or less fresh. So it may not have made him sick, but it was still risky.

A big thing that stood out to me is that Watney never performs a soil test. In fact, it’s only mentioned once in the entire book, when NASA mentions sending a care package with soil tests. Are we to assume that NASA sent a master botanist to Mars with no means of performing tests on the Martian soil?

Soil testing is an important agricultural practice. Simple at-home tests can quickly give you a sense of your NPK concentrations, and more advanced ones can give you exactly breakdowns of various nutrients. You can then use this information to know whether you have enough of any given nutrient to grow crops. When your crops literally mean life or death, and you have very limited resources, those tests are critical.

It’s also hard to believe that NASA would send a master botanist to Mars, along with seeds and Earth soil to perform botanical experiments, and not send along any sort of fertilizers or other amendments. All they sent was Earth soil and a handful of seeds:

That’s why I have a small amount of Earth soil and a bunch of plant seeds with me. I can’t get too excited, however. It’s about the amount of soil you’d put in a window box, and the only seeds I have are a few species of grass and ferns. They’re the most rugged and easily grown plants on Earth, so NASA picked them as the test subjects.

Then again, Europeans of the early modern period didn’t have fancy soil tests or fertilizers. They mostly used poop — human poop at that — so maybe it would work.

Finally, let’s discuss a few of Watney’s potato-specific strategies. Referring back to the debate about whether whole or sliced potatoes give higher yields, Watney chose to slice his potatoes:

I cut each potato into four pieces, making sure each piece had at least two eyes. The eyes are where they sprout from. I let them sit for a few hours to harden a bit, then planted them, well spaced apart, in the corner. Godspeed, little taters. My life depends on you.

It takes longer than a “few hours” for sliced potato ends to cure, more like two to three days. And if those sliced potatoes don’t cure, they’re likely to rot in the wet ground.

I also found this curious:

Also, as their flowering bodies breach the surface, I can replant them deeper, then plant younger plants above them.

I doubt this would work for a few reasons. One is that replanting potatoes would run the risk of disturbing tuber formation. He could hill soil on top of those potatoes and plant more on top, but if you want large potatoes, they need room to grow. Cram in a bunch of potato plants together, and you’re going to get small potatoes.

The viability of Martian agriculture

The commercial success of The Martian led to a number of studies and analyses of whether Martian soil can support plants. In short: yes, it can!

NASA worked with Peru’s University of Engineering and Technology to set up a Martian simulator a year after the film’s 2015 release with below-zero temperatures, high carbon monoxide levels, low air pressure, and salty Mars-like soil.

The first experiment with 65 different varieties was an utter failure, with only four plants sprouting. But the scientists eventually found one that thrived, a cultivar called Unique.

But Watney didn’t grow potatoes in full Martian conditions. He grew them in a human habitat with water, oxygen, and other amenities found on Earth. He just happened to use Martian soil for the bulk of his growing medium.

The key factor would be the heavy amount of Martian soil used. The good news is that outside of nitrogen, soil on Mars has sufficient nutrients to grow plants. The bad news is that Martian soil is high in salt content, which is bad for most plants. Worse news, and something Andy Weir couldn’t possibly have known about at the time because it was discovered after the book was published, is the presence of perchlorates in Martian soil, which would have to be removed by rinsing and soaking in water. Otherwise, the potatoes could be toxic.

That seems like a simple problem, but fans of The Martian will remember the extraordinary lengths Watney had to go to to procure water. And then the water would have those perchlorates in them. Presumably, Watney could filter those out, but it would be an additional complication.

The cold equations of Martian potatoes

In theory, growing potatoes on Mars might work, but does the story’s math check out?

All in all, Watney had 126 square meters of growing space or 1,356.25 square feet. We’ll round that down to 1,350 square feet for the sake of simplicity. We know from the movie that Watney planted his spuds in rows, and typically you want rows to be three feet apart. To get a rough estimate of actual growing area, divide 1,350 by 3 to get 450 square feet of growing space.

Martian soil is pretty poor, so fertilizer is a must. Here’s what Texas A&M University prescribes for fertilizing potatoes:

Use 2 to 3 pounds of complete fertilizer (such as 10-20-10) for each 30 feet of row in bands 2 inches to each side and 1 inch below the seed piece. Do not allow the fertilizer to touch the seed piece.

So we need at least 2 pounds of fertilizer for every 30 feet of planting row. Given that humanure is 6-4-2 instead of the specified 10-20-10, we’d probably best lean toward at least 3 pounds every 30 feet. So 450 feet of planting row divided by 30 feet is 15, multiplying that by 3 pounds means Watney needed 45 pounds of poo for his potatoes.

The novel doesn’t specify how much poop Watney had to work with. Ares III had a six-person crew, four men and two women. Men produce about 1 pound of poop per day, women a little less, but we’ll give them full credit for the sake of equality and easier math. So, six pounds of poo per day.

The rest of the crew was only on Mars for six days before blasting off, so they presumably deposited 36 pounds of poop before it hit the fan.

It was day 14 on Mars before Watney seriously started contemplating his potato plan, so by then, he should have had about 50 pounds of poop — just enough to work.

The math works out for fertilizer, but did Watney grow enough potatoes? Watney estimates that he grew 2,000 potatoes and needed 1,100 calories per day (1,500 total, but he factored in his other rations). Assuming the potatoes had 150 calories each, he would need to eat 7.3 potatoes per day to keep up his calories. We’ll round that down to 7 potatoes. So he had enough potatoes for about 285 days, not even close to the year-and-a-half mark he hit in the book.

To be fair, Weir addresses this point in the book:

“Sorry, but no,” Keller said. “He’s already at a minimal calorie count. In fact, considering the amount of physical labor he does, he’s eating far less than he should. And it’s only going to get worse. Soon his entire diet will be potatoes and vitamin supplements. He’s been saving protein-rich rations for later use, but he’ll still be malnourished.”

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