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Should We Poop on our Plants?

Thoughts on Growing Food on Mars Inspired by the book "The Martian", by Andy Weir.

Humans are a curious species. We are born to question and explore all that lies before us. Space is one of those frontiers that have attracted many humans ever since our eyes wandered upward to ponder stars in the night sky. As one would expect, it is only a matter of time before we begin taking other species from our home planet as we learn how to live extended periods off-planet. Chiefly, plants! Upon reading the recent best-selling novel, The Martian, I was delighted to see an astronaut whose specialty in botany plays a significant role. There may be hope for my astronaut career after all!

The premise of the book involves a group of astronauts on a mission to Mars. Six days after landing on the red planet an unexpected storm forces the team to abort the mission and evacuate the planet, leaving presumed dead astronaut Mark Watney behind. After surviving the storm Watney must use his botanical knowledge to "Grow food on a planet where nothing grows".

Author of The Martian, Andy Wier, spent an incredible amount of research to base his story on sound scientific knowledge. Let's take a look at Wier's story and the horticultural science behind Watney's survival by growing food on Mars.

Disclaimer: Despite my fervent obsession with science fiction/non-fiction, I am not an expert in astronomy, astrobiology, or anything 'space'. My capabilities lie predominantly in terrestrial horticulture. That being said, this might be the most fun I've had writing a blog post. Oh, and potential spoilers ahead. There, now you've all been warned.


To grow food in Martian soil, the first hurdle the character Mark Watney must overcome is…soil. Soil is a media that contains nutrients, water, and living organisms. Mars has two out of three, not bad, but that does not qualify it as the type of soil we find here on Earth. The term regolith is often used when referring to Martian soil. Regolith is defined as a layer of unconsolidated materials covering solid rock. The Moon has lunar regolith and Earth's soil is a type of regolith. After some research it seems scientists use the terms 'soil' and 'regolith' interchangeably and in fact argue quite often on their definitions. I also use the terms interchangeably below.

Ideally, hydroponics would be the way to go because Watney must grow the plants indoors. Watney doesn't have access to hydroponic equipment, so he must use Mars's regolith. Like Earth soils, the Martian regolith will be used to support the plant and plant roots, however, unlike Earth, the regolith on Mars is inert and lacks any biological activity. Living organisms have a set of criteria in order to survive. According to NASA's Jet Propulsion Laboratory, Mars is a self-sterilizing planet. A saturation of solar ultraviolet radiation, extreme dryness of the soil, and the oxidizing nature of the soil chemistry precludes the presence of life in Martian soil.

Aside from the biology, my assumption is that soils on both Earth and Mars are relatively similar in their elemental composition. NASA scientists use a mixture of volcanic soils to simulate Martian regolith. The basic elements for plant growth are there; only difference being on Earth we have microorganisms (such as bacteria) that convert those elemental nutrients into a plant available form.

Watney must inoculate the Martian soil with bacteria to jump start the processes of life to begin growing food. What can he use as an inoculant on a sterile planet? His excrement of course! Is this a recipe for gastronomic disaster? Absolutely! Yet, the novel's author states that because the flora in his feces is from Watney's own gut, he can't get sick from contaminated food. My question- is soil bacteria and the bacteria found in a person's gut comparable? Plus, I'm pretty sure there are bacteria in your lower GI that wreak havoc when introduced to your upper GI. These are questions for a biologist or your gastroenterologist, depending on who you bump into first.

The idea of using feces to grow plants has been around since the dawn of agriculture. On Earth we use manures from various species to improve soil biology, soil structure, and plant nutrition. Manures can be detrimental to crops (and human health) if they are not allowed to be composted prior to application. If Watney hadn't composted his waste prior to use, he would have encountered issues akin to what we see on Earth in regards to the use of fresh manure.

Reading Watney's attempt to activate the Martian soil, conjures memories of certain soils on our own Terra. On Earth there are similar situations of depleted soils in post-construction environments. Typically, in a housing development the topsoil is stripped exposing the near-inert subsoil. The developer should come back in to spread the topsoil that was set aside, alas that is often not the case. To facilitate proper plant growth a homeowner could inoculate the subsoil in some way. There are a variety of methods some being: hauling in new topsoil, compost, or applying biostimulants.

One final thing to consider when it comes to the soil of Mars is that 1% of the soil composition is perchlorates. Calcium perchlorate is a compound that is caustic to humans. If we touch or inhale this substance it would burn our skin or lungs, respectively. Since Mars is a windy, dust-laden planet, every piece of equipment would be covered in a red-film. Watney, may not have survived shoveling Martian soil around his Hab (short for Habitation Module) due to stirring up the caustic perchlorates. To the author's credit, perchlorates were only recently discovered on the red planet. Luckily, researchers have developed means to deal with perchlorates for our future non-fiction Mars mission.


In the book, astronaut Mark Watney surmises that the existing lighting within the Hab will be sufficient to grow crops of potatoes. This statement prompted some head scratching because the key factor here is light. Despite our common vernacular, we do not feed plants like we do kids and ourselves. Plants have the wonderful ability to make their own food through photosynthesis. Photosynthesis is a complex process where plants capture light energy and along with CO2 and water, convert these elements to basic sugars to be used in plant development. It is not precisely stated what type of lighting or how bright of light Watney has in the Hab, but I have a suspicion it would not be enough to grow substantially sized potatoes.

Potatoes are a full sun crop, requiring a minimum of six hours of sun per day to yield effectively. With even the best indoor lighting, potatoes would become leggy with diminutive development of the underground tubers (in other words- small potatoes). Upon harvest, Watney's inspection of the potatoes indicates they are smaller than he had hoped. So did the author take insufficient lighting into consideration? I think so.

Though not in the book, if perhaps you're wondering about a Mars greenhouse, keep in mind the distance to Sun has increased to 1.52 AUs. (AU – Astronomical Unit is the distance from the Sun to Earth, represented as 1 AU) The diminished sunlight would most certainly play a role in light requirements for plants. Martian sunlight is about 44% as intense as it is here on Earth. Even if a Mars colony constructed some type of greenhouse, supplemental lighting would still likely be needed for fruiting crops. Just like here on Earth, when to grow food on Mars may be dependent on the season, location on the planet, and axis tilt.


There's little for me to add about Watney's water woes that he didn't already solve. Some discoveries that have come to light after the books publishing:

  • Water was discovered on Mars's poles.
  • Scientists believe there are significant traces of water beneath the Martian surface.

So could astronaut Watney have simply brought in soil to let the Hab's water reclaimer extract that water as it evaporated into the module's atmosphere? It depends on the amount of water content in the soil. Is it parts per thousand, million, perhaps billion?

On Earth we describe the amount of water in our soils with various terms: saturated, field capacity, plant available, water holding capacity, permanent wilting point, and oven dry. We call the last one oven dry because there comes a point where plant roots or gravity cannot separate a water molecule from a soil particle. The chemical bonds are simply too strong, making that water unavailable to plants. It takes a lot of energy in the form of a hot oven to break those bonds.

Watney brought in only the minimum amount of soil he needed. Excess soil in the Hab might draw humidity out of the air, binding it to Mars's dry soil particles, thereby reducing the critical amount of water Watney needs to grow his potatoes.

There was some astounding engineering/chemistry at play when Mark Watney made his required water out of rocket fuel.

Concluding Thoughts

Could Watney have survived on Mars by growing potatoes? Based upon what we know right now, perhaps. The Irish did this for centuries until disease overtook their potato stock. I have not seen the movie, but a colleague mentioned Watney used peeled and pressure packed potatoes. While the potatoes may still be moist, without skin and eyes, they wouldn't have grown. Another area to suspend disbelief, but it's Hollywood.

There are still many more discoveries to be made. The Martian is about surviving on Mars, but reading it made me think about how we survive here on Earth. The more we learn in space, the more informed we are about our own planet.

The Martian is a good story, and I believe it will inspire many, to think about how we grow food right here on Earth. Plus the book speaks to the innate curiosity of humans. Whether you're exploring distant planets or the infinite intricacy of the soil beneath our own feet, don't forget to bring potatoes. But please, don't poop on the plants. Compost it first.

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