24

In The Martian, main protagonist Mark Watney

has some bad luck on Mars at some point, after which he says he cannot grow anymore potatoes.

What prevents him from

gathering more soil on Mars, collecting his feces and water like he did before, and reusing fuel from one of the vehicles that are still around to replace the hydrogen in the artificial garden heater?

Note: I am not a native English speaker and saw the film in a US cinema, which might be why I missed some plot details.

26

One problem is the loss of a third of his water. He used up the hydrazine fuel left in the lander - it was the only hydrazine he had available. There's only one other MAV on Mars, and he needs to leave that one intact to be able to leave Mars. There is no other source of hydrazine.

The Martian, Ch. 15:

   As for water supply, I have 620L remaining. I started with 900L (300 to start with, 600 more from reducing hydrazine). So I lost almost 300L to sublimation.

The other problem isn't completely s-p-e-l-l-e-d out in any one place in the book, but it's the lack of Earth soil bacteria. He only had a small amount of Earth soil to start with, and after the accident all of the enriched soil he made was sterilized. In hindsight he probably should have kept a small sample of the enriched soil stored in a safe place, but nobody can think of everything.

The Martian, Ch. 14:

   With a complete loss of pressure, most of the water boiled off. Also, the temperature is well below freezing. Not even the bacteria in the soil can survive a catastrophe like that. Some of the crops were in pop-tents off the Hab. But they’re dead, too. I had them connected directly to the Hab via hoses to maintain air supply and temperature. When the Hab blew, the pop-tents depressurized as well. Even if they hadn’t, the freezing cold would have killed them.
   Potatoes are now extinct on Mars.
   So are earthworms and soil bacteria. I’ll never grow another plant so long I’m here. (incidentally, this is the only mention of earthworms in the whole book)

Later, he finds out that some of the soil bacteria had survived and was replenishing the soil. But by then he was hard at work on the new rescue plan and didn't have time to go back to farming.

The Martian, Ch. 17:

There's still soil everywhere. No point in lugging it back outside. Lacking anything better to do, I ran some tests on it. Amazingly, some of the bacteria survived. The population is strong and growing. That's pretty impressive, when you consider it was exposed to near-vacuum and sub-arctic temperatures for over 24 hours.

  • 3
    But he also didn't have any seed to reseed his farm with... – Aron Oct 6 '15 at 16:23
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    TLDR; all of the potatoes, which were plants, were killed by exposure to the Martian atmosphere. You can't grow a crop with dead plants. – Chris B. Behrens Oct 8 '15 at 18:56
  • 1
    @fractalspawn. That is exactly my point. The potatoes he has at that point are dead. Freeze dried. Caput. Dodo. Extinct. The Martian atmosphere is nothing like your fridge. – Aron Oct 13 '15 at 0:05
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    @fractalspawn It's not made clear in the film, but to store the potatoes he is eating on the way to the MAV, he effectively freeze dries them by storing them outside the Hab. These potatoes are, as Aron said, dead, sterile, and will not grow. – AerusDar Oct 13 '15 at 4:03
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    ah, I get what's being said now; sorry. The potatoes that he has/had are only good as food, and nothing else. I was just falsely assuming that if you have a potato, you could cut and re-grow from it. Thanks for clarifying @AerusDar. I had a dummy moment. :) – coblr Oct 13 '15 at 19:51
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The sudden exposure to atmosphere (the cold and extremely low air pressure) killed the plants, the bacteria, and the earthworms entirely. The potato plants won't sprout. The earthworms are a bust, and Watney probably isn't carrying the right sort of bacteria.

The farm is dead.

With a complete loss of pressure, most of the water boiled off. Also, the temperature is well below freezing. Not even the bacteria in the soil can survive a catastrophe like that. Some of the crops were in pop-tents off the Hab. But they’re dead, too. I had them connected directly to the Hab via hoses to maintain air supply and temperature. When the Hab blew, the pop-tents depressurized as well. Even if they hadn’t, the freezing cold would have killed them.

Potatoes are now extinct on Mars.

So are earthworms and soil bacteria. I’ll never grow another plant so long I’m here.

As per this discussion on potato shipping:

Potato tubers kept in in dry environment will tolerate some frost, but not outright freezing.

  • 1
    Interesting about the earthworms. I was reading a book about native North American trees recently and it made the claim that earthworms are not native to North America. I decided to dig a bit and found this article: The hardwood forests of New England and the upper Midwest, for instance, have no native earthworms—they were apparently wiped out in the last Ice Age. So it's questionable if you actually need earthworms. The book I was reading claimed recent research indicated they do more harm than good. – Scott Whitlock Oct 5 '15 at 16:32
  • Interesting. It might make a difference, though, that he had to have more rapid results. From what I gleaned from your article, without earthworms, the decay back into nutrients is more gradual. – FuzzyBoots Oct 5 '15 at 16:41
  • I agree it's probably different on Mars or in a greenhouse. One of the problems with earthworms is they apparently drag organic material down lower in the ground, which isn't as helpful. Might not be a problem when you have a "floor" under your dirt. – Scott Whitlock Oct 6 '15 at 9:27
  • @Scott Whitlock: Interesting. In those forests earthworms are invasive species. – Peter Mortensen Oct 10 '15 at 9:12
  • First paragraph: Oxygen is a waste-product for plants. (Photosynthesis). It's not the lack of oxygen, it's the lack of pressure that's damaging, because it's lower than the vapour pressure of water even at Mars temperature, so any water boils if it doesn't freeze first. (And then it sublimates). – Peter Cordes Jan 28 '17 at 5:23

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