Research suggests Martian soil has some of the nutrients plants need to grow and survive (see “Plants’ Nutrients,” right). But because of Mars’s extremely cold conditions, plants such as Watney’s potatoes would need to grow inside a controlled environment, such as his Hab. Also, just like on Earth, nutrients in Martian soil may vary from place to place. So, people stranded on Mars should be prepared to turn to ingenious ways for making the soil more suitable for plant growth—even if the only option is using their own feces, as Watney did.
When soils are rich in nutrients—such as nitrogen, phosphorus, and potassium—crops grow well. But when the soils are not as rich—with even just one of the needed nutrients in low supply—plants would not grow as well. Fertilizers help farmers to double or triple their crop yields and contain 5% or more of primary plant nutrients. These fertilizers also supply nutrients to the crops that some soils do not have.
Here on Earth, experts encourage using organic waste, or manure, to fertilize soil, although U.S. environmental agencies regulate the use of manure to avoid transmission of viruses and bacteria that could contaminate harvests. Other sources of nutrients, such as organic food waste, are also useful. That is why, for example, some people mix banana peels or coffee grounds into soil in their gardens.
On Mars, Watney did not have any man-made fertilizers available to him. He was not planning to stay there for a long time, let alone having to farm there, so his feces acted as organic waste that contained nutrients. In fact, in earlier times, when technology was less advanced, farmers used their own sewage to provide important nutrients, such as nitrogen and phosphorous, to their land.