Mark Watney farms potatoes in Martian soil and his own waste, because the bottleneck is calories.
That is the whole engine of The Martian: nearly every problem becomes a measurement. Watney counts food, energy, water, and time instead of waiting for luck; each fix buys days and opens a new constraint somewhere else.
The suspense never hangs on courage. It hangs on whether the math leaves him enough days to be rescued.