Trivia: The secret project created to use the Hermes to return to Mars to rescue Watney was called Project Elrond, a reference from the Lord of the Rings (also used in the original book of The Martian). Mitch Henderson, played by Sean Bean, was an attendee at the Project Elrond meeting. Sean Bean also played Boromir, who was an attendee at the Council of Elrond in the LOTR movie.
Trivia: When discussing "Operation Elrond", the Director says he wants his codename to be "Glorfindel." This shows he has read the book, as Glorfindel was cut out of the movie and replaced with Arwen.
Answer: Part of his job, aside from also being a mechanical engineer, was to use soil taken from Earth to Mars, mix it with Martian soil then grow seeds in it to see how Martian soil is for growing crops. This would be preparing for a longer term mission where growing full crops to feed the crew would be part of the mission.