95% carbon dioxide, 3% nitrogen, 1.6% argon. The sabatier reaction take CO2, Hydrogen and energy to produce methane and water. Methane is useful as a fuel, but the hydrogen has to go back into the process. So rather than producing water this actually produces oxygen. Of course, oxygen has it's uses.
Plants need CO2, water and (filtered on mars) sunlight and produce oxygen, food, nice smells and a pleasant environment. Some will also require ants or bees for pollination. Plants also require nitrogen and both nitrogen and argon are useful buffer gasses. Now the martian soil.
Water is the first thing colonists are going to look for. The good news is that an orbiting Gamma Ray Spectrometer found ice just under the surface almost everywhere including around the equator. So colonists will need to be able to drill wells, heat the ice and pump it to the surface. Mars even has snow and liquid water rain. Glaciers may exist underground on mars but would probably be a bad place for a colony because it may have less mineral diversity than other places. Industry needs more than just water. Update: They don't need wells. They can get water just by heating dirt from the surface which is the Mars One plan.
While many will talk a good game, we don't really know that much about the composition of martian soil. We know that everything we need for an industrial society exists on mars. We just don't know the concentration and distribution beyond a few simple things. We're just going to have to go and find out.
We do know that iron (15% of soil and everywhere) is abundant and steel will be needed to produce most tools and industrial machines. Steel making is going to be an essential knowledge and specialty for some of the colonists.
Aluminum (4% of mars soil) can come from alumina silicate clay when there is no bauxite. It does require a lot of energy to produce.
Silicon dioxide (45% of soil) provides silicon for making solar panels.
Copper sulfide can be found at the base of lava flows.