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Do current plans for people going to Mars by SpaceX depend only on consumables loaded from Earth or do they also include obtaining some materials from Mars fairly soon after arrival?
Does the plan for the return trip then include bringing along and using something produced on Mars? (water, oxygen, etc.)
The basic model that SpaceX is following is similar to Robert Zubrin's Mars Direct model.
In Mars Direct, you send a ISRU (In Situ Resource Utilization) plant first, then by the time the next Mars window comes around either you have a fully fueled return vehicle waiting, or you do not. But you know before you go.
SpaceX plans to use 2022 if they are ready to launch a cargo only mission to Mars, that would likely carry an ISRU plant to process the fuel and oxidizer. This is why SpaceX selected Methane as their future fuel, as on Mars the atmosphere is primarily carbon dioxide, which can be chemically processed (Sabatier process) to Methane if you add in some hydrogen (which you get from water, or bring with you).
Food wise, you bring everything with you.
Water wise, you need recycling anyway, so you bring enough for the mission.
Fuel/Oxidizer you expect to manufacture on Mars.
Nitrogen is possibly a problem.
Indoor hydroponic farming with LEDs is being done commercially. That method needs no soil, very little water, it is compact and productive. Plant nutrients in the soil could be derived from human waste. It can also be made from organic sources. Martian soil has plant nutrients. So it could become a source of food directly, or indirectly through hydroponics. For perspective, the average person eats just under 2K pounds of food a year. https://www.reference.com/food/many-pounds-food-average-adult-eat-day-3f49d34cd3d872cd So one might compare the weight of food against the wait of farming gear. The advantage of farming is that it can produce food indefinitely, whereas when food runs out, it's out.
