Over at Popular Science they provide some insight into the technical hurdles which must be overcome in order to establish a colony on Mars. Here’s a small taste:

Growing crops on Mars isn’t just for feeding hungry astronauts; plants will serve as a vital source of renewable oxygen for the habitat. It’s a much better option than consistently sending heavy oxygen tanks to the red planet, which will take up too much precious space on resupply missions and cost a lot of money to transport.

Studies have shown plants may be able to grow in Martian soil, however crops have never been grown in the Mars gravity environment, so further testing is required to see if vegetation can survive at all. But if that works, the plants required to feed a multi-person crew will be producing a lot of oxygen. And that’s not necessarily a good thing.

According to Do’s report, too much oxygen in a closed environment can lead to an increased risk of oxygen toxicity for the crew, and even worse, spontaneous explosions. So O₂ will have to be vented from the habitat. To do this, the astronauts would need a specialized method for separating oxygen from the gas stream. There are a number of methods for doing so here on Earth (cryogenic distillation and pressure swing adsorption) but none of these technologies have been tested for a Martian environment, and considerable research and development would be needed to make these techniques viable on another planet.

[Popular Science: How You’ll Die On Mars]