Nobody ever said it'd be easy.

I think you're right that Mars, and any other world, presents its own set of unique challenges. But I also think GM crops are an inevitability here and elsewhere.

There may not be much moisture on Mars - unless you're talking about that trapped in a martian ice-cap - but there may be quite a bit under Mars. Many people think there is. They may turn out to be wrong. But if they're right, martian agriculture may involve crops that tap that moisture.

In any event, I wouldn't underestimate human ingenuity. I'm not sure humans will decide to establish colonies on other worlds, but if they do they will no doubt tackle these problems one at a time. Maybe they'll slam a few comets into Mars - centuries or millennia from now - as part of a long-term terraforming effort. Comets contain some of the organic material farmers would require to grow their crops. Before that, though, other methods for dealing with harsh martian conditions could be found.

Earth-soil was transformed over time into something plants could use - maybe humans will find a way to help that process along on Mars.

With the right amount of blood, sweat, tears and dollars, that is. :-D

Martian Soil

Zeolite Soil

Lunar Agriculture

Life Sciences

Lunar Biosphere

Crops for Space Bases

Lunar Hydroponics

NASA Returns to the Moon