Since soil samples on mars seem to be strongly oxidizing, even containing perchlorate, the vegetables are likely to catch fire upon touching martian soil.
EDIT: OK, for the benefit of whoever didn't have a sense of humor, I was suggesting that the astronauts might be planning on bringing some soil with them.
You could have a really good plan to convert martian soil for plant use, but then again, that might fail. This obviously means you can't depend on your garden. Also, what you would have would be a laboratory, not a garden. You would spend a lot more time analysing the food than eating it.
On the other hand, you could carry out a sample return mission, bring some mars dust back, run tests, and land with a solid plan. Only thing is, sample return from mars would make moon rocks look cheap.
Finally, bringing the soil with you would be the most reliable method. It's moon rocks in reverse though! That soil would be, kilo for kilo, the most expensive soil in the history of mankind.
This might require slightly better AI than currently exists, and a lot of patience from telepresence operators, but it seems achievable in the near future.
(N2 also exists in the atmosphere, but at low levels.)