Nitrates on earth range from 10-50 mg/kg in soil, on mars they make up 1100ppm = 0.1% = 1mg/kg, so in the extreme you can convert 10% of martian land to soil. Seems like plenty.

We just don’t know if the 1/3 gravity will cause bone loss problems - going to the moon is the best way to study it, but who knows. Probably, people will just age 3x faster in terms of bone mass on mars. https://en.m.wikipedia.org/wiki/Spaceflight_osteopenia

And yeah, the first years of a mars colony are gonna be dependent on earth. We’d definitely make sure we have enough rockets that they can abort if shit goes south on supply missions, but there’s no reason earth can’t supply them at first.