Yes, you'd have to mine the nutrients there for this to be effective. As you say, there's no point lifting it all from Earth. Organic life is largely made out

Yes, you'd have to mine the nutrients there for this to be effective. As you say, there's no point lifting it all from Earth. Organic life is largely made out of carbon, hydrogen, oxygen and nitrogen (the so-called CHON). There's not much carbon or nitrogen. This is one reason why people are so interested in hypothetical ice deposits at the poles. There's a reasonable chance that there'll be stocks of this elements there (in the form of ammonia or hydrocarbons).