Is Mars Ready for Human Agriculture?

It’s looking more and more like Mars might be ready for some terraforming:

Martian soil could grow turnips, Phoenix finds

“Some Martian dirt has the same basic chemistry as garden soil, a new analysis from the Phoenix lander suggests. The find widens the range of organisms that might be able to live on Mars. Although the analysis is not yet complete, the lander has already found trace levels of nutrients like magnesium, sodium, potassium and chloride. Although these ingredients were known to exist in Martian soil, until now no one was sure whether they would be soluble in water and thus potentially available for life.”