The devil is in the details. Even though all minerals contain impurities, NPK fertilizer is processed to reduce them to acceptable levels for agriculture. If they did not do this, places dosed with large quantities of it (year after year after…) would become superfund sites. It is the same reason coal is so nasty: the CO2 is nothing compared to the ash—which is loaded with heavy metals. If the ash retaining ponds around a coal plant ever broke, the land would be uninhabitable for centuries, so the the ppms and ppbs are crucial information here.