The formation of paramagnetic iron sulfate hydrate salts containing iron in the di- and trivalent states from black magnetic iron oxides of high iron content and their use as fertilizers effective on both acid and alkaline soils is disclosed. Such oxides are comminuted, wetted with a specified amount of water, reacted with concentrated sulfuric acid, and dried. The resultant products are paramagnetic and contain iron in both the di- and trivalent states. These products have fertilizing and soil conditioning properties superior to non-magnetic iron compounds containing iron only in either the di- or trivalent state.
