The most commonly used form of Ferrous Sulfate is dried Heptahydrate because it has consistent iron, sulfur, water and oxygen contents, goes into solution rapidly, is free-flowing, and can be shipped in 50 lb. bags, bulk bags, dump trucks, pneumatic trucks, rail hopper and rail box cars.

Although dried Ferrous Sulfate Heptahydrate is free-flowing when produced and shipped by QC Corporation, this material has a tendency to compact and/or cake (becomes hard) under various shipping and storage conditions. These hardness problems occur due to the instability of the sixth and seventh molecules of interior water (H₂O), which have a tendency of releasing from each Heptahydrate crystal when stored in temperatures in excess of 75° Fahrenheit. Once interior water is released in a closed environment such as a 50 lb. bag, a rail hopper car or a storage hopper, the water becomes free or excess and causes a knitting or caking effect on the crystals. Compaction occurs in bulk storage and in 50 lb. bags that are stored under the weight of other bags due to the large amount of interior moisture (~44%) of each Heptahydrate crystal. Usually, compacted material can be broken up with some type of vibration as opposed to cake material that requires excessive handling. Upon request, and at no additional cost, QC Corporation adds up to 1% by weight of calcium carbonate which acts as an anti-caking agent to extend storage times of dried Heptahydrate and help to prevent caking or compaction. The addition of calcium carbonate is not advised if Ferrous Sulfate is being used as a reducing agent or is being pumped or sprayed in a liquid form because calcium carbonate is insoluble and can clog filters, pumps and spray nozzles.