I thought Ferrous Sulfate was water soluble?

Iron is absorbed into the body through the stomach. During this process of absorption, oxygen combines with iron and is transported into the plasma portion of blood by binding to a plasma protein called transferri. From there, iron and transferri are used in the production of hemoglobin (the molecule that transports oxygen in the blood), stored in the liver, spleen, and bone marrow, and utilized as needed by all body cells.
Almost two-thirds of the iron in your body is found in hemoglobin.

The human body needs iron for oxygen transport. That oxygen is required for the production and survival of all cells in our bodies.

The greatest influence on iron absorption is the amount stored in your body. Iron absorption significantly increases when body stores are low. When iron stores are high, absorption decreases to help protect against iron overload.

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Iron supplementation is indicated when an iron deficiency is diagnosed and diet alone cannot restore bodily iron content to normal levels within an acceptable time frame. Iron in supplements comes in two forms: ferrous and ferric. The ferrous form is better absorbed and is usually the preferred form when iron deficiency has been diagnosed.

Ferrous sulphate is partially water soluble.