What does adding fish parts to soil actually do to the soil?

Question

Looking for the chemicals that it adds, and affects on plants.

Answer 1

C HOPKNS CaFe Mg B Mn CuZn MoCl.

The sixteen essential nutrients required for plant growth.  Addition of any organic matter (including flesh) will add at least some, if not all, of these nutrients, in the plant available form needed for plant growth.  This is soil fertility management at it's simplest.

Answer 2

The proteins in the fish break down. This adds nitrogen to the soil. You also get some phosphorus and potassium, but mainly nitrogen.