If most powder detergents contain sodium carbonate, and if sodium carbonate causes the calcium in water to form a precipitate which will settle back into clothes and washer parts, why do you use a powder detergent?
Liquid detergents don't have the same "fallout" effect from whatever are the water softening agents in them?
I add STPP to the wash when using powders (and liquids, for that matter).
Looking at a box of Biokleen powder. Sodium carbonate is the 1st ingredient. It is highly alkaline and there for that purpose. Zeolites are the water softner.