Soda can be used to influence the hardness of the water / dyebath. It increases the ph level and most plant-dyes give a different shade or colour which this hardened bath. It usually darkens the colour where acid such as cream of tartar or simple vinegar does the opposite. Look for a good recipy or do some testing with it yourself!
This is very fine pure Soda which easily dissolves in hot water. Perfect for (plant) dyeing !