The soluble grit is Limestone Grit & Crushed Sea Shells which provides a source of Calcium that is essential in a birds diet.
The insoluble grit is Flint Grit which the birds store in the crop/gizzard where it acts as a grinding agent to assist digestion of seed & kernels which are swallowed whole.
Charcoal assists in sweetening of the crop by enhancing the palatability.
The resealable 3 Litre plastic bucket helps avoids waste & mess and stops the grit becoming contaminated.