soap powder 
noun [mass noun]
Detergent in the form of a powder, used for washing clothes.Items of hygiene including toothpaste, toothbrushes, soap, soap powder, toilet rolls, brushes and combs, cotton wool and first aid packs....
- It removes stains better than any other soap powder.
- It is a short drive from here to the site of the Great Geyser, which first started erupting in the 14th century and finally gave up early last century after years of tourists pouring soap powder in it to make it erupt.