Potassium sulphate should be fairly white if pure but its quite common for the agricultural grade to contain some black specks or impurities. It still works just fine. I don't know what the impurities are; perhaps it says on the box. I can't examine mine at the moment. Anyone know? Steve ------------------ To unsubscribe from this list, please send mail to majordomo@thekrib.com with "Unsubscribe aga-member" in the body of the message. Archives of this list can be found at http://lists.thekrib.com/aga-member/