Sulphate of Potash Potash is very important fertilizer  Solubility of potash.... Potash solubility SOP Sop is widely used potash fertilizer in Pakistan. Its chemical formula is K2SO4 and it contains 54% K2O and 18% S, if pure. But for commercial use it is available with 50% K2O and 18% S, as it contains some impurities like Magnesium , ammonium,...