Potassium and sulphur are both vital nutrients that plants take up from the soil; consequently, potassium sulphate (K2SO4) is a compound that finds common use as a fertiliser. Like other potassium compounds, potassium sulphate is soluble in water.
Potassium sulphate is a salt of sulphuric acid; a crystal of potassium sulphate contains two positively charged potassium ions for each negatively charged sulphate ion. When it dissolves in water, these ions dissociate, and the interactions between them are replaced by interactions between water molecules (which have a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atom) and the ions.
A solution of potassium sulphate dissolved in water has neutral pH, so potassium sulphate used as fertiliser should not affect soil pH. Since potassium is one of the most important soil nutrients -- and since it can also be leached from coarse-textured soil by water -- potassium sulphate plays an important dual role in providing both potassium and sulphur for the plant in a form it can use.
Potassium sulphate dissolved in water is not believed to be toxic to aquatic organisms. Unlike nitrate fertilisers, it will not increase nitrogen concentration in runoff or promote eutrophication -- an increase in the amount of nutrients -- in surface water. It's especially useful for crops like potatoes or tobacco that are sensitive to high levels of chloride, since potassium chloride would be an inappropriate choice for these plants.