I can't really tell from the image, but do you have a swirl of precipitated compound? It probably isn't your K+, or SO4-- ions, that have come out of solution, because all <"potassium, and nearly all sulphate, salts are soluble">.K2SO4?
Potassium oxide (K2O) would be easy to check for via a pH measurement, it is a base. Potassium sulphate and potassium chloride (KCl) are neutral salts and won't change pH. All agricultural fertilisers have their K content expressed as potash (K2O), even though the compound doesn't contain any K2O. Potassium oxides are yellow compounds, but they are highly reactive and I'm pretty sure that isn't what you've bought.Poured out it’s golden in colour. First guess was K2SO4. Alternative is K2O due to its pale yellow colour in solution.
The cheapest potassium salt to buy is potassium chloride (KCl), there are actually <"mineable deposits"> of this (<"even in the UK">) where it is mixed with iron, and that would be my guess.It’s an over the counter product sold as a source of potassium for aquarium plants
I'd be interested in any more details, the UK potash source is a <"polyhalite" mine">, so it has a lot strange <"evaporite minerals"> present.No other details (as per usual) and would like to have a guess at what it is.