Potash comes form your fireplace or as a chemical form. By definition, potash is any compound that contains a large quantity of potassium and, as a side benefit,  it raises the PH of soils.

Place potash near the roots in the form of humus to improve the general well-being of plants deficient in potassium. Potash is ideal for feeding tomato plants especially as they begin to set and fruit. Both roses and tomatoes love postash.

Wood ashes from you fireplace are a great source of potash, but if you leave the ashes out in the weather the chemical nutrients will seep out and loose their effectiveness. Sheltering the wood ashes out of the rain will preserve it's potency for later application.

Chemical forms of potash are potassium nitrate or potassium sulphate. Sulphate of potash will keep blackspot at bay in roses. Potassium is the chemical that helps your plants flower and it's very important for the structural strength of plants.

5-10-10 - Standard commercial fertilizer used for vegetable gardening. All fertilizers have three numbers. The first is nitrogen, the second is phosphorus, and the third is potash.