the pH scales the concentration of hydrogen ions in a solution, you can use a pOH to measure the concentration of hydroxide ions.
It is important to realise that pH is a property of water. Acids and alkalis are solutions formed when substances dissolve in water, these substances which dissolve can change the concentration of the hydrogen ions and the hydroxide ions originally present in the water and form either acids or alkaline solutions. Bases are mostly insoluble in water and so do not have a pH value. You cannot measure the pH of a solid, bases which do dissolve in water will form alkalis, which since they are solutions in water and can have a pH value, but you really mentioned this in your question above.
Not sure if this answers your question or not!!