In any garden, your soil should be balanced between nitrogen, phosphate and potassium. If you discover your soil is too low in potassium for your growing purposes, potash fertilizers are the best way to fix the problem. You can apply any of several types of potash, including muriate of potash, the more popular sulfate of potash or a stand-alone source like greensand or seaweed.
Dissolve the potash fertilizer in warm water. It is much easier to use liquid fertilizer on container gardens than powder or granule forms. Use about two cups of fertilizer to a gallon of water to make a solution. Label this jug clearly as potash solution.
Mix the potash solution with water. Use about 2 tbsp. of the solution in a gallon of water.
Water your container garden with the lighter potash solution and water mix. Do this once a day, depending on the needs of your plants.
After a week, check the soil again. If it is now adequate in potash, you can begin using a more balanced liquid fertilizer that includes nitrogen and phosphates. You can prepare this in the same way as the potash-only liquid fertilizer.