You should only apply potash fertilizer to soils that are lacking in potassium. You can determine soil potassium levels using a home soil test kit available from many local gardening stores. Potassium levels between zero and 60 require 3 pounds of potash for every 1,000 square feet.
Levels that fall between 61 and 120 parts per million need 2 pounds of potash fertilizer for every 1,000 square feet, and 121 to 181 parts per million requires only 1 pound of potash fertilizer. Soil that has levels greater than 181 parts per million already has enough potassium and does not require potash fertilizer.
It is best to apply potash fertilizer to a garden before the actual planting process. This is because potash fertilizer generally sits on the top portion of the soil and does not easily permeate down to the deeper levels on its own. Therefore, it is important that gardeners till and turn the fertilizer and the dirt together so that the potash is distributed evenly throughout the soil. This process makes it easier for the potash fertilizer to reach the root systems of the plants.
You should generally only apply potash once every year in order to avoid damaging or killing plants. Excess potash can cause an abundance of potassium salt to form in the soil, which is detrimental to the root systems of the plant.