Plants use potassium to help build strong cell walls, improve photosynthesis, and maintain proper cellular levels of electrolytes. Potassium is an essential nutrient for plants and is considered a macronutrient. Like many other nutrients, potassium is used by plants in small amounts. Most plants will use potassium at the rate of 1% of their dry weight. To make liquid potassium fertilizer, mix a 1-pound bag of granular potassium chloride with water and stir to dissolve. Add 1/4 pound of magnesium sulfate per gallon of water and stir again. Filter the mixture through a fine mesh strainer to remove any remaining solids. Store the fertilizer in a glass container.