In hard water areas, add 1 cup (225ml) of vinegar to 1 gallon (3.75 liters) of tap water for watering acid-loving plants such as rhododendrons or azaleas. The vinegar will release iron in the soil for the plants to use. Azaleas will benefit from an occasional watering with a mixture of two tablespoons of vinegar to 4 cups (1 liter) of water.