The basic pH scale runs from 0 to 14. The pH of a substance can be categorized by 0 being acidic, 14 being alkaline and 7 being neutral. Pure water has a pH of 7, but the normal pH range for tap water is 6.5 to 8.5. The pH of water is very important when you are gardening. Avid gardeners, especially hydroponic growers, use natural chemicals to lower the pH of water when it becomes too basic, or alkaline.
Use phosphoric acid to lower the pH in water. Add the chemical slowly and be careful not to let it come in contact with your skin. You can purchase a diluted solution of phosphoric acid made especially for gardeners at most hardware stores with garden centers. The amount of phosphoric acid to add to water depends on the concentration level of phosphoric acid in the product purchased. Follow the instructions included with your purchase.
Use nitric acid or sulfuric acid as another option to lower pH levels in water. These chemicals can be very dangerous if not used carefully. Add the chemicals slowly following the directions that came with them.
Add citric acid to your water to lower the pH if you are growing an organic garden. Make sure the citric acid is food grade.
Use goggles and gloves to protect yourself while dealing with chemicals.