Measure the alkalinity and pH of what you wish to adjust. A pH above 7 indicates that something is basic. The pH scale is log-based, so a pH of 9 is 10 times more basic than a pH of 8. If alkalinity is being measured in a swimming pool, the test may be as simple as dropping some of the water onto a pH strip or pool alkalinity test kit.
Increase soil alkalinity by adding basic solids such as lime. Sprinkle lime over the soil, wash it in with water, and wait until at least the next day to test it again. The adjustment process should be done slowly and carefully; adding too much at a time could kill your plants.
Reduce soil alkalinity by supplementing the soil with acidic organic material such as coffee grounds, pine needles, waste orange juice or vinegar. Acidic fertilizers, such as ammonium sulfate, give quick results. Elemental sulfur works but is slower acting.
Increase the alkalinity of your swimming pool by adding sodium bicarbonate (baking soda). Use 14 grams of sodium bicarbonate for every 1,000 liters of pool volume to raise the alkalinity ppm (parts per million) by 10. The total alkalinity should be between 80 and 120 ppm and have a pH of 7.2 according to your pool's test kit.
Reduce your pool's alkalinity by adding hydrochloric acid (also known as muriatic acid), which can be purchased from a pool store, or for much cheaper from a hardware store. The stuff from the hardware store is exactly the same. Use 13 mL of acid (hydrochloric, 30 percent) for every 1,000 liters of pool volume to reduce the alkalinity ppm by 10. Hydrochloric acid reacts with sodium bicarbonate to produce carbon dioxide, which is eventually released into the air, effectively removing it.