If soil is added to water, the pH of water will change based on the pH of the soil. Adding soil to water is a way to assess the pH of the soil which is an important property. Plants can live in a variety of pH conditions as low as 4.5 for orchids and hydrangeas up to 8.0 for lilacs. The pH of the soil affects the availability of nutrients for the plant and controls other important chemical processes. The pH also affects the microorganisms in the soil that the plant depends on. What can make soil more acidic (lower pH) is rainfall, fertilizer use, and decaying organisms. More basic soil (higher pH) is the result of a high salt concentration.

Different minerals, when dissolved in water, behave as either acids or bases. Carbonate rocks in particular are bases because they soak up protons to make hydrogen carbonate, which then dissociates into carbon dioxide and water. A lot of organic materials, on the other hand, tend to give up protons, making them acid.