Soil Acidity and Liming

TLDR: The pH is a measure of the concentration of hydrogen ions (symbol = H) in the water contained in the soil as mentioned in this paper, and it is used to indicate acidity and alkalinity.

Abstract: Soil pH, soil acidity, and their effects on plants The pH scale, ranging from 0 to 14, is used to indicate acidity and alkalinity. pH is a measure of the concentration of hydrogen ions (symbol = H) in the water contained in the soil. A pH of 7.0 is neutral, values below 7 are acidic, and those above 7 are alkaline (basic). The lower the pH, the more acid the soil. Each unit pH drop indicates ten times more acidity. For example, pH 5 has 10 times more acidity than pH 6, and 100 times more acidity than pH 7. Most Hawaii soils have pH ranging from 4 to 9. For comparison, here are the pH values of some common liquids: • pure water, 7.0 • “city” water (tap water), 7.5–8 • clean rain water, about 5.6 (because of CO2 presence) • “acid rain” water, 3.5–5.5 • lemon juice, 2.2–2.4 • orange juice, 3.4–4 • vinegar, 4–4.5 • fresh milk, 6.3–6.6 • mild soap solution, 8.5–10.