A scale to measure the alkalinity or acidity. Substances with a pH below 7. 0 are acidic (high in free hydrogen ions), and those above 7. 0 are alkaline, with 7. 0 neutral. Acids tend to be sour and corrosives, and bases (alkaline compounds) the opposite, and slippery. Rainy climates tend to produce acidic soil, dry climates alkaline soil. For growing food, soil pH should fall between 6. 3 and 6. 8. Lime is sometimes added to acidic soil and sulfur to alkaline soil. The more humus in the soil to buffer it, the less pH impacts fertility.