Soil acidity is measured by the pH scale. This measures the concentration of hydrogen ions in the soil solution. A pH value of 4 - 7 is acid, pH 7 is neutral, and a value from 7 - 8.5 is alkaline. Acidity has considerable influence on nutrient availability and soil organisms. Different crops have different pH tolerances. Soils tend to become more acidic with cultivation due to the release of organic acids on the breakdown of organic material.