General relationship between soil pH and acidity. Soil pH, which is the negative logarithm of the soil hydrogen concentration, is expressed on a scale from 1 to 14. Because the pH scale is logarithmic, soil with a pH of 6 is 10 times more acidic and soil with a pH of 5 is 100 times more acidic than soil with a pH of 7. Remember that the lower the pH number, the more acid the soil and therefore the greater the need for lime. This relationship is shown in Figure 1 .
Plants also remove calcium and magnesium.
Decay of crop residue or the addition of animal waste or other organic matter increases soil acidity. Widespread use of fertilizer nitrogen also increases soil acidity. This is the combination of aluminum and hydrogen in soils and is used to predict lime needs. Lime recommendations must take into account differences in acidity between soils and differences among various crops' tolerance to acidity. The reason for the difference is that soils high in organic matter generally contain less aluminum and are thus less toxic to plant roots at a relatively low pH. Furthermore, crops differ in their ability to tolerate a low pH. Plants such as blueberries and azaleas are known to be especially tolerant, whereas others such as alfalfa, cotton, and tomatoes grow better at a higher pH. Because of the differences in crops and soils, the North Carolina soil test report recommends varying rates of lime to achieve the best pH for the particular soil class and crop combination under consideration.