Lime Potential

The properties of hydrogen and aluminum ions have been examined in Chapters 10 and 11. These two ion species are ions that directly induce the acid reaction in soils. In soils devoid of soluble salts, the content of cations is constant and the negative surface charges are saturated by, besides hydrogen and aluminum ions, alkali metal and alkaline earth metal ions. These ions are called base ions. The acidity of a soil is determined chiefly by the ratio of the quantity of hydrogen and aluminum ions to that of base ions. Among these base ions, calcium ions occupy the most important position, because they generally account for 65-80% of the total amount of base ions in variable charge soils. Therefore, calcium is an ion species closely related to the acidity of soils. In addition to the parameter pH that directly reflects the concentration of hydrogen ions, one other desirable way is to find a parameter that can reflect the ratio of the hydrogen ions to the calcium ions. This parameter is the lime potential. Since the introduction of the concept of lime potential 40 years ago, little practical application has been made in soil science, although some further theoretical considerations were advanced in the 1950s and the 1960s. Actually, as shall be seen in this chapter, for strongly acid soils, such as variable charge soils, because the quantity of hydrogen ions is too high and at the same time the quantity of calcium ions is too low, lime potential that can reflect the relative ratio of these two ion species is of significance not only in theory but also in practice. The mathematical expression of lime potential is pH-0.5pCa. Lime potential is a simple function of the chemical potential of calcium hydroxide, lime. Hence it may be called lime potential. The physical meaning of pH-0.5pCa can be derived as follows.