<P> The accumulation of alkalinity in a soil (as Na, K, Ca and Mg bicarbonates and carbonates) occurs when there is insufficient water flowing through the soils to leach soluble salts . This may be due to arid conditions, or poor internal soil drainage; in these situations most of the water that enters the soil is transpired (taken up by plants) or evaporates, rather than flowing through the soil . </P> <P> The soil pH is usually increased when total alkalinity increases, but the balance of the added cations also has a marked effect on the soil pH--for example, increasing the amount of sodium in an alkaline soil will tend to induce dissolution of calcium carbonate, which will increase the pH . Calcareous soils may vary in pH from 7.0 to 9.5, depending on the degree to which Ca or Na dominate the soluble cations . </P> <P> Plants grown in acid soils can experience a variety of stresses including aluminium (Al), hydrogen (H), and / or manganese (Mn) toxicity, as well as nutrient deficiencies of calcium (Ca) and magnesium (Mg). </P> <P> Aluminium toxicity is the most widespread problem in acid soils . Aluminium is present in all soils, but dissolved Al is toxic to plants; Al is most soluble at low pH; above pH 5.0, there is little Al in soluble form in most soils . Aluminium is not a plant nutrient, and as such, is not actively taken up by the plants, but enters plant roots passively through osmosis . Aluminium inhibits root growth; lateral roots and root tips become thickened and roots lack fine branching; root tips may turn brown . In the root, the initial effect of Al is the inhibition of the expansion of the cells of the rhizodermis, leading to their rupture; thereafter it is known to interfere with many physiological processes including the uptake and transport of calcium and other essential nutrients, cell division, cell wall formation, and enzyme activity . </P>

Use a ph scale to indicate acid and alkaline soil