pH is a
scale that chemists use to measure acidity. Values below 7 are
considered acidic, values above 7 are alkaline ( the opposite of
acidic) and 7 is neutral.
plants can tolerate a wide pH range in solution culture, but they
cannot tolerate a wide range of acidity in the soil.
soil acidity changes, the solubility of a number of metal ions also
change. Plant growth is really affected by the varying concentration
of these metals in solution rather than by the acidity itself.
acidic conditions, many soil minerals dissolve and increase the
concentration of metal ions to toxic levels. The primary toxic metal
is aluminum, but high levels of manganese and iron can also inhibit
plant growth under these conditions. The nutrients phosphorus and
molybdenum are less available in acidic soils and calcium and/or
magnesium may also be deficient.
alkaline conditions, the
solubility of minerals decrease to the point that nutrient
deficiencies occur. Plant growth is therefore limited by
deficiencies in iron, manganese, zinc, copper and boron. Phosphorus
is also less available in alkaline soils and high levels of calcium
may inhibit the uptake of potassium and magnesium.
aim in managing soil pH is not to achieve a particular pH value, but
to adjust the acidity to the point where there are no toxic metals
in solution and the availability of nutrients is at its maximum.
This condition is usually achieved when the soil pH is between 5.8
and 6.5, however some plants have special acidity requirements.
is used to treat acidic soils, but the soil pH value alone does not
indicate the amount needed. An exchangeable acidity analysis must
also be done to determine the amount of limestone required, and the
soil calcium and magnesium levels must be analyzed to determine
which type of limestone (dolomitic or calcitic) is required.
alkaline soils can be acidified using sulfur or acid forming
fertilizers, but soils with free calcium carbonate cannot be easily
acidified. It is often easier to manage
the nutrient deficiencies that occur on alkaline soils than to
acidify the soil.