The pH or potential hydrogen is the negative logarithm
or logarithm of the reciprocal of the hydrogen ion
activity. pH is also called hydrogen ion concentration or
proton concentration.
where aH+ is the activity of the hydrogen ion and pH is the
hydrogen ion concentration, indicating the degree of the
soil acidity or alkalinity.
Specific processes that influence the pH of the soil
solution are those relating to (a) dissociation of carbonic
acid, (b) interaction between soil humus and aluminum
hydroxy polymers, and (c) mineral weathering.
An instrument called pH meter with a 0 to14 scale,
measures the pH value. A value below 7 represents
acidic, above 7 represents alkaline and 7 represents the
neutral point.
Proton concentration is vital for all living organisms.
It also has an impact on soils and soil constituents. A high
hydrogen ion (H+) concentration (pH<4) of soil
constituents attack soil minerals, dissolve the metal
cations out of the crystal lattice, and eventually lead to
mineral degradation. Low soil pH affects root growth,
and also affects the decomposition of organic matter.
High pH conditions ( >5) suppress bacterial life, and
favor fungal life. In many cases, high pH conditions lead
to higher solubility of Al+ and Mn2+ ions, causing
toxicity in the soil. Such a situation leads to hampered
plant growth as also the inhibited formation of ammonia
and nitrate.
Aluminum reduces phosphate availability and is toxic
to most crops, but some plantation crops (for example,
tea and rubber trees) tolerate acidic conditions down to
pH 4. Under alkaline conditions above 7.5, nutrients
such as Fe, Mn, Zn, Cu and B are strongly bound to the
soil and their availability is consequently reduced,
causing deficiencies. The soil pH should
be adjusted to the range appropriate for the soil and the
crop.
There are two kinds of acidity - the actual acidity and
potential acidity. The actual or active acidity is
determined by the hydrogen ion (H+) concentration in the
soil solution, as measured by hydrogen ion electrodes.
Actually, many more hydrogen ions are stored than those
determined by hydrogen electrodes. This stored acidity is
known as potential acidity. The solution and adsorbed
hydrogen ions (H+) are determined by titration with a
base. Low pH levels can be easily overcome by liming
with calcium-magnesium (Ca/Mg) oxides, carbonates
and silicates.
The soil acidity is problematic in humid zones. Here,
the hydrogen ion (H+) formed in the upper soil layer
replaces the adsorbed metal cations (Ca2+, Mg2+, K+)
which are then leached. The soil pH is determined in
slurries with the soil to water ratio of 1:l or 1:2.5.
For example, 10 g of soil is added to 10 ml of
distilled water in a beaker and stirred. The pH is recorded
using glass and calomel electrodes before the
suspension settles. There are variations in the pH values
due to changes in the soil to water ratio; but the
discrepancy is overcome by measuring pH in 1M
potassium chloride (KCl) or 0.01M calcium chloride
(CaCl2).
