pH

Definition

pH is a measure of the acidity of a solution. pH is equal to the negative logarithm of the concentration of hydrogen ions in a solution.

Agricultural Uses

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 a_H+ 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).