SOIL AND PLANT ANALYSIS (1+1)

Sources of soil acidity

Leaching due to heavy rainfall

Acid soils are common in all regions where rainfall or precipitation is high enough to leach appreciable amounts of exchangeable bases from the surface soils and relatively insoluble compounds of Al and Fe remains in soil. The nature of these compounds is acidic and its oxides and hydroxides react with water and release hydrogen ions in soil solution and make soil acidic. Besides, when the soluble bases are lost, the H+ ions of the carbonic acid and other acids developed in the soil replace the basic cations of the colloidal complex. As the soil gets gradually depletes of its exchangeable bases through constant leaching, it gets desaturated and becomes increasingly acid.

CO₂ + H₂O H₂CO₃
(Carbonic acid)
H₂CO₃+ CaCO₃ Ca (HCO₃)₂
Calcite (Insoluble) readily soulble and leachable

Acidic parent material

Some soils have developed from parent materials which are acid such as granite and that may contribute to some extent soil acidity.

Acid forming fertilizers and soluble salts
The use of ammonium sulphate and ammonium nitrate increases soil acidity. Ammonium ions from ammonium sulphate when applied to the soil replace calcium ions from the exchange complex and the calcium sulphate is formed and finally leached out.



Besides, basic portion of ammonium sulphate is NH4+ and it undergoes biological transformation in the soil and form acid forming nitrate ions. Similarly, sulphur also produces acid forming sulphate ions through oxidation. Divalent cations of soluble salts usually have a greater effect on lowering soil pH than monovalent metal cations.

Humus and other organic acids
During organic matter decomposition, humus, organic acids and different acid salts may also be produced and also concentration of CO2 increased. The increased concentration of CO2, hydrolysis of acid salts and various organic acids increased the total acidity of soil.

Aluminosilicate minerals
At low pH values most of the Al is present as the hydrated Aluminium ions, which undergoes hydrolysis and release hydrogen ions in the soil solution.

Carbon dioxide
Soil containing high concentration of CO2, the pH value of such soil will be low and the soil becomes acidic. Root activity and metabolism may also serve as sources of CO2, which ultimately helps the soil to become acidic.

Hydrous oxides
These are mainly oxides of iron and aluminium. Under favourable conditions they undergo stepwise hydrolysis with the release of hydrogen ions in the soil solution and develop soil acidity.

Production constraints

• Increased solubility and toxicity of Al, Mn and Fe
• Deficiency of Ca and Mg,
• Reduced availability of P and Mo and
• Reduced microbial activity