1. Leaching of basic cation by heavy rainfall: CO₂ dissolve in water to produce carbonic acid (H₂CO₃). These H₂CO₃ removes the basic cations (i.e. Mg⁺, Na⁺, Ca²⁺, K⁺) from alkaline soil and form bi-carbonate which are soluble in water. These bi-carbonate are leach down due to heavy rainfall and causes soil acidity.

CO₂+ H2O —->H₂CO₃

H₂CO₃ —–>H⁺ + HCO₃

2. Acidic parent material: Some soil have developed from acidic parent material, so the soil are acidic soil. E.g. Acid rock (granite) weathering    Acid soil.

 

3. Increase acid cations (Al³⁺, Fe³⁺, Mn⁴⁺): These are ions are hydrolysis produce H⁺ ion in soil and thus causes soil acidity.

 

Al³⁺+ H₂O —> Al(OH)²⁺+ H⁺ (for strong acid)

Similar reaction will be occur in case of Fe³⁺ and Mn⁴⁺. Al³⁺ and H⁺ are two acidic cations which arc mainly responsible for soil acidic.

 

4. Alumino-silicate minerals: At low P” values most of the Aluminium is present as the hydrate. Aluminium ions (Al3+) which undergoes hydrolysis and release hydrogen Oil ions in the soil solution that develop soil acidity.

Al³⁺+ H₂O —> Al(OH)²⁺+ H⁺

5. Application of manures and fertilizers: Manures- by decomposition of different types of organic matter in soil produce different types of organic acid. e.g. pulmic acid. These organic acid produce H⁺ which can create soil acidity.

 

Cowdung—-> Protein —-> R-NH₂  —>      NH⁴⁺oxygen —->NO₂^–  oxygen  —–> NO₃^– + acidic soil.

 

Ammonium sulphate decrease PH more than Urea. So, it is-usually apply in tea garden.

 

Urea CO(NH₂)₂  —-> NH₄+ oxygen —->  NO₂^–+ H⁺   oxygen —>   NO₃^– (Soil acidic)

 

6. Carbon-dioxide (CO₂):Root activity and metabolism may serve as source as CO, which dissolved in soil water and produce Carbonic acid (H₂CO₃). As a result, soil acidity occurs.

CO₂+ H₂O —–> H₂CO₃

7. Acid rain: In industrial area SO₂ and NH₃ etc. are present in high amount. These mix with rain water and come down to the earth and causes soil acidity.

NH₃oxygen      NO₃^–water     HNO₃   rain        Acidic soil.

SO₂   water      H₂SO₄  rain      Acidic soil

 

8. Acid sulphate soil: Oxidized form of SO₄^2-and Fe³⁺ converted into reduced form when the land receive tidal flood. These reduced form react together to form FeS2. This FeS2 is Oxidized to Fe₂(SO₄)₃. As a result, decreases and acidity of soil occur.

 

  –>Fe³⁺ —          Fe²⁺, SO₄^2-   —–> H₂S

 

H₂S + Fe²⁺ —-> FeS₂ oxygen —–>   Fe₂(SO₄)₃   (acidic sulphate)

 

9. Nutrient Uptake:In rhizosphore cation and anion ratio 1:1 always same. If H+ uptake increases then soil acidity increases i.e. soil PH decreases. When OH- Uptake increases then soil alkalinity increases i.e. PH increases.