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 havedeveloped 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.
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.
SO2water H2SO4 rain Acidic soil
8. Acid sulphate soil: Oxidized form of SO42-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.
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.
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