Unit 2: Soil acidity and liming

Krishna Poudel

Senior Agriculture Instructor

Department of Plant Science

Shree Triveni Secondary School

Province 1, Nepal

Contact: +9779847016830

Krishna Poudel (researchgate.net)�E-mail: poudelkrishna051@gmail.com

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Soil Acidity:

• Soil acidity is a condition in which the soil pH is lower than a neutral pH (less than 7).
• Soil pH is a measure of the hydrogen (H+) ion concentration expressed as the negative common logarithm of H+ concentration.

i.e. Soil pH = -log (H⁺)

• When soil becomes too acidic it can: decrease the availability of essential nutrients, increase the impact of toxic elements, decrease plant production and water use.
• The main cause of soil acidification is inefficient use of nitrogen, followed by the export of alkalinity in produce. Ammonium based fertilizers are major contributors to soil acidification.

• Soil acidity is common in all regions of moderate to heavy rainfall (e.g. eastern Terai of Nepal) where precipitation is sufficient to leach appreciable amt. of exchangeable non-acid cations) like Na⁺, K⁺, Mg^2+, Ca^2+.
• As basic cations are removed, soil tends to become acidic in reaction.
• Predominant acid causing cations are H⁺, Al³⁺ .

Causes of soil acidity

1. Characteristics of parent materials

• Soils formed from the rocks having acidic ions are acidic eg Granite/Quartz

• Soils formed from rocks having basic ions are basic eg. Basalt

• Clay minerals such as Kaolinite, montmorillonite, Fe and Al are acidic in nature

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2. Accumulation of OM & their decomposition

• OM contains numerous acid functional groups from which H⁺ ions can dissociate.

• During decomposition, many (in)organic acids are released.

• Slow & persistent action of weak inorganic acids especially of carbonic acid on mineral constituent of soil is responsible for the removal of base forming cations like Ca^2+, Mg^2+, etc. by dissolution & leaching.

3. High rainfall & low evaporation

• Basic cations are more soluble than acidic cations

• Under high precipitation, basic cations (Ca^2+, Mg^2+, K⁺ , Na⁺) leach out but the acid cations such as Al³⁺ & H⁺ tends to retain in the soil surface.

• So surface soil becomes acidic & consequently the sub - surface soil is basic.

4. Acid rain

• Rain with pH value <5 is termed an acid rain.

• The acid rain has pH of about 4-4.5 but sometime as low as 2.

• When raindrops falls through unpolluted air, water reacts with CO2 & forms weak acids H₂CO₃ which dissociates & release H⁺

• In polluted air with N & S gases coming from industrial combustion, forest fires, volcanic eruptions, N & S reacts with water forming HNO₃ & H₂SO₄ .

• These strong acids dissociates in soil and release H⁺, N₂, N₂O, H₂O HNO₃

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8. Plant uptake of basic cations

• Plants uptake more of certain cations (e.g. K⁺, NH⁴⁺ & Ca²⁺ ).

• For every +ve charge taken in on a cation, a root maintain charge balance either by taking up anion or by exuding cations.

• When they take up far more of cations (Ca, Mg, NH₄, K ) than they do of anions (SO₄^2-, NO₃^-) (for eg Legumes), plant usually exude H⁺ ions into the soil sol to maintain charge balance & contribute to acidification.

• When plant root maintain charge balance by up taking the same quantity of opposite charged ions, it does not change soil P.

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9. H⁺ ions (Root exudates)

• Some H⁺ ions excreted by plants are exchanged for nutritive cations such as Ca^2+, process known as contact cation exchange.

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10. Crop removal of basic cations

• Some dicot plants such as legumes absorb basic cations thus indirectly causing soil to be acidic.

• Eg. Yield of 13 ton/ha alfa-alfa removes 45 kg Ca and 9 kg Mg

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Liming material and their uses

• Liming materials are found in 3 major forms :
• Oxide form: eg. CaO (unslaked, burned, quick lime)
• Carbonate form: eg.Calcite (CaCO₃), dolomite CaMg(CO3)₂
• Hydroxide form: eg. Ca(OH)₂ (slaked, hydrated lime)

Oxide form: eg. CaO

• Commercial oxide of lime is normally referred to as unslaked (no water molecule) lime, or burned lime or quick lime.
• It is the white powder & more expensive than other limes.
• It is produced by heating calcite (CaCO3) or dolomite in oven or furnace, in which the following reaction takes place :

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• It has caustic properties; reacts much rapidly with soil.
• Thorough/well mixing with soil is necessary otherwise there will be caking problem (difficult to handle).
• It is much more effective among all liming materials, NV is 178% as compare to pure CaCO₃

Factors affecting the liming

1. Amount:

• Crops differ in sensitivity to soil and added lime in soil.
• Amount of lime to be applied depends on inherent properties of soil i.e. texture, clay type and OM content.
• In a coarse textured with low OM soil, the lime requirement will be less than for a fine textured, high OM soils.

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2. Timing:

• Liming reacts slowly with soil acidity.
• Lime may not have adequate time to react with soil if applied just before seeding.
• Due to its gradual effect, lime should be spread about 6-12 Months ahead of crop that has the highest pH and Ca requirement.
• For rotations that include leguminous crops, lime should be applied 3-6 months before the seeding time.

3. Frequency:

• The lime requirement depends on soil texture, N source and rate, crop removal, precipitation patterns and lime rate.

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• On sandy soils, frequent light applications are preferable whereas on fine textured soils, large amounts may e applied less frequently.

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• For humid regions, where the forces of acidification proceed excessively, application to arable soils is not done once but must be repeated every 3-5 years.

Soil salinity

• The condition of soil where there will be the higher concentration of dissolved/soluble salts(ions) that is sufficient to interfere the growth of most plants is known as soil salinity.

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• Salts present are typically dominated by carbonate (HCO₃^-/CO₃^–), sulfate (SO₄^2–) & chloride (Cl^–) of Ca, Mg, Na, K

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• In general, NaCl is the most abundant.

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• The salinity is measure primarily by measuring the Total Dissolved Solids (TDS) and Electrical Conductivity (EC).

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THANK YOU