Soil Reaction

Significance of Soil pH:

1. Nutrient availability: Soil pH affects the availability of essential plant nutrients. Most nutrients are most readily available to plants in slightly acidic to neutral soil pH ranges (around 6.0 to 7.0).
2. Microbial activity: Soil pH influences the activity and diversity of soil microorganisms, which play a vital role in nutrient cycling and organic matter decomposition.
3. Plant growth: Different plants have varying preferences for soil pH, and the optimal pH range varies among species. Soil pH can affect root growth, nutrient uptake, and overall plant health.
4. Soil structure: Soil pH can influence soil structure, affecting factors such as soil aggregation, water-holding capacity, and susceptibility to compaction.

Factors Affecting Soil pH:

1. Parent material: The geological origin and composition of the parent material from which the soil is formed can influence the inherent pH of the soil.
2. Precipitation and leaching: In areas with high precipitation, basic cations (such as calcium, magnesium, and potassium) can be leached from the soil, leading to the formation of more acidic soils.
3. Organic matter decomposition: The decomposition of organic matter can produce organic acids, which can lower the soil pH.
4. Fertilizer application: The use of certain fertilizers, such as ammonium-based fertilizers, can increase soil acidity over time.
5. Atmospheric deposition: Acid rain and other forms of atmospheric pollution can contribute to soil acidification.
6. Microbial activity: Certain microorganisms, such as nitrifying bacteria, can produce acids that can lower soil pH.
7. Plant uptake of nutrients: The uptake of certain nutrients by plants can also influence soil pH. For example, the uptake of cations (such as calcium and magnesium) can increase soil pH, while the uptake of anions (such as nitrate) can decrease soil pH.

Testing Soil PH

1. Laboratory soil testing:
   
   • This involves sending a soil sample to a soil testing laboratory for analysis.
   • The laboratory will use standardized methods, such as a glass electrode pH meter, to determine the soil's pH value.
   • Laboratory testing provides the most accurate and reliable results, as they use precise equipment and standardized procedures.
   • This method is often preferred for more detailed soil analysis and recommendations.
2. Field soil pH test kits:
   
   • These are portable kits that allow you to test the soil pH on-site, without sending a sample to a laboratory.
   • They typically use colorimetric or electronic methods to determine the soil pH.
   • Colorimetric kits involve adding a chemical reagent to a soil-water mixture and comparing the resulting color to a pH scale.
   • Electronic pH meters use a probe that is inserted into the soil to measure the pH directly.
   • Field test kits are convenient and provide relatively quick results, though they may not be as accurate as laboratory testing.

Soil Acidity

Soil acidity refers to the concentration of hydrogen ions (H⁺) present in the soil.

Acid soils are the ones in which H⁺ ions are dominant in soil solution and in soil colloids. They have a PH below 7.

1. Active Acidity
   
   • Active acidity refers to the concentration of hydrogen ions (H+) present in the soil solution.
   • It is the immediate source of acidity that plants and microorganisms experience.
   • Active acidity is measured by the pH (potential of hydrogen) value of the soil, which ranges from 0 to 14.
   • Soils with a pH below 7 are considered acidic, with lower pH values indicating higher acidity.
   • The active acidity of a soil can be easily measured and is the most commonly used indicator of soil acidity.
   • Plants and soil organisms are directly affected by the active acidity of the soil, as it influences the availability of nutrients and the suitability of the environment for their growth and development.
2. Potential Acidity also is known as Reserve Acidity
   
   • Potential acidity refers to the acidity that is not immediately available in the soil solution but can be released over time.
   • It is primarily associated with the presence of aluminum (Al³⁺), iron (Fe³⁺), and hydrogen (H⁺) ions that are adsorbed on the surface of soil colloids or present in the soil organic matter.
   • Potential acidity is not directly measured by the pH value but can be determined through chemical analysis, such as the exchange acidity
   • Potential acidity represents the long-term or reserve acidity in the soil that can be released and contribute to the active acidity over time.
   • The release of potential acidity can occur through various processes, such as weathering, decomposition of organic matter, or the application of acidifying fertilizers.

Causes of Soil Acidity

• Leaching of basic cations: Rainfall or irrigation can leach away positively charged basic nutrients like calcium, magnesium and potassium leaving behind more hydrogen ions.
• Weathering of acidic parent materials: Rocks and minerals that contain elements like sulphur, iron and aluminium can weather and release hydrogen ions, increasing soil acidity.
• Acid rain: Acid rain and other air pollutants can deposit acidic compounds in the soil.
• Decomposition of organic matter: As plant and animal matter decomposes, it can release organic acids that increase soil acidity.
• Microbial activities (or nitrification): The microbial conversion of ammonium to nitrate can release hydrogen ions.
• Application of nitrogeneous fertilizers: Many nitrogeneous fertilizers like ammonium sulphate and urea release hydrogen ions as they break down.
• Manure and compost application:Depending on the source, these organic inputs can contribute to soil acidity.
• Mining and smelting: These activities can release acidic substances into the environment.

Effects of Too Much Soil Acidity

• Toxicity of Certain Elements:
High soil acidity can increase the solubility of toxic elements like aluminum (Al³⁺) and manganese (Mn²⁺). Aluminum toxicity is particularly harmful as it stunts root growth and hinders nutrient and water uptake.
Al(OH)₃ + 3H⁺ → Al³⁺ + 3H₂O
• Reduced Availability of Essential Nutrients: Elements like phosphorus, calcium, and magnesium become less available in acidic soils, leading to deficiencies in plants. CaCO₃ + H ⁺ → Ca²⁺ + CO₂ + H₂O
• Decreased Microbial Activity: Beneficial soil microbes, such as nitrogen-fixing bacteria and decomposers, are less active in highly acidic soils, reducing the organic matter breakdown and nutrient cycling.
• Poor Soil Structure: Acidic conditions can deteriorate soil structure, reducing aeration and water infiltration, which can lead to soil erosion and compaction.
• Stunted Growth and Poor Yield: Plants grown in acidic soils may exhibit stunted growth, chlorosis (yellowing of leaves), and poor yield due to nutrient deficiencies and root damage.

Control Measures for Soil Acidity

1. Liming: refers to the process of applying various calcium or magnesium-containing materials to the soil to reduce soil acidity and increase the PH.
   Examples of common liming materials include:
   
   • Limestone(Calcium Carbonate, CaCO₃): Adding agricultural limestone to the soil can neutralize acidity by increasing the pH. The lime reacts with hydrogen ions (H⁺) in the soil, forming water and carbon dioxide. CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂
     
   • Dolomitic limestone (Calcium Magnesium Carbonate - CaMg(CO₃)₂): This type of lime also adds magnesium to the soil, which can be beneficial if the soil is deficient in this nutrient.
     CaMg(CO₃)₂ + 4H⁺ → Ca²⁺ + Mg ²⁺ + 2H₂ O + 2CO₂ ​
   • Quicklime (Calcium oxide, CaO) and slaked lime also known as hydrated lime ( Ca(OH)₂): More reactive than limestone, as they do not require dissolution to neutralize acidity. Quicklime and slaked lime can be used when a faster response is desired, but they are more expensive and can be more difficult to handle.
   • Wood ash: Produced from the burning of wood or other plant biomass. Contains calcium, potassium and other beneficial minerals that can help neutralize soil acidity.
2. Incoporation of green manures and cover crops:
Growing and incorporating green manures and cover crops can increase organic matter content and enhance microbial activity, leading to a more balanced soil pH over time.
→Legumes such as clover, alfalfa and peas are more commonly used as green manures and cover crops.
→ Legumes have the ability to fix atmospheric nitrogen through a symbiotic relationship with rhizobia bacteria, adding nitrogen to the soil.
→ As legumes decompose, they release compounds that can help neutralize soil acidity.

3. Avoiding Acidifying Fertilizers: Certain fertilizers, like ammonium sulphate and Urea, can further acidify the soil. Applying basic fertilizers that have liming effect, such as those containig calcium, magnesium and potassium , can help neutralize soil acidity. Examples include calcium nitrate, magnesium sulphate or potassium carbonate. These fertilizers can help raise the soil PH and provide essential plant nutrients.
   (NH₄)₂SO₄ + 2O₂ → 2NO₃ ⁻ + 2H⁺ + 2H₂O
4. Monitoring Soil pH: Regular soil testing helps track pH levels and nutrient availability, allowing for timely interventions with lime or other amendments.

Soil Alkalinity

Causes

• Parent materials: Soils formed from alkaline rock types like limestone and dolomite tend to have higher PH levels.
• Excessive use of alkaline fertilizers:

Control Measure

• Soil amendment: To adjust the soil with acidic amendments like sulphur, iron sulphate and aluminium sulphate.
• Application of ammonium-based fertilizers such as urea and ammonium sulphates which are usually oxidized by soil microorganisms, producing hydrogen ions (H⁺).