chapter_4c_-_pH_v3_-_HANDOUT

Soil pH Overview

Definition:

Soil pH, often referred to as soil reaction, is a crucial indicator of the acidity or alkalinity of soil, quantified as the negative logarithm of the total hydrogen ion (H+) concentration in a solution. This measurement is vital for understanding soil chemistry and its impact on plant growth.

pH Scale:

The pH scale ranges from 0 to 14, where:

• Acidic: pH < 7

• Neutral: pH = 7

• Alkaline: pH > 7 This scale is logarithmic, meaning each whole number change represents a tenfold change in acidity or alkalinity.

Acidity or Alkalinity:

Soil pH reflects the balance of H+ (hydrogen) and OH- (hydroxide) ions present in the soil solution. The relationship can be expressed mathematically as:

• pH = -log[H+]

Sample Problem:

For example, if the concentration of hydrogen ions [H+] is 0.01 M, the pH is calculated as:

• pH = -log(0.01) = 2. This indicates a highly acidic condition for the soil.

Calculating pH and [H+]

Increasing pH:

As the pH value increases, the concentration of hydrogen ions [H+] decreases inversely.

One Unit Change:

A change of one unit on the pH scale corresponds to a tenfold increase or decrease in hydrogen ion concentration. For instance, at a pH of 5.0, the [H+] is approximately 10^-5 moles/liter, illustrating the relationship between pH and acidity.

Methods to Determine pH:

1. Electrometric Method:

   • Utilizes a pH meter for direct and accurate pH readings in soil solutions.

2. Colorimetric Method:

   • Involves the use of organic dyes and litmus paper, which change color based on pH, though this method is less precise compared to electrometric readings.

Effects of Soil pH:

Microbial Activity:

• Fungi: Generally remain unaffected by fluctuations in pH, thriving in various pH levels.

• Bacteria & Actinomycetes: These organisms face inhibition at pH levels lower than 5, which can adversely affect soil health.

Nutrient Availability:

Soil pH significantly influences the availability of essential nutrients:

• Nitrogen (N): Availability is markedly reduced below pH 5.5.

• Calcium (Ca) & Magnesium (Mg): Both nutrients become less available below pH 6.0.

• Phosphorus (P): Limited availability at pH levels below 6.0 and above 7.0 due to chemical reactions that render it less accessible to plants.

• Potassium (K): Becomes deficient when soil pH is below 6.0.

Metal Availability:

Certain metals like Aluminum (Al), Iron (Fe), Manganese (Mn), and Zinc (Zn) can become toxic to plants at low pH levels, leading to stunted growth or even plant death.

Origins of Soil Acidity:

Natural Chemical Processes:

• Carbon dioxide (CO2) reacts with soil moisture forming carbonic acid, contributing to acidity.

• Decomposition of organic matter releases organic acids into the soil.

• Oxidation of ammonium, sulfur, and iron compounds can lead to increased acidity in the soil.

Natural Deposition:

• Lightning: Produces nitric acid which can acidify soils during rainfall.

• Volcanic Activity: Releases sulfur dioxide, which when converted to sulfuric acid, can further acidify nearby soils.

• Mineral Weathering: The weathering of sulfide minerals like pyrite generates acid sulfate soils, enriching soil acidity naturally.

Human Factors:

• Application of ammonium-based fertilizers can contribute to soil acidification over time.

• Nutrients deposited by acid rain can also exacerbate soil acidity issues.

Forms of Soil Acidity:

• Active Acidity: Directly measured by hydrogen ions in the soil solution, usually presented as pH.

• Reserve Acidity: Consists of hydrogen ions and aluminum ions (Al³+) held in solid phase; reflects the soil's buffering capacity against pH changes.

• Buffering Capacity: Soils enriched with clay, organic matter, and high cation exchange capacity (CEC) tend to have higher buffering capacities, allowing them to resist pH fluctuations.

Management Strategies for Acid Soils:

1. Minimize Acidification:

   • Reduce the use of acidifying fertilizers.

   • Employ split applications of fertilizers to minimize leaching effects on soil pH.

   • Return plant residues and organic matter to the soil to retain pH stability.

2. Use Acid-Tolerant Crops:

   • Plant species such as sweet potatoes, coffee, and various fruits can thrive in more acidic soils, providing sustainable agricultural options.

Common Agricultural Crop pH Preferences:

• Crops with Optimal pH Ranges:

  • Abaca: 6.0-7.0

  • Alfalfa: 6.5-7.2

  • Sweet Potato: 5.0-7.0

  • Cacao: 6.0-7.0

• Crops Sensitive to pH:

  • Beans: 5.5-6.5

  • Mango: 5.5-7.5

  • Pepper (Sweet): 5.8-6.8

Acidic Soils - Constraints:

• Aluminum Toxicity: Can severely damage root systems, inhibiting nutrient uptake.

• Manganese Toxicity: Although less frequent, it can lead to similar issues as aluminum toxicity.

• Nutrient Deficiencies: Common deficiencies include phosphorus and calcium, leading to reduced crop yields and health.

• Microbial Activity Decrease: A drop in microbial activity adversely affects essential nitrogen mineralization and fixation processes, crucial for plant growth.

• Plant Diseases: Increased risks of diseases such as damping off and root rot are associated with acidic soils, affecting crop viability.

Correction of Soil Acidity:

• Liming: The application of lime (e.g., Calcium Carbonate CaCO3 or Calcium-Magnesium Carbonate CaMg(CO3)2) is a fundamental practice to neutralize soil acidity.

  • Chemical Reactions: Lime reacts with H+ ions to form neutral compounds, effectively lowering acidity levels and improving soil health.

Salinity and Sodicity:

• Salinity: Excess salts can restrict the roots' ability to absorb water, impacting plant growth and survival. High salt irrigation or proximity to saltwater bodies often contribute to this issue.

• Sodic Soils: Defined as soils exhibiting more than 15% sodium (Na+) on exchangeable cation sites, leading to soil structure dispersal and diminished drainage efficiency.

Electrical Conductivity (EC):

• Definition: A measure of the ability of soils to conduct electric current, which indicates ionic concentration in the soil solution.

• Conductivity Classifications: Saline conducts >4 dS/m while sodic conducts <4 dS/m; these values guide soil management decisions.

Correcting Salinity and Sodicity:

• Salinity: Can often be corrected through leaching with adequate irrigation practices, effectively flushing excess salts from the root zone.

• Sodicity: Addressing this issue is more complex and may require the application of gypsum or sulfur to improve soil structure and fertility.