Lecture 6 Soil Reaction

Fundamentals of Soil Science

Soil Chemical Properties

• Focus on soil reaction, salinity, and sodicity.

Soil Reaction

• Definition: Refers to acidity and alkalinity of soils.

• Examples:

  • Forest soil has a pH of 5 (acidic).

  • Grassland soil has a pH of 8 (alkaline).

  • Organic soil has a pH of 4.5 (acidic).

Acidic Soil Classification

• Extremely Acid: pH < 3.5

• Highly Acid: pH 3.5 - 4.5

• Moderately Acid: pH 4.5 - 5.5

• Slightly Acid: pH 5.5 - 6.5

• Not Acid: pH 6.5 or higher.

• World Distribution: Shows areas where acidic soils are prevalent.

Regional Observations

• Alberta has limited areas needing liming; however, the Peace region exhibits more acidic soils.

Soil Acidity Influences

• Acidity: Related to number of H+ ions in the soil.

• Alkalinity: Related to number of OH- ions.

• Acidity influences:

  • Mineral weathering.

  • Solubility and availability of elements.

  • Microbial activity.

  • Plant growth.

Soil pH Reference

• Soil pH is categorized as:

  • Acidic: pH < 7

  • Alkaline: pH > 7

  • Neutral: pH = 7

Factors Affecting Soil pH

• Vegetation: Different litter types (forest vs. grassland) affect soil pH.

• Climate: Wetter environments generally lead to more acidic soils due to leaching of base cations like calcium, magnesium, and potassium.

Characteristics of Acid-Sulfate Soils

• Formed under waterlogged conditions with iron sulfide (pyrite).

• Concerns arise when drained, leading sulfur compounds to react and produce sulfuric acid, causing ecological damage like fish kills and plant death.

Plant Responses to Soil pH

• High or low soil pH directly impacts plant health by affecting nutrient availability.

  • Preferable pH for most crops is between 5.5 and 7.0.

Nutrient Availability Chart

• Nutrient availability peaks at certain pH levels:

  • Calcium: Most available at pH 7.

  • Iron: Most available between pH 4-6, benefiting forest vegetation.

Crop Tolerance to Acidity

• Many cereal crops (e.g., oats) don't tolerate soils with pH < 5.5 due to aluminum toxicity.

• Some plants are adapted to acidic conditions, such as blueberries and cranberries.

Soil Acidity Sources

• H+ ions primarily come from:

  • Carbonic Acid: Developed from rainwater contact with CO2.

  • Nitric Acid: Resulting from nitrification processes.

Acidic Compounds and Fertilizers

• Certain fertilizers, like ammonium sulfate, contribute to soil acidity.

Acid Rain Contribution

• Caused by combustion of fossil fuels releasing sulfur and nitrogen gases that form acids, leading to ecosystem impacts.

Basic Cations and Soil pH

• Strength of cation adsorption affects soil pH:

  • Al3+ and H+ > Ca2+ > Mg2+ > K+ > NH4+ > Na+.

Buffering Capacity

• Defines the soil's ability to resist pH changes.

• Higher CEC indicates better buffering capacity, allowing stable pH despite cation exchanges.

Soil Management Techniques

• Liming and growing acid-tolerant plants are primary methods for managing acidity.

Active and Reserve Acidity

• Active Acidity: H+ ions in soil solution, easily measured.

• Reserve Acidity: H+ ions adsorbed to soil colloids, crucial for understanding potential acidification.

Types of Salt-Affected Soils

• Saline Soils: High neutral salt concentration, common in Alberta.

• Sodic Soils: High Na+ concentration, leading to poor soil structure due to dispersion.

• Saline-Sodic Soils: Combination of both types, often problematic for agriculture.

Effects of Salinity on Plant Growth

• Increased salinity correlates with reduced plant health and growth.

• Response varies among crops based on their salinity tolerance.

Soil Salinity Measurement

• Electrical conductivity (EC) assesses salt problems, where values > 4 dS/m indicate a concern.

Types of Salt-Affected Soils

• Saline, saline-sodic, and sodic soils affect agricultural practices.

Sodium Adsorption Ratio (SAR)

• Indicator of sodic soil health, with a ratio > 12 signaling issues.