Soil Chemistry, Processes, and Properties

• Definitions, soil functions, soil forming factors and processes, parent material,weathering, soil composition, and soil sampling are important aspects of soil study.
• Chemical processes and properties are key characteristics of soil.
• Physical processes and properties influence soil behavior.
• Biological properties and Soil Organic Matter (SOM) impact soil health.

Chemical Processes

• Chemical processes encompass dissolution, carbonation, hydrolysis/protolysis, hydration, oxidation-reduction (redox), complexation, exchangeable ions, and isomorphous substitution.

Chemical Properties

• Important chemical properties include soil reaction (pH; acidity or basicity), buffering capacity (BC), and electrical conductivity (EC; salinity).
• The Sodium Adsorption Ratio (SAR), Cation Exchange Capacity (CEC), Exchangeable Sodium Percentage (ESP), and Base Saturation (BS) are also key chemical properties.
• Concentrations of nitrogen (N), phosphorus (P), calcium (Ca), magnesium (Mn), sodium (Na), potassium (K), sulfur (S), and carbon (C) as well as trace elements and heavy metals, and the C/N ratio are important.
• These properties influence processes like nutrient cycling, biologic activity, pollutant fate, erosion, and degradation.

The Periodic Table of Elements

• The periodic table organizes elements by atomic number, electron configuration, and recurring chemical properties.
• Elements are arranged in rows (periods) and columns (groups) based on their electron structure.
• Metals, nonmetals, and metalloids exhibit different properties and behaviors.

Ionic Radius

• Ionic radius refers to the size of an ion in a crystal structure.
• When a metal loses electrons to form an ion, the radius shrinks.
• More electron shielding gives a larger radius.
• Gray circles indicate the sizes of the ions shown; colored circles indicate the sizes of the neutral atoms.

Ionic vs. Hydration Radius

• Ions have different radii when dry versus when hydrated in soil.
• Hydrated radii are the effective sizes of ions when surrounded by water molecules.
• Ionic mobility is affected by the hydrated radius, with smaller hydrated ions generally having higher mobility.

Clay Minerals

• Clay minerals are composed of basic units of silica tetrahedra and alumina octahedra.
• Tetrahedron and Octahedron units form Tetrahedral and Octahedral sheets.
• These sheets combine in different arrangements to form various clay minerals, such as 1:1 and 2:1 structures.

Classification of Clay Minerals

• Clay minerals are classified into layer silicates, chain silicates, sesquioxides, and other minerals like carbonates and sulfates.
• Layer silicates consist of 1:1 type (Kaolinite, Halloysite), 2:1 type (Vermiculite, Mica, Illite), and 2:1:1 type (Chlorite) minerals.
• Crystalline metal oxides and metal hydroxides, as well as amorphous allophane and imogolite, are also clay minerals.

Clay Structure

• Clay minerals have layered structures with tetrahedral and octahedral sheets.
• Kaolinite has a 1:1 structure, while Illite, Vermiculite, and Smectite have 2:1 structures.
• Chlorite has a unique 2:1:1 structure.
• Clay layer expansion occurs due to water adsorption between layers.

Charges in Clay Minerals

• Charges in clay minerals arise from isomorphic substitution and broken edges.
• Negative charges result from substituting lower-charged ions for higher-charged ions.
• Positive charges result from substituting higher-charged ions for lower-charged ions.
• Variable charges depend on functional groups that release or accept hydrogen ions (H+1).

Soil Reaction (pH)

• Soil pH is a measure of acidity or basicity, expressed as the negative logarithm of H+ ion concentration.
• pH affects nutrient availability and soil fertility.
• pH can be measured in the field or laboratory.

Soil pH Levels

• Soil pH is measured on a scale from 0-14
• A pH of 7 signifies neutral conditions
• A pH below 7 signifies acidic conditions
• A pH above 7 signifies alkaline conditions

Sources of H+ Ions

• Carbonic acid forms from root and soil respiration.
• Organic acids result from microbial breakdown of organic matter.
• Oxidation of nitrogen (nitrification) and sulfides generates H+ ions.
• Plants release H+ during cation uptake to maintain charge balance.

Buffering Capacity (BC)

• BC is a soil's ability to resist pH changes and maintain a stable pH level under acidic or alkaline inputs.
• Soils with high BC resist pH changes.
• Soils with high BC require larger amounts of amendments to significantly alter the soil pH
• Soils with low BC are more susceptible to rapid pH changes.
• Higher CEC, organic matter, and Ca2+/Mg2+ carbonates results in high BC.

Exchangeable Cations

• Exchangeable cations such as Ca, Mg, K, and Na influence soil properties.

Electrical Conductivity (EC; Salinity)

• EC measures a soil's ability to conduct electrical current and indicates salinity.
• EC is measured in deciSiemens per meter (dS/m).
• EC levels classify soil as non-saline, slightly saline, moderately saline, very saline, or extremely saline.

EC Impacts

• Salinity negatively impacts the soil and agriculture.
• High Salinity has adverse affects on the soil biogeochemistry by affecting the GHG emissions, soil organic carbon reduction and microbial communities.
• Increased desertification is caused by loss of vegetation cover and soil organic matter

Sodium Adsorption Ratio (SAR)

• SAR measures relative sodium concentration compared to calcium and magnesium in soil solution.
• SAR values classify soil as normal, slightly sodic, moderately sodic, strongly sodic, or very strongly sodic.

Cation Exchange Capacity (CEC)

• CEC is the total amount of exchangeable cations a soil can hold, expressed in centimoles per kilogram (cmol/kg).
• CEC values classify soil as very low, low, moderately, high, or very high.
• Soil pH influences CEC levels, with higher pH generally leading to greater CEC.
• Organic matter content is closely related to CEC.

Exchangeable Sodium Percentage (ESP)

• ESP is the percentage of CEC occupied by sodium cations.
• ESP values classify soil as normal, slightly sodic, moderately sodic, strongly sodic, or very strongly sodic.
• ESP is calculated as (Exchangeable Na+ / CEC) x 100.

Base Saturation (BS)

• BS is the percentage of CEC occupied by base cations (Ca+2, Mg+2, K+).
• BS values classify soil as low, moderate, or high.
• BS is calculated as (Sum of base cations / CEC) × 100
• Non-sodic soil are soils with low exchange of sodium ions, whereas sodic soils are soils with very high exchange of sodium ions

Integrating EC, SAR, ESP

• Normal soil has no osmotic stress.
• A Sodic soil has no osmotic stress, but can be dispersed with high aggregation
• Saline soil has osmotic stress, it can be well aggregated
• Saline-sodic soil has osmotic stress and has possibility of being dispersive.

Plant Growth

• EC: Plant growth responses vary with salinity levels
• ESP: Plant growth responses depend on sodicity levels.
• Alterations in ion and water imbalances lead to decreased Germination, shoot morphological traits, physiological and biochemical activity and root morphological traits.