Soil Chemical Properties

Soil Colloids

• Size: 1 um or <0.001mm

• Seat of soil chemical reactions- most chemically active fraction of the soil because of its charges

• May either be organic (humus) and Inorganic (crystalline or amorphous)

Origin of Soil colloids

• Slight physical and chemical alteration of certain primary minerals

• Decomposition of primary minerals with subsequent recrystallization of their products into silicate clays

Structural Unit of Crystalline Silicate Clay

• Tetrahedral Sheet ( Tetrahedron)

• Octahedral (Octahedron)

Properties of Soil Colloids

Soil colloids = very small particles (<0.001 mm) of clay minerals and humus with large surface area and unique chemical properties.

Properties of Soil Colloids

• Small size

• Large area surface

• Prescence of surface charge

• Ion absorption

• Adsorption of Water

Sources of charges

• Permanent charge

• pH Dependant

• Protonation

Type of Soil Colloid

• Crystalline Silicate Clays

• Non-Crystalline Silicate Clays

• Iron and aluminum Oxides

• Humus

Crystalline Silicate Clays

• 1:1 Type of silicate clay (Non-expanding type of clay) e.g. Kaolinite

• 2:1 Type of silicate clay (e.g. Smectite Group, ex. of Smectite: Montmorillonite (Expanding type of clay)(ex. Smectite: Vermiculite (Limited Expanding Type of clay)

• 2:1 Type of Silicate Clay (Non-expanding type of clay) e.g. Fine grained mica

• 2:1:1 (Non-expanding Type of clay)

Montmorillonite

High CEC

Kaolinite

Low CEC

Organic matter (humus)

Has the highest CEC among colloids

Cation Exchange Capacity (CEC)

Ability of soil to hold and exchange positively charged ions (Ca²⁺, Mg²⁺, K⁺, NH₄⁺).

Higher in clays and organic matter → more fertile soils.

Soil Nutrient Status

Macronutrients: N, P, K, Ca, Mg, S.

Micronutrients: Fe, Mn, Zn, Cu, B, Mo, Cl.

Their availability depends on soil pH, CEC, and redox conditions.

pH

pH = -log(H+) = 1/log(H+)