Soil Structure: Flocculation

Flocculation

• Flocculation is the clustering together of soil particles to form larger structures called floccules.
• Improvement of soil structure is promoted by the presence of cations.
• Soil particles have a negative charge, with smaller particles having more negative charges.
• Cations (Ca2+, Mg2+) are attracted to these negative charges and are adsorbed to the surface of soil particles.
• Polarized water holds soil particles together, trapping larger sand and silt in a floccule structure.

Benefits of High Humus Content

• Humus is more fertile than clay.
• Humus holds more water than clay.
• Humus has a higher Cation Exchange Capacity (CEC) than clay.
• Humus encourages more earthworm activity than clay.
• Humus has more negative charges, resulting in an increased rate of flocculation.

Cation Exchange

• Cation exchange is the ability of the soil to adsorb cations onto its surface and exchange them for other cations.
• Cations are attracted to the negative charge of clay/humus particles and are adsorbed on the surface.
• Cations are removed from soil solution by plant roots.
• Colloidal humus particles have a higher CEC than colloidal clay particles.
• Cation Exchange Capacity (CEC) is the ability of the soil to adsorb cations onto its surface.
• Main cations are Ca2+, Mg2+, Al3+, H+, K+.
• Nitrate () cannot be held by the soil.
• Sandy soils have the lowest CEC.

Soil pH

• Soil pH is the acid-base scale, measuring the concentration of H+ ions in a solution.
• Acidic: pH < 7, Neutral: pH = 7, Basic: pH > 7.
• The concentration of acidic ions adsorbed onto the surface of the soil determines the acidity of the soil.
• Importance of Soil pH:
  • Lime increases Ca cations available for exchange (H, K, Al).
  • Liming reduces 'acid leaching'.
  • Key points to note: Lime moisture content must be suitable for application.