Soil Science: Flocculation and Cations

Flocculation

• Flocculation is the clustering of soil particles to form larger structures called floccules, which improves soil structure.
• Cations such as Ca2+ and Mg2+ promote flocculation by being attracted to the negative charges on soil particles.
• Soil particles have a negative charge, with smaller particles having more negative charges.
• Cations are adsorbed to the surface of soil particles, and polarized water holds soil particles together, trapping larger sand and silt in a floccule structure.
• High humus content is more beneficial than high clay content because humus is more fertile, holds more water, has a higher Cation Exchange Capacity, encourages more earthworm activity, and 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 can be released into soil water (soil solution) and be replaced by others, supplying minerals to plants.
• A cation with a +2 charge (Ca2+) can replace two separate cations each with +1 charge (H+).
• Cations are removed from soil solution by plant roots.
• Colloidal humus particles have three times higher cation exchange capacity than colloidal clay particles.
• Adding organic matter to a soil with poor CEC (like sand) will increase its CEC.
• Main cations involved in cation exchange are Ca2+, Mg2+, Al3+, H+, and K+.
• Note that nitrate (NO3-) cannot be held, and 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 soils have a pH of 0-6, neutral soils have a pH of 7, and basic soils have a pH of 8-14.
• The concentration of acidic ions adsorbed onto the surface of the soil determines the acidity of the soil.
• Acidic ions include H+ and Al3+, while alkaline/basic ions include Ca2+ and Mg2+.
• Importance of Soil pH:
  • Lime increases Ca cations available for exchange (H, K, Al).
  • As pH increases, the amount of negative charges on soil colloids increases, leading to greater CEC.
  • Liming reduces ‘acid leaching’.

Flocculation

• Flocculation is the clustering of soil particles to form larger structures called floccules (aggregates = peds).
• Cations (Ca2+, Mg2+) promote flocculation by being attracted to the negative charges on soil particles.
• Soil particles have a negative charge, with smaller particles having more negative charges.
• Cations are adsorbed to the surface of soil particles, and polarized water holds soil particles together, trapping larger sand and silt in a floccule structure.
• Flocculation improves soil structure.

Benefits of High Humus Content

• Humus is more fertile.
• Humus holds more water.
• Humus has a higher Cation Exchange Capacity (CEC).
• Humus encourages more earthworm activity.
• Humus has more negative charges, increasing the 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 can be released into soil water (soil solution) and be replaced by others, supplying minerals to the plant.
• A Cation with a +2 charge (Ca2+) can replace two separate cations each with +1 charge (H+).
• Cations are removed from soil solution by plant roots.

Cation Exchange Capacity (CEC)

• Colloidal humus particles have three times higher CEC than colloidal clay particles.
• CEC is the ability of the soil to perform cation exchange, measured by the quantity of cations that the soil can adsorb onto its surface.
• Adding Organic Matter (slurry/FYM/Seaweed) to a soil with poor CEC (like sand) increases CEC.
• Main cations involved are Ca2+, Mg2+, Al3+, H+, K+.
• Nitrate (NO3-) cannot be held.
• Sandy soils have the lowest CEC.

Soil pH

• Soil pH measures the concentration of H+ ions in a solution, ranging from acidic (0-6) to neutral (7) to basic (8-14).
• The concentration of acidic ions adsorbed onto the surface of the soil determines the acidity of the soil.
• Acidic ions include H+, Al3+, while alkaline/basic ions include Ca2+, Mg2+.

Importance of Soil pH

• Lime increases Ca cations available for exchange (H, K, Al).
• As pH increases, the amount of negative charges on soil colloids increases, leading to greater CEC.
• Liming reduces ‘acid leaching’.
• Key points to note: Lime moisture content must be controlled. Optimum pH range is 5.5-7.5, as few cations are available below 5.