Soil Chemical Properties

Study Notes

Flocculation is the clustering of soil particles to form larger structures called floccules, improving soil structure.
Cations (Ca2+, Mg2+) promote flocculation by being attracted to the negative charges of soil particles, which have a higher negative charge due to smaller particle size.
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.

Cation exchange is the ability of 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.
Colloidal humus particles have three times higher cation exchange capacity than colloidal clay particles.
Cation Exchange Capacity (CEC) is the ability of soil to adsorb cations onto its surface, measured by the quantity of cations that can be adsorbed.
Main cations involved in cation exchange are Ca2+, Mg2+, Al3+, H+, and K+.
Nitrate (NO3-) cannot be held by cation exchange.
Sandy soils have the lowest CEC.

Soil pH measures the concentration of hydrogen ions in a solution, with acidic pH having a high concentration of H+ ions, neutral pH having a equal concentration of H+ and OH- ions, and basic pH having a high concentration of OH- ions.
The concentration of acidic ions adsorbed onto the surface of soil determines the acidity of the soil.
Importance of soil pH:
- pH affects nutrient availability and toxicity.
- pH affects microbial activity and populations.
- pH affects the availability of some nutrients.

Lime increases Ca cations available for exchange, reducing the amount of acidic ions and increasing the amount of negative charges on soil colloids.
Liming reduces 'acid leaching' and increases CEC.
Key points to note:
- Lime moisture content must be considered when applying lime.
- Optimum pH for cation exchange is between 5.5 and 7.5.

Flocculation is the clustering of soil particles to form larger structures called floccules, which improves soil structure.
It is promoted by the presence of cations, which are attracted to the negative charges on soil particles.
Soil particles have a negative charge, with smaller particles having more negative charges.
Cations (Ca2+, Mg2+) are adsorbed to the surface of soil particles, and polarized water holds them together, trapping larger sand and silt in a floccule structure.
High humus content is more beneficial than high clay content because it is more fertile, holds more water, has a higher Cation Exchange Capacity, encourages more earthworm activity, and has more negative charges, leading to an increased rate of flocculation.

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 the soil solution by plant roots.
Colloidal humus particles have three times higher cation exchange capacity 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+, and K+.
Note: Nitrate (NO3-) cannot be held by the soil.
Sandy soils have the lowest CEC.
Optimum pH for cation exchange is 5.5-7.5, as few cations are available below 5.

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).
- 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 considered.