L5 Soil Cations/Anions

Physical and chemical

Physical properties of soils

1. Texture

2. Structure

3. Density

4. Porosity

5. Permeability

6. Soil colour

7. Temperature of soil

8. Soil plasticity, Compressibility and Erodibility

Soil texture

A relative amount of sand, silt and clay

Sand: Gritty

Silt: Floury when dry, silky when wet

Clay: Velvety

Soil structure

Arrangement of soil particles into aggregates (peds)

• Granular

• Blocky

• Platy

• Columnar and prismatic

• Massive (non-structure)

Effect of Soil structure on water permeability

Soil Density and Pore Space

Loose, well-aggregated, porous soils and those rich in organic matter have lower bulk density

Soil colour

Inherited from parental material or due to soil forming processes

Organic substances impact - black or geryish-black colour. 

Iron compounds → brown, red and yellow coloured soil

Iron oxide in combo with organic substances → brown colour which is the most common soil colour

Silica, lime → white and grey tinges to soil

Chemical properties of soil

• Inorganic matter composition

• Organic matter composition

• Cation Exchange Capacity of Soils

• Buffering Capacity of Soils

Inorganic matter composition

Aluminosilicates and oxides, because surface electrochemical properties these minerals control the adsorption and transformation and behaviour of other chemical constituents. 

Aluminosilicates → clay particles, colloidal in nature

Colloids

Thin, plate-like shape that reflect their layered chemical crystal structure

Colloid surfaces are negatively charged because of their molecular structure. 

They attract and hold positively charged ions that include nutrient bases like calcium and magnesium. 

Ion

An ion is an atom with positive or negative charge

Cation

Positively charged ion that has more protons than electrons, meaning it has lost one or more electrons. 

Common +2 cations include mg and ca

Anion

Negatively charged ion that has more electrons than protons, meaning it has gained one or more electrons

Cation Exchange Capacity (CEC)

The total capacity of soil to hold exchangeable cations

• Clay and organic matter have negative charges that hold and release positively charged nutrients. (Cations absorbed onto the surface of the clay or humus)

• The static charge keeps the nutrients from being washed away and holds them so they are available to plant roots and soil microorganisms. 

Buffer capacity

The amount of acid or base buffered solution can soak up before its pH will start to change significantly. 

Bioavailablity

Available to Biological Organisms

• May refer to ingestion or uptake of a given compound

• May also refer to biological or physiological effects

• By definition → chemical measurements remain proxy and mist be linked to a biological organism