L5 Physical and Chemical properties of Soil

some definitions and keywords

Physical properties of Soils

1. texture

2. Structure

3. Density 

4. Porosity 

5. Permeability 

6. Colour

7. Temperature

8. Plasticity, Compressibility and Erodibility

Soil texture

Relative amount of sand, silt and clay

Soil fractions (soils separates → mineral part of soil)

Sand: gritty

Silt: floury when dry, silky when wet

Clay: Velvety

Equilateral triangle used bu USDA for textural classes of soil

Sand

• Particles seen by unaided eye

• Less important role in physiochemical activities 

• Sand increases size of pores between soil particles facilitating movement of air and water. 

Silt

• Coarse shows little physiochemical activity but finer grades play important role in some chemical processes

• Silty soil has larger exposed surface area than sandy soil. 

• Contains sufficient quantities of nutrients, both organic and inorganic

• Soils rich in silt possess high water holding capacity

Clays

• Plasticity and smoothness when wet and harness when dry. Smallest size and colloidal nature when clay particles are exposed to extremely large surface area. 

• Take very active part in physiochemical reactions in soil

• Have fine pores, poor drainage and aeration.

• Acts as store house for water and nutrients. 

Soil structure

Arrangement of soil particles into aggregates (peds)

• Granular

• Blocky (angular and sub angular

• Platy

• Columnar and prismatic 

• Massive (non structure)

Soil structure associated with its functions

• Fertility 

• Biodiversity

• Rootability

• Carbon sequestration 

• Nutrient cycling 

• Water cycling

Degraded soil structure is a decrease in soil services and increase in hazards. 

structure and texture together regulate porosity, density, compactness, retention and movement of water and air in soil 

Effect of soil structure on water permeability

Particle density

A measure of the mass per unit volume of soil expressed with regard to volume of solids only Ranges from 1 – 3 mg m^-3

Bulk density

• A measure of the mass per unit volume of the entire soil sample (usually undisturbed) –  a measure of the solids + pore spaces

• Soil bulk density is influenced by both texture (sands are more dense) and structure

• Bulk density is indicative of the pore space in a soil and ease with which roots may penetrate

Soil density and Pore space

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

Soil colour

• Inherited from the parental material (i.e., lithochromic) Or may be due to soil forming processes 

• Organic substances impart - black or dark greyish-black colour 

• Iron compounds a - brown, red and yellow colours of soils Iron oxides in combination with organic substances -  brown colour which is most common soil colour

• Silica, lime - light white and grey tinges to the soil.

Chemical properties of Soil

• Inorganic matter composition 

• Organic matter composition

• Cation Exchange Capacity of Soils

• Buffering Capacity of Soils

Inorganic matter composition

• Composed chiefly of aluminosilicates and oxides. 

Through their surface electrochemical properties these minerals control the adsorption  and transformation and behaviour of other chemical constituents. 

• Aluminosilicates are commonly called clay particles 

• Clay particles are colloidal in nature

Colloids

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

• Colloid surfaces tend to be negatively charged because of their molecular structure  -  they attract and hold positively charged ions which often include nutrient bases such as calcium and magnesium

Organic matter

Organic matter - partially decomposed plant residues and humus 

Humus - resistant product of decomposition and colloidal in nature

Humus contains the following organic molecules:

Amino acids – glycine, serine, alanine, glutamic acid

Proteins – purines, guanine, adenine

Aromatic molecules - Aromatic carboxylic acids

Aminosugars – glucosamine

Hexose sugars – glucose

The humus fraction also contains fats, oils, waxes, resins, tannin, lignin and some pigments.

Also contains colloidal structures that are resistant to decay.

Ion

An ion is an atom with a positive or negative charge

Cation

A positively charged ion that has more protons than electrons, meaning it has lost one or more electrons

A +2 charge on an ion means it has lost two electrons, resulting in a net positive charge. This occurs when the atom transitions from a neutral state to a positively charged state, known as a cation.

Common +2 cations include magnesium (Mg²⁺) and calcium (Ca²⁺)

Anion

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

Cation Exchange Capacity (CEC)

Is the total capacity of a soil to hold exchangeable cations

• Clay and organic matter have negative charges that can hold and release positively charged nutrients. (The cations are adsorbed onto the surface of the clay or humus)

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

• Negatively charged colloids attract cations

Buffer capacity

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

• The buffer capacity of a soil is important in determining how its pH will change. Various minerals in soil help to buffer against changes in pH when an acid or base is added. 

• At high pH, calcium, magnesium and potassium oxides, together with carbonates, help to buffer pH changes

• At acidic pH, aluminium oxides and iron hydroxides act as buffering agents; at intermediate pH levels, soil organic matter, mineral weathering and exchange reactions help to buffer the soil. 

• A higher buffer capacity means that the soil can absorb more acid and/or base without a significant change in pH. 

• In general, clay soils have higher buffer capacity than sandy soils, and a higher organic matter content tends to increase buffering capacity.

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- must be linked to a biological organism