Question

3. What are the soil chemical properties and its primary focus on colloids as a way to understand CEC, soil acidity, and the concept of soil as a buffer?

Given adequate physical properties, the capacity of the soil to provide nutrients depends on soil colloids and the capacity to cycle nutrients through decomposition of organic matter and mineralization (recall the N cycle and our model of plant nutrient availability). Explain this concept in details?

Answer 1

Soil Texture:

The proportions of sand, silt, and clay particles in soils:
Sand 2 to 0.05 mm effective diameter
Silt 0.05 to 0.002 mm
Clay < 0.002 mm

Soil Colloids: Soil particles < 0.001 mm in diameter. These Are the most reactive of soil particles because of surface area and electrical charge.Types of soil colloids:

• Inorganic: Clay minerals, Oxide minerals
• Organic: Soil Organic Matter

Sources of Charge on Common Soil Clays:

• 2:1 clays (smectites, vermiculite, etc.)
  • Most charge is due to isomorphous substitution (always negative)
  • Little pH-dependent charge
• 1:1 clays (kaolinite)
  • Little isomorphous substitution
  • Most charge is due to pH-dependent charge (positive or negative)

Cation Exchange:The exchange of cations adsorbed (attached) onto colloid surfaces with cations in solution. Exchangeable cations are those attached to colloid surfaces. Cations in solution and on colloid surfaces tend toward a state of equilibrium. Exchangeable cations can be manipulated.

Cation Exchange Capacity: The mass of exchangeable cations that a given soil can retain per unit weight. Units are cmol(+)/kg soil or meq/100g. Soils have CEC because of: Soils have many more exchangeable cations than cations in solution (buffering capacity).

Cations attraction to clays is a function of charge and size. Strength of attraction:

Al3+ > Ca2+ > Mg2+ > K+ = NH4+ > Na+

CEC is commonly measured in laboratories by:
1. Saturating soil cation exchange sites with a cation (e.g. NH4+)
2. Extracting the soil with another cation to remove the NH4+
3. Measure NH4+ extracted

The exchangeable cations have very important influences on soil properties:

• Ca2+ is the dominant exchangeable cation in most soils.
• Soils become acidic when they contain significant amounts of exchangeable Al3+.
• Soils have poor structure when they contain significant amounts of exchangeable Na+.

Buffering Capacity: The resistance of the soil solution to a change in composition. The soil solids control or “buffer” the composition of the soil solution. Caused by dissolution of minerals, adsorption/desorption of exchangeable cations.

The amount of buffering capacity is:

• Proportional to minerals present (e.g. soils high in K-feldspars will be highly buffered with respect to K).
• Proportional to amount of exchangeable cations (e.g. soils high in exchangeable Ca will be highly buffered with respect to Ca)
• Typically, highly-weathered soils are less well-buffered with respect to nutrients than are lightly-weathered soils (more CEC, more primary minerals)