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Bonding and Aggregation

Soil Colour | Soil Particles | Bonding and Aggregation | Porosity | Changing Soil Structure | Soil Strength

Principal Binding Agents | Soil Texture | Dispersion | Slaking | Dispersion Animation

The mineral component (sand, silt and clay) of a soil has the primary influence on how soil particles bind together and therefore what sort of aggregation results. Soil dominated by sand tends to bond weakly. Clay soil may bond strongly but different clays give different strengths of bonding. The organic component then becomes critical in augmenting, modifying and improving this bonding.

What causes aggregation?
Aggregation occurs when particles are held together by gravity, electrostatic forces, various chemical glues (such as iron oxides), "fibrous" organic materials and a range of gels and gums.

Thus aggregation is the result of the interaction of:

  • Soil texture
  • The type of clay mineral
  • The soluble and exchangeable ions in soil fluids
  • Organic matter
  • The accumulation of chemical cements
  • Time and the environment

Different clays produce quite different types of aggregation. Some (e.g. kaolins) will produce quite stable bonding and retain their integrity through many wet and drying cycles, even with considerable mechanical disturbance. Some (e.g. smectite) are quite susceptible to dispersion on wetting (and "working") and need to be used and managed with care.

Sandy or silty soil (low clay content) is usually weakly aggregated, whereas high clay soil is usually strongly aggregated.

Plant material and root exudates are the primary source of organic matter in soil. Living organisms and roots might comprise up to 15 % of total organic matter. Good levels of
organic matter (especially of the humus fraction) give strength to soil aggregates.

An active microbial population will produce many adhesive by-products which will help form and stabilise aggregates. Below is a model of soil aggregation adapted from
Tisdall and Oades (1982). Each step of the aggregation model demonstrates the bonding agents and aggregation of soil as size decreases.

Soil aggregation model

Why is clay so important?
Clays are tiny, tiny, plate like minerals ranging in diameter from about 0.02 of a micron to about 2 micron. They have a very large surface area relative to their weight and they carry a negative charge on much of their surfaces. Depending on the shell of cations (positively charged ions) surrounding the negatively charged parts of the clay surface, the clays may cluster together, or they may repel each other (disperse) and act as independent particles.
Dispersion can result in low permeability, crusting, poor drainage, low water availability, and increased likelihood of erosion.

Resilience of soil aggregation
The strength of bonding between soil aggregates will determine
soil strength; how the soil consolidates under pressure; the capacity of the soil to absorb energy without the aggregates breaking down; disintegration under mechanical cultivation (related to final tilth), and resistance of the soil to erosion.

Evaluating aggregation
One of the simplest ways of assessing aggregation is to measure by how well individual aggregates hold together in the presences of water. This is known as aggregate stability. Soils with poor aggregate stability can often
disperse or slake. If dispersion occurs the clay fraction of the soil can become mobile. Quite simply, aggregates of soil (approximately the size of a pea) should be placed carefully into a glass bottomed vessel containing just enough distilled water (or rain water) to completely cover the aggregate. The aggregate will then remain unchanged, swell, slake or disperse. For further information, the Practical Note and Quick Reference Guide attached provide details on evaluating aggregation.

Practical Note: Aggregate Stability

Well aggregated soil is important. It has pores between aggregates and within the aggregate. Large pores allow for the exchange of oxygen and other gases with the atmosphere, while small pores hold plant available water and dissolved nutrients.

Quick Reference Guide: Assessing aggregate stability

When a fragment of soil is immersed in fresh water, there are four things that can happen:
  • It can remain unchanged
  • It can swell
  • It can fall apart into smaller fragments (it slakes)
  • It can disperse into a fine milky suspension

Further information
More technical information on aggregation and bonding (otherwise known as soil coalescence) is available on the Soil Coalescence webpage.
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