Soil Science

Structural Stability

Aggregate stability is the ability of compound particles of soil to maintain their integrity when a disruptive force is applied.

Action of Water - Dispersion

Clay particles are subject to forces of attraction and repulsion. Repulsion is manifested in swelling of clay on wetting and ultimately in dispersion of clay particles.

Repulsion can be visualized as being due to water attracted between clay particles in response to osmotic activity of large monovalent cations between the particles. Van der Waals forces are short-range forces of attraction between molecules and atoms. These forces decrease rapidly with increasing distance of separation between clay particles.

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Action of Water - Slaking 

Rapid wetting of dry soil can cause damage due to air trapped by water that fills the outer pores.

If this compressed entrapped air reaches a pressure greater than the tensile strength of the soil aggregate, it escapes explosively, breaking off aggregate fragments.

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Action of Water - Dissolution of cementing substances

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The most important forces holding particles and aggregates together at field water content are bonds due to inorganic or organic compounds.

During high rainfall periods and with a low concentration of cementing substances in the soil solution, such bonds can weaken.

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External Forces - Raindrop impact

The impact forces of raindrops hitting the soil surface can cause aggregates to break into smaller fragments, which, combined with clay dispersion, can lead to surface sealing. 

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External Forces - Vehicle traffic

Heavy vehicle traffic over relatively wet soil can cause severe soil compaction, with subsoil compaction being related to the width of the tires. The larger the area over which the load is applied, the greater the depth of potential compaction damage.

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External Forces - Animal treading

Soil compaction under animal hooves is similar to that under vehicle traffic, with comparable compactive pressures in both cases. However, due to the relatively small area over which the load is applied by hooves, compaction damage is typically limited to the top 10 centimeters of the soil profile.

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