Factors involved in genesis and stability of soil aggregate

a. Physical process

Any action that will shift the particles back and forth and force contact between particles will encourage aggregation. Alternate wetting and drying, freezing and thawing, the physical effect of root extension, and mixing action of soil organism and of tillage implements encourage such contacts and therefore stimulate aggregate formation.

Spring season plowing of fine textured soils result in breaking of clods into smaller aggregates under the influence of light rain and support seed bed preparation and aggregation of soil. Aggregation influence of plant roots, earthworm and other organism are also considerably important for soil structure development and its stability.

b. Influence of organic matter

Organic matter is the major agent that stimulates the formation and stabilization of granules and crumb type aggregates. As organic residue decomposes, gel and other viscous microbial products along with associated bacteria and fungi encourage crumb formation. Organic exudates from plant roots also participate in this aggregating action. Organic compounds such as polysaccharides then chemically interact with particles of silicate clays, iron and aluminum oxides. The organic compounds orient the clay in a common plane and then form bridges between individual soil particles, thereby binding then together in water stable aggregates. The complexity of organic materials found in humus makes possible this binding process.

c. Effect of adsorbed cations

Aggregate formation is definitely influenced by the nature of the cations adsorbed by soil colloids. For instance, when Na+ is a prominent adsorbed ion, as in some soil of arid and semiarid areas, the particles are dispersed and an undesirable soil structure result. By contrast, the adsorption of ions such as Ca2+, Mg2+, or Al3+ may encourage aggregate formation sating with process of flocculation.

These ions encourage the individual colloidal particles to come together in small aggregates called floccules. Flocculation however itself is only the first step because it alone doesn’t provide for the stabilization of the aggregates.

d. Influence of tillage and the compaction

Tillage has both favorable and unfavorable effect on aggregation. If the soil moisture level is favorable, the short time effect of tillage is generally favorable because the implements break up the clods, incorporate the organic matter into the soil, kill weeds, and create a more favorable seed bed. Tillage is thus considered necessary in normal management of some soils.