Soil Structure

Soil structure is determined by how individual soil granules clump or bind together and aggregate, and therefore, the arrangement of soil pores between them. Soil structure has a major influence on water and air movement, biological activity, root growth and seedling emergence.

Only about 50% of soil is solid material. The remainder is pore space. It is in these spaces that the action happens. Water is stored there. Organisms live there. Organic matter and nutrients accumulate there.

Small pores within the aggregates provide storage and refuge. The larger pores (and fissures) between the aggregates are the pathways for liquids, gases, roots and organisms.

Soil Structure

Soil structure is the arrangement of soil particles into groupings. These groupings are called peds or aggregates, which often form distinctive shapes typically found within certain soil horizons. For example, granular soil particles are characteristic of the surface horizon.

TEXTURAL TRIANGLE

Soil texture is the relative proportions of sand, silt, or clay in a soil. The soil textural class is a grouping of soils based upon these relative proportions. Soils with the finest texture are called clay soils, while soils with the coarsest texture are called sands. However, a soil that has a relatively even mixture of sand, silt, and clay and exhibits the properties from each separate is called a loam. There are different types of loams, based upon which soil separate is most abundantly present.

SOIL AGGREGATES

Generally, only the very small particles form aggregates, which includes silicate clays, volcanic ash minerals, organic matter, and oxides. There are various mechanisms of soil aggregation.

Mechanisms of soil aggregation

• Soil microorganisms excrete substances that act as cementing agents and bind soil particles together.
• Fungi have filaments, called hyphae, which extend into the soil and tie soil particles together.
• Roots also excrete sugars into the soil that help bind minerals.
• Oxides also act as glue and join particles together. This aggregation process is very common to many highly weathered tropical soils and is especially prevalent in Hawaii.
• Finally, soil particles may naturally be attracted one another through electrostatic forces, much like the attraction between hair and a balloon.

Aggregate Stability

Stable soil aggregation is a very valuable property of productive soils. Yet, the stability of soil aggregation is very reliant on the type of minerals present in the soil. Certain clay minerals form very stable aggregates, while other clay minerals form weak aggregates that fall apart very easily.

·         Highly weathered silicate clays, oxides, and amorphous volcanic materials tend to form the most stable aggregates. The presence of organic matter with these materials improves stable aggregate formation. 

·         In contrast, less weathered silicate clays, such as montmorillonite, form weak aggregates. Some silicate clays are said to have a shrink-swell potential. This means that the soil minerals expand, or swell, when wet, causing the soil to become sticky and drain poorly. When dry, these soils shrink and form cracks.