Soil Structure

Soil Structure the arrangement of soil particles into aggregates and the subsequent arrangement of these aggregates in the soil profile. The term structure relates to the grouping or order of soil particles. It describes the gross or overall combination or arrangement of the primary soil separates into secondary groupings called aggregates or peds. A single type of structure may dominate a profile. More often, however, several types are encountered in the different horizons. Soil conditions and characteristics such as water movement, heat transfer, aeration, and porosity are much influenced by structure.

The dominant shapes of peds or aggregates in a horizon determine its structure type. The four principal types of soil structure are spheroidal, platy, prism-like and block-like. A brief description of each of these structural types (and appropriate subtypes) follows:

Spheroidal (granular and crumb subtypes) Rounded peds or aggregates are placed in this category. They usually lie loosely and are separated from each other. Relatively nonporous aggregates are called granules and the pattern granular. However, when the granules are especially porous, the term crumb is applied.Granular and crumb structures are characteristic of many surface soils, particularly those high in organic matter, and are especially prominent in grassland soils.

Plate-like (platy) The aggregates (peds) are arranged in relatively thin horizontal plates, leaflets, or lenses. A platy structure is found in the surface layers of some virgin soils but may characterise the lower horizons as well. In some soils the ploughpan has a coarse platy structure. The well-drained soil developed under the sal forest in the madhupur tract shows a platy structure.

Prism-like (columnar and prismatic subtypes) This subtype is characterised by vertically oriented aggregates or pillars that vary in height with different soils and may reach a diameter of 15 cm or more. Prism-like structures usually occur in subsurface horizons in arid and semiarid regions and, when well developed, are very striking features of the profile. They also occur in some poorly drained soils of humid regions. When the tops of the prisms are rounded, the term columnar is used. When the tops of the prisms are plane, level and clean cut the structural pattern is designated prismatic. A prismatic structure is typical of most loamy and clayey floodplain soils of Bangladesh.

Block-like (blocky and subangular blocky subtypes) In this case the aggregates have been reduced to blocks, irregularly six-faced, with their three dimensions more or less equal. These fragments range from about 1 to 10 cm in thickness. In general the design is so individualistic that identification is easy. When the edges of the cubes are sharp and the rectangular faces distinct the subtype is designated blocky. When subrounding has occurred, the aggregates are referred to as subangular blocky. Block-like structures are usually confined to the subsoil, and their stage of development and other characteristics have much to do with soil drainage, aeration, and root penetration. Blocky structure is common in most loamy and clayey floodplain soils. Prismatic structural units may also be broken up by soil animals and roots, so that angular blocky or subangular blocky structures may also be developed in less clayey materials. [Md Mizanur Rahman Bhuiyan]