   #copyright

Soil structure

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Soil structure is determined by how individual soil granules clump or
   bind together and aggregate.

Overview

   Natural aggregates that can be clearly seen in the field are called
   peds. Clods, on the other hand, are aggregates that are broken into
   shape by artificial actions such as tillage. The surfaces of peds
   persist through cycles of wetting and drying in place. Commonly, the
   surface of the ped and its interior differ as to composition or
   organization, or both, because of soil development. Earthy clods and
   fragments stand in contrast to peds, for which soil forming processes
   exert weak or no control on the boundaries.

   Structure is very important since (along with soil texture) it affects
   the porosity of the soil. A dense structure will greatly reduce the
   amount of air and water than can move freely through the soil. Also, it
   will affect the plant's ability to propagate roots through the soil.

Types

   There are five major classes of structure seen in soils: platy,
   prismatic, columnar, granular, and blocky. There are also structureless
   conditions. Some soils have simple structure, each unit being an entity
   without component smaller units. Others have compound structure, in
   which large units are composed of smaller units separated by persistent
   planes of weakness.

Platy

   In platy structure, the units are flat and platelike. They are
   generally oriented horizontally. A special form, lenticular platy
   structure, is recognized for plates that are thickest in the middle and
   thin toward the edges. Platy structure is usually found in subsurface
   soils that have been subject to leaching or compaction by animals or
   machinery. The plates can be separated with a little effort by prying
   the horizontal layers with a pen knife. Platy structure tends to impede
   the downward movement of water and plant roots through the soil.

Prismatic

   In prismatic structure, the individual units are bounded by flat to
   rounded vertical faces. Units are distinctly longer vertically, and the
   faces are typically casts or molds of adjoining units. Vertices are
   angular or subrounded; the tops of the prisms are somewhat indistinct
   and normally flat. Prismatic structures are characteristic of the B
   horizons or subsoils. The vertical cracks result from freezing and
   thawing and wetting and drying as well as the downward movement of
   water and roots.

Columnar

   In columnar structure, the units are similar to prisms and are bounded
   by flat or slightly rounded vertical faces. The tops of columns, in
   contrast to those of prisms, are very distinct and normally rounded.
   Columnar structure is common in the subsoil of sodium affected soils.
   Columnar structure is very dense and it is very difficult for plant
   roots to penetrate these layers. Techniques such as deep plowing have
   help to restore some degree of fertility to these soils.

Blocky

   In blocky structure, the structural units are blocklike or polyhedral.
   They are bounded by flat or slightly rounded surfaces that are casts of
   the faces of surrounding peds. Typically, blocky structural units are
   nearly equidimensional but grade to prisms and to plates. The structure
   is described as angular blocky if the faces intersect at relatively
   sharp angles; as subangular blocky if the faces are a mixture of
   rounded and plane faces and the corners are mostly rounded. Blocky
   structures are common in subsoil but also occur in surface soils that
   have a high clay content. The strongest blocky structure is formed as a
   result of swelling and shrinking of the clay minerals which produce
   cracks. Sometimes the surface of dried-up sloughs and ponds shows
   characteristic cracking and peeling due to clays.

Granular

   In granular structure, the structural units are approximately spherical
   or polyhedral and are bounded by curved or very irregular faces that
   are not casts of adjoining peds. In other words, they look like cookie
   crumbs. Granular structure is common in the surface soils of rich
   grasslands and highly amended garden soils with high organic matter
   content. Soil mineral particles are both separated and bridged by
   organic matter breakdown products, and soil biota exudates, making the
   soil easy to work. Cultivation, earthworms, frost action and rodents
   mix the soil and decreases the size of the peds. This structure allows
   for good porosity and easy movement of air and water. This combination
   of ease in tillage, good moisture and air handling capabilities, and
   good structure for planting and germination, are definitive of the
   phrase good tilth.

Structureless

   Some soils lack structure and are referred to as structureless. In
   structureless layers or horizons, no units are observable in place or
   after the soil has been gently disturbed, such as by tapping a spade
   containing a slice of soil against a hard surface or dropping a large
   fragment on the ground. When structureless soils are ruptured, soil
   fragments, single grains, or both result. Structureless soil material
   may be either single grain or massive. Soil material of single grains
   lacks structure. In addition, it is loose.
   Retrieved from " http://en.wikipedia.org/wiki/Soil_structure"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
