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Encyclopedia > Soil structure

Soil structure is determined by how individual soil granules clump or bind together and aggregate. Granule is a generic term used for a small particle or grain. ...

Contents

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. To meet Wikipedias quality standards, this article or section may require cleanup. ... Pedogenesis or soil evolution (formation) is the process by which soil is created. ...


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. Soil texture triangle, showing the 12 major textural classes, and particle size scales. ... Porosity is a measure of the void spaces in a material, and is measured as a fraction, between 0–1, or as a percent between 0–100%. The term porosity is used in multiple fields including manufacturing, earth sciences and construction. ...


Structures

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 the 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 the 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 the 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. A polyhedron is a geometric shape which in mathematics is defined by three related meanings. ... An Inner Mongolia Grassland. ... A garden is a planned space, usually outdoors, set aside for the display, cultivation, and enjoyment of plants and other forms of nature. ... Organic has several meanings and related topics. ... Tillage (American English), or cultivation (UK) is the agricultural preparation of the soil to receive seeds. ... Earthworm is the common reference for the larger members of the Oligochaeta (which is either a class or subclass depending on the author) in the phylum Annelida. ... Weathering is the process of disintegration of rocks, soils and their minerals through natural, chemical, and biological processes. ... In oceanography and limnology, the displacement and mixing of sediment particles by benthic fauna (animals) or flora (plants) is termed bioturbation. ... Porosity is a measure of the void spaces in a material, and is measured as a fraction, between 0–1, or as a percent between 0–100%. The term porosity is used in multiple fields including manufacturing, earth sciences and construction. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... It has been suggested that Germination rate be merged into this article or section. ...


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.


References

  • Soil Survey Division Staff (1993). Soil Structure. Handbook 18. Soil survey manual. Retrieved on 2006-04-11.
  • Transient 3D Soil/Structure Interaction Analyses Including Nonlinear Effects, Mohammad Firuziaan. ISBN: 3-504-43892-0

This article incorporates text from http://soils.usda.gov/technical/manual/contents/chapter3g.html#60, a public domain work of the United States Government. For the Manfred Mann album, see 2006 (album). ... April 11 is the 101st day of the year in the Gregorian calendar (102nd in leap years). ... The public domain comprises the body of all creative works and other knowledge—writing, artwork, music, science, inventions, and others—in which no person or organization has any proprietary interest. ... A work of the United States Government is, as defined by United States copyright law, a work prepared by an officer or employee of the United States Government as part of that persons official duties. ...


  Results from FactBites:
 
SSM - Chapter Three (Part 7 of 9) | NRCS Soils (0 words)
The term "structural unit" is used for any repetitive soil body that is commonly bounded by planes or zones of weakness that are not an apparent consequence of compositional differences.
In soils that have structure, the shape, size, and grade (distinctness) of the units are described.
They extend upward to the soil surface unless there is a relatively thick overlying horizon that is very weakly compacted (loose or very friable) and does not permit the propagation of cracks (mechanically bulked subzones (fig.
Soil structure - Wikipedia, the free encyclopedia (382 words)
Soil structure is a sub-discipline of Soil Science concerned with the description of how particles in a given soil adhere - how they stick together.
Soil structure describes the size and shape of these individual peds, and attempts to both interpret the causative factors behind the structure, and evaluate the most productive and healthy structural characteristics of soil.
A major focus of study in the area of soil conservation, from at least the Dust Bowl era in the United States, is the effect of agricultural tillage methods on soil structure.
  More results at FactBites »

 
 

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