Microstructure is considered to encompass all aspects of soil structure that are revealed when soil material is examined at a magnification of 5X or greater. Thin sections of heterogeneous soil material may contain more than one type of microstructure. In aggregated soil material, the aggregates themselves are described together with voids between and within aggregates; in non-aggregated soil material the microstructure is described by the void pattern present.
Peds: Four main types of peds have been recognized in field studies. All four can be composed of compound smaller units which may not be readily evident in the field but can be recognized microscopically.
Spheroidal Peds: Particles are arranged into more or less equant peds that are bounded by rounded faces and, as a consequence, are not accommodated to the adjoining peds.Blocky Peds: Particles are arranged into more or less equant peds which are angular if bounded by flat faces and subangular if bounded by flat and rounded faces. Faces of the angular blocks are more or less mirror images of the faces of the surrounding peds except where the soil had been disturbed by cultivation or where the peds accumulated invoids as the result of natural fracturing and/or gravitational transfer. Surfaces of subangular blocky peds can also be molds of each other, but due to high biological activity or cultivation they may be loosely packed and only touch each other in parts.
Plates: Particles are arranged into generally elongate horizontal peds. There are three main sub-types: a) more or less straight plates; b) wavy plates as present in some frost affected soils; c) lenticular plates which are thick in the middle and thin towards their edges.
Prisms: Particles are arranged into more or less vertically elongate peds bounded by surfaces that are usually flat, but can be also rather rough. They are subdivided into prisms with caps, and columns without caps.
Grade of Pedality: When a ped is totally surrounded by voids, it is easy to identify. When peds are only partially surrounded by voids, interpretation is more difficult since identifying planes of weakness is difficult. The following grades of pedality are proposed: (for moderate and weakly developed pedality, the voids are inferred to be the points at which aggregates will separate if pressure is applied)
Strongly Developed: The soil material is divided up into a number of units each of which is entirely surrounded by a void.
Moderately Developed: The soil material is divided into a number of units which are surrounded on at least two thirs of their tow-dimensional surface area by planar voids and/or are linked to other peds by links less than one third the size of the main diameter of the ped.
Weakly Developed: The soil material is divided into a number of units separable on the basis of being partially surrounded by planar voids (1/3 to 2/3 of the surface area of the proposed unit) and/or by liinks between units which are one third to two thirds the maximum width of the unit.
Accommodation of Peds: Accommodation of aggregates is a measure of the degree to which adjacent faces are molds of each other.
Accommodated: All the faces are accommodated by faces of adjoining aggregates.
Partially Accommodated: Some faces are accommodated by faces of adjoining aggregates.
Unaccommodated: Virtually none of the faces is accommodated by the faces of adjoining aggregates; adjoining aggregates touch at points.
This page (http://www.ces.ncsu.edu/plymouth/programs/microstructure.html) created by
Vera MacConnell, Research Technician, I on January 27, 1998.
Last Updated on January 27, 1998.
