A ped is a soil particle.

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.

They are found most frequently in the C, E, Bs and K horizons as well as in sesquioxide

Sesquioxide

A sesquioxide is an oxide containing three atoms of oxygen with two atoms of another element. For example, aluminium oxide is a sesquioxide.Many sesquioxides contain the metal in the +3 oxidation state and the oxide ion, e.g., Al2O3, La2O3...

s (very old soils that are rich in iron and magnesium).

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 and soils rich in swelling clays such as the semectites and the kandite Halloysite

Halloysite

Halloysite is a 1:1 aluminosilicate clay mineral with the empirical formula Al2Si2O54. Its main constituents are aluminium , silicon , and hydrogen . Halloysite typically forms by hydrothermal alteration of alumino-silicate minerals. It can occur intermixed with dickite, kaolinite, montmorillonite...

. 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

Polyhedron

In elementary geometry a polyhedron is a geometric solid in three dimensions with flat faces and straight edges...

 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

Organic matter

Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...

 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

Tillage

Tillage is the agricultural preparation of the soil by mechanical agitation of various types, such as digging, stirring, and overturning. Examples of human-powered tilling methods using hand tools include shovelling, picking, mattock work, hoeing, and raking...

, earthworms, frost action and rodents mix

Bioturbation

In oceanography, limnology, pedology, geology , and archaeology, bioturbation is the displacement and mixing of sediment particles and solutes by fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves In oceanography, limnology, pedology, geology (especially...

 the soil and decreases the size of the peds. This structure allows for good porosity

Porosity

Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...

 and easy movement of air and water. This combination of ease in tillage

Tillage

Tillage is the agricultural preparation of the soil by mechanical agitation of various types, such as digging, stirring, and overturning. Examples of human-powered tilling methods using hand tools include shovelling, picking, mattock work, hoeing, and raking...

, good moisture and air handling capabilities, and good structure for planting and germination

Germination

Germination is the process in which a plant or fungus emerges from a seed or spore, respectively, and begins growth. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. However the growth of a sporeling from a spore, for example the...

, are definitive of the phrase good tilth.