Last week we looked at soil horizons, the vertical layers that soil forms naturally when undisturbed. This week we’ll zoom in a little closer and look at the soil peds, the natural aggregates of soil within those horizons.
A soil ped is an individual natural soil particle. You’d recognise these as the ‘crumbs’ you can see when crumbling garden soil gently between your fingers. A soil mostly made up of visible peds when moist is pedal (pronounced ‘pea-dal’), and a soil with indistinguishable peds when moist is apedal (A-pea-dal).
Peds differ to clods, which form from disturbances such as tillage, which cause soils to fragment along natural planes of weakness. You may see clods form when digging very wet soil for example, and should you pick that clod up you may well be able to break it along yet another fault line into two smaller clods. Any clods that form in that same soil, dug when drier, will probably be different — smaller perhaps, and/or maybe harder to break apart.
In short, peds are the natural, permanent aggregates of soil that persist through wet and dry cycles, while clods are neither of these.
The shape, size and arrangement of peds gives a soil its characteristic structure — and weak peds may cause a soil’s structure to collapse during even gentle rain, leaving a fine crust on the surface. There are five general ped shapes, which are best determined in dry or slightly moist soil:
Granular peds are less than 5 mm in diameter, resemble biscuit crumbs and are found in the A horizon (surface soil). These ped types retain water and organic matter well, are easy for plant roots to push through, and enable air and water to flow through a soil profile easily. Friable (’easily crumbled or pulverised’) and good tilth (’suited to plant growth and cultivation’) are words that describe soils made of granular peds.
Blocky (angular and subangular)
Blocky peds resemble blocks and are up to 50 mm in diameter. They are roughly the same width in all dimensions. Ones with sides joining at sharp angles, as in the top shape at left, are angular blocky. Those with more rounded corners, as in the bottom shape at left, are subangular blocky. These peds are often in the B horizon subsoil where the clay content is higher, but can also be found in surface soils with a high clay content, as these images show. These blocky surface peds form when clay minerals swell when wet and shrink when dry, leading to surface cracks forming.
These have a flat, plate-like shape and are usually arranged horizontally. They are more likely to be found in subsoil that has been leached or compacted. Their wide, horizontal alignment hinders the downward movement of water and plant roots.
This ped type resembles an elongated blocky ped, with faces joining at an angle. The tops are usually flat. These peds are found in the B horizon and can be several centimetres long. Prismatic peds can break into smaller, blocky peds — this is known as a compound structure, where one structure type resides in another.
Columnar peds are similar to prismatic ones, but with smoother edges and rounded tops. They are found mostly in subsoils subjected to salinity or high in swelling clays. Plants struggle to penetrate these peds.
Not surprisingly, the soils with good structure for plants will typically have many peds of about 0.2 mm to 3 mm diameter — the granular peds. These are the type we most want, for their properties of water and organic matter retention, good drainage and aeration, friability and tilth.
While we are stuck with the soils we have, we aren’t stuck with their structure if poor. Next week we shall cover the ways to encourage good ped formation and improve soil structure.
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