Soil structure in the field

Whats is soil structure?

Soil structure refers to the three-dimensional arrangement of primary particles within the soil profile. Due to its significance in influencing soil physical properties, Oz Soils dedicates an entire section to Soil Structure.

Here, we aim to demonstrate how to make valuable observations of soil structure in the field.

Soil structure is vital for plant growth and agriculture. In 1987, it was estimated that degradation of soil structure was responsible for $145 million per year of lost agricultural production in the Murray-Darling Basin.

Soil structure impacts plants by determining the size and distribution of soil pores.

Peds and their shapes

Frequently, soil primary particles are clustered to form aggregates or peds, which can be identified in the soil profile.

Three characteristics of peds are observed in the field: shape, size, and distinctness.

Platy

Platy peds are flat and horizontal. Near the soil surface, platy peds may indicate a compacted layer or pan.

Prismatic

Prismatic peds are taller than wide with relatively flat faces and sharp edges. They are mostly observed in clay subsoils where deep shrinkage cracks create the vertical ped faces.

Columnar

Columnar peds are taller than wide with relatively flat faces and sharp edges, except for the top which is domed. They occur in the upper part of the B2 horizons of some duplex soils.

Lenticular

Lenticular peds are lens-shaped. They often occur in the subsoils of cracking clays (Vertosols) and may have striations along the ped surface (called slickensides) which result from adjacent peds rubbing during shrink-swell cycles.

Angular Blocky

Angular blocky peds are approximately equidimensional with six relatively flat faces that meet at sharp edges and corners.

Subangular Blocky

Subangular blocky peds are approximately equidimensional with six somewhat rounded faces and rounded corners.

Polyhedral

Polyhedral peds are approximately equidimensional and have more than six relatively flat faces with angular corners. Re-entrant angles are common.

Granular

Granular peds are spheroidal and have little accommodation to the faces of adjacent peds. They are often small.

Size & Distinctness of Peds

The size of peds is estimated simply in millimeters for a typical ped. The least dimension (height or width) is reported. Describing the distinctness (or grade) of peds is a bit more subtle. The first distinction to make is between ped structures (which have peds) and apedal structures (which do not).

Strong 50mm angular blocky

Strongly pedal structures have peds that are quite distinct in the soil profile, and when disturbed more than two thirds of the soil material comprises peds. Note that strong used in this context refers to the distinctness of the peds, not how hard the soil is. Some strongly pedal soils (eg krasnozems) can be quite soft and friable.

Weak 20 mm subangular blocky

Weakly pedal structures have only indistinct peds which are hard to see in the undisturbed profile. When disturbed less than one third of the soil material consists of entire peds. Note that weak here refers to the pedality, not the hardness of the soil.

Moderate 50 mm angular blocky

Moderately pedal structures have peds that are well formed and evident, but not distinct. When the soil is disturbed about half of the material comprises entire peds. In the clay B horizon shown here, the shapes of the peds are more apparent when you prise them out!

Massive

Soil with a massive structure lacks peds, but is coherent (ie the soil material forms a united mass). The massive structure here is the A2 horizon below the sandy clay loam A1 horizon shown as the example of a weakly pedal structure.

Single grain

Single grain structures are apedal and loose rather than coherent. This is the typical structure seen in sand dunes.

Practical applications

One application of field soil structure assessment is the diagnosis of compaction in clay soils used for cotton production. The soil is a Vertosol (cracking clay) from near Narrabri, NSW.

A sample from an undisturbed vegetated area is contrasted with soil which has been badly compacted.

The effects of compaction are outlined below.

"Good" structure

This sample comes from an uncultivated grassed area.

The high biological activity in the soil is apparent.

Shrinking and swelling of the montmorillonite rich clay, coupled with the binding action of plant roots and other biological agents has led to the creation of a fine, granular structure and low bulk density. There are many macropores between the peds.

"Bad" structure

This clod was taken from the surface in a cotton field where the soil structure had been damaged. The wheels of agricultural machinery compact soil if they drive over it when it is moist. Note the absence of macropores, particularly in the lower part of the clod, which has a massive or platy structure. This compacted layer is called a traffic pan, and has a high bulk density.