Testing soil structure

What is soil structure?

The soil matrix is composed of different sized structures composed of sand, silt and clay particles (as well as larger pieces such as gravel and iron oxide nodules), organic matter in varying stages of decay, fine plant roots, soil biota and the pore spaces that exist between them that allow for gas and water exchange. The arrangement and size of these aggregates and pores in a soil, result in different soils having different types of soil structure.

Determining soil structure

To determine the structure of your soil, carefully break apart each layer and match its characteristics with the appropriate structural type described above.

Soil composition, porosity, texture, dispersion and slaking are also used to describe aspects, or properties of soil contributing to structure. Soil structure should be assessed both at the surface and for each subsequent horizon in the profile (particularly over the entire rooting depth). On-farm, the most easily assessable soil properties are outlined below and may be useful as indicators to undertake more comprehensive, quantitative measures.

Soil composition

Half fill a jar with soil, top up with water and replace the lid. Shake vigorously and set the jar on a flat surface. After 24-48 hours, the different mineral components will generally settle into layers, with the heaviest particles settling first at the bottom (sand, then silt, then clay). Clay may also remain suspended in the water and gives the suspension its colour. Floating, or partially submerged particles are likely to be pieces of organic matter in various stages of decay – the least decayed is generally floating at the surface and sinks as it decomposes. The relative proportion (percentage) of sand, silt and clay is used to name soil texture.

Slaking and dispersion

Place three soil aggregates 6–8 mm in size onto a shallow dish of rainwater ensuring they are covered – and observe. If the aggregates collapse in less than five seconds then the soils are highly unstable (slaking), but not necessarily dispersive. If the water subsequently becomes cloudy around each aggregate, then the soil is dispersive. Stable aggregates neither slake nor disperse.

Infiltration rate

Using either a plastic or metal tube of known volume, place on top of and push at least 5 cm into the soil and fill with water to a predetermined depth. Use a known volume of water (i.e. 500 ml). Measure the amount of time taken for all the water to infiltrate.

Surface soil crusting

Observe whether there is a hardened crust on the surface of the soil. Poke the surface with a sharp tool. Can intact pieces of crust be lifted off the soil? If the answer is yes, this confirms crusting. During the growing season, plant germination can be affected with plants struggling to emerge under the soil crust at the surface.

Soil permeability

Observe whether water ponds on the soil surface during and after rainfall events. Ponding will be at its most obvious after high intensity rainfall events. Water ponding can be the result of shallow or deep compaction layers, a dense soil layer, or be the result of a water repellent surface. Further investigation can help to determine the primary cause.

Compaction

Use a push probe or penetrometer to determine whether there are compacted layers and if so at what depth/s. This should be done when the soil is sufficiently moist (at field capacity), not bone dry. Gravel or stones present in soil can act as a physical barrier to a probe, making this approach unsuitable in these soil types.

Dig a hole with a narrow spade to about 30 cm and carefully create a pit where the face of the soil is relatively undisturbed. Observe soil for visual indications of a dense subsoil and identify whether there is a visible change in root growth (i.e. sideways growth of main root, stubby roots, less root growth evident below a given depth).

Soil bulk density

Use a core with a minimum diameter of 50 mm to take a known volume of soil, dry the core for 24 h at 105°C, weigh and calculate the bulk density (BD = weight of dry soil/cm³ volume of soil). Increasing density is often indicative of a tighter structured soil, which, as it increases plant roots find it increasingly difficult to penetrate. Bulk density increases with compaction and tends to increase with soil depth – those soils with a bulk density higher than 1.6 g/cm³ can restrict root growth.