SubVESS Subsoil Visual Evaluation of Soil Structure – Method Description

*Use SubVESS as an add-on to VESS if you suspect that subsoil compaction is a problem.*

If you suspect that there is a compact layer in the subsoil and are considering improving it by subsoiling then it is important to confirm this by digging a hole and assessing the soil. SubVESS is ideal for this because it enables you to score the quality of the subsoil and helps to decide whether it is worth remediating and also what type of remedial action is needed. Often a compact layer detected at the bottom of the topsoil when using VESS provides a clue that it is necessary to assess soil quality deeper in the soil by using SubVESS.

Soil inspection

SubVESS differs from VESS by describing the structure of the subsoil only. (VESS works soley for topsoil). Subsoil description starts below spade depth, where the soil is often compacted. Physical differences are less obvious in the subsoil than in the topsoil so assessment is more progressive and comprehensive than VESS. The method is summarized in the SubVESS chart

When to do a SubVESS assessment

Sample when the soil is moist – if the soil is too dry or too wet it can be difficult to dig a pit and it will fill with water if the soil is waterlogged. After harvest in autumn is a good time because crop roots are still visible and crop damage during excavation is avoided.

Soil picture

Where to sample

Choose sites from areas where you expect good structure (uniform crop growth or near a fence line) and from areas where you expect poor structure (poor crop growth or areas near gates) to allow you to see structural differences. Dig the pit across the direction of tillage or crop rows to cut through any compacted areas.

Three steps to assess subsoil structure using SubVESS

Step one: Digging a soil pit

Often it is enough to dig down below the hole from where a VESS sample was taken. This can expose subsoil down to at least 40 cm, where most subsoil compaction can be detected.

Optional step for detailed assessment: dig a trench.

Dig a trench, preferably using a mechanical digger. Make it al least 60 cm wide, 2 m long and 50 cm deep. For safety reasons the depth should not exceed 1.4 m. If there are roots or soil compaction below trench depth, use a spade to dig a small hole in the base of the trench or extract soil with an open-threaded auger.

Step two: Identify the layer to be assessed

Soil structure

Remove soil from any surfaces smeared during digging the pit using a spade, spatula or knife. Observe the soil below the topsoil and identify any hard layer by prodding with the point of a knife or a pen. It is important to distinguish man-made features of the subsoil from natural features. The signs of compaction are hardness due to high density, reduced hydraulic conductivity leading to accumulation of water above the compact layer, and a marked change in structure with horizontal, laminated or platy aggregates and low or absent porosity. More information on identifying a compacted layer can be found in the supplementary material download.

Optional step for detailed assessment: measure the depth and thickness of all distinct subsoil layers to 1.4 m.

Usually there are only two or three layers. Mark the layers with a knife or, preferably, by inserting plastic tags and measure their depths. Each layer is then scored separately. Take a picture of the profile. This may need to be split into two or more sections to cover the whole depth. Use labels for subsequent identification. An example of a labelled, compacted soil profile is shown below. The top label shows the beginning of the subsoil at 25 cm depth.

Step three: Soil scoring

Aim to record information on the score sheet (see examples of a blank SubVESS score sheet and a completed SubVESS score sheet describing the above labelled profile). SubVESS uses a separate and distinct assessment scale from VESS and the scores are not interchangeable. Score the layer below the topsoil or, if making a detailed assessment, score each layer, starting with the top layer. Use the SubVESS chart. Score subsoil quality by assessing (a) mottling, (b) strength, (c) porosity, (d) root depth and pattern and (e) aggregate size and shape. Give an assessment between 1 and 5 for each factor where 1 is best and 5 is worst. Some adjacent assessments are combined if there is no means of distinguishing between them e.g. 1a-3a. Assessment is helped by observing the profile face or by extracting fragments of soil (~5- 10 cm deep by 2-5 cm thick) using a knife (see the SubVESS chart for how to extract these).

Mottling (a) refers to colouring of the soil due to waterlogging. The best score (1a-3a) is where there is either no mottling (colours are bright and uniform) or where mottling is faint and diffuse. Poorer scores (4a-5a) indicate some waterlogging and correspond to progressively more distinct mottling in a layer that is uniformly grey/blue with rust coloured mottles only present around pores or blocked channels.

Strength (b) refers to the resistance of the soil to penetration by a knife or to the removal of soil fragments. High soil resistance can restrict roots. Soil resistance is assessed either by jabbing the side of the profile with a knife progressively moving from the top to the bottom of the layer or by extracting fragments of the layer. Alternatively, break these fragments apart by holding the ends and snapping like a twig. Fragments may fall apart when removed (1b-2b), may keep their shape but are readily broken (3b) or have an angular shape and are difficult to extract and break (4b-5b).

Porosity (c) is either between sand grains or aggregates or within fragments (< 0.5 mm diameter), tube-like worm, and/or root channels (1-10 mm diameter) or vertical cracks (a few mm wide). Look for smaller pores on an unsmeared surface such as a freshly broken fragment. To help distinguish internal pores from surface imperfections, hold fragments in the shade. Picking at the vertical face of the soil profile with a knife will reveal channels and cracks. The soil can have high porosities, with pores throughout (1c) or with occasional less porous zones (2c). Pores are larger and less abundant - mainly cracks and earthworm holes in 3c. Porosity is low (<5 pores 100 cm-3 of profile face) in 4c and not visible in 5c.

Roots (d) can appear to grow throughout the layer with no signs of restriction (1d-2d) or can be localised within cracks and worm channels (3d) and include thickening (4d). A layer with no roots where they are expected is 5d. A dense interlocking web of roots may develop on the surface of a compact layer. Rooting depth is also important as it indicates whether a compact layer has been successfully penetrated and is assessed by looking for roots at depth. To find such roots, pick a few mm into the vertical face of the soil profile with a knife. The rooting depth corresponds to the presence of the deepest root detected.

Aggregate shape and size are determined by looking at the profile face and by crumbling fragments. Aggregates can be small, rounded and friable (1e), larger and more angular (2e and 3e) or poorly defined such that extraction of fragments leaves knife marks visible on the profile face (4e and 5e). Single grain structures such as distinct, coarse sand grains are included as structureless (4e). The poorest structures are tough, massive, plastic and hard to break up.

More information on mottling, porosity, rooting depths and soil structural forms and sizes is provided in a sheet of supplementary material.

Produce a score (Ssq) for subsoil quality based on the dominant assessment of the five factors. For example, a heavily compacted layer from 25-30 cm may have assessments of Mottling 1a-3a, Strength 4b, Porosity 5c, Roots 4d-5d and Aggregate size/shape 5e giving a score of Ssq 5. It is important to determine if the score is due to natural conditions or is due to management, usually compaction. It may be necessary to dig deeper to discover if the quality improves with depth (see Optional step for detailed assessment below) or to look at a soil under natural conditions e.g. under nearby woodland.

Optional step for detailed assessment: do an overall subsoil assessment:

Where there are two layers present (occasionally there are three), score each layer separately

For example, the complete assessment of the compacted profile shown above covered 25-130 cm depth with three layers, Ssq 5 from 25-40 cm and from 40-75 cm and Ssq 3 from 75-130 cm giving an overall profile score of Ssq 5/5/3. Commonly, in a uniform profile such as this, if a compact layer (higher score) overlies a looser layer, this is evidence of subsoil compaction by machinery. If a compact layer is observed in an agricultural field then this may be detectable using a penetrometer. If it is, the penetrometer can be used elsewhere in the field to rapidly check out the depth and extent of the compact layer thus avoiding the need to dig further pits.

Assessing the need for remedial action

For management purposes, scores of Ssq1, Ssq2 or Ssq 3 are good and should only require further yearly monitoring. A score of Ssq 4 is fair and may require medium to long-term remediation. A score of Ssq 5 is poor and requires more immediate remedial action.

Aim to improve the structure of this layer by, for example, subsoiling when conditions are suitable. Compact layers occur not only in fields, especially in turning or stock feeding areas, but also in rows between trees in orchards and vineyards and under tramlines. Further practical details of the use of subsoilers and controlled traffic for conserving subsoil structure are given in: Godwin, R.J. and Spoor, G. 2015. Choosing and evaluating soil improvements by subsoiling and compaction control. In: Ball, B.C. and Munkholm, L.J. (eds.). Visual soil evaluation: realizing potential crop production with minimum environmental impact. CABI, Wallingford, Oxon., pp. 66-85.

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