SubVESS as an add-on to VESS if you suspect that subsoil compaction is a
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 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.
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.
step for detailed assessment: dig a
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
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.
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
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
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.
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.,