3rd European Summer School on Soil Survey
Co-organized by TAIEX, JRC and SZIU
VISUAL SOIL ASSESSMENT
Beata Houšková
18 – 22 July, 2005
Introduction
1. Principles and Purposes of Visual Soil Assessment (VSA)
VSA is based on evaluation of soil properties and soil indicators, mainly
morphological, physical, biological and partly chemical which are visible or
possible to distinguish without laboratory analyses. VSA can be used as support
tool in:



Soil survey;
Soil quality assessment;
Soil conditions evaluation.
Plant cover, single plants and their stand are supporting indicators- plant
indicators for soil conditions and quality evaluation.
2. Soil properties and indicators evaluated in VSA
1. Soil texture and stoniness;
2. Soil structure, shape of soil aggregates;
3. Degree of clod development;
4. Consistence;
5. Soil porosity, bioporosity;
6. Crust and cracks formation;
7. Soil color;
8. Number and color of soil mottles;
9. Soil depth and thickness of humus horizon
10. Number of earthworms;
11. Tillage pan presence, thickness and depth;
12. Presence and percentage of soil wetting, water stagnation on soil surface;
13. Presence and degree of soil erosion;
14. Soil hydrofobicity;
15. Depth of groundwater table
16. Presence of carbonates.
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How to use VSA
1. Soil Texture and Stoniness
1.1 Soil Texture
Soil texture is configuration of single inorganic particles of different size.
According to size these particles are divided into sand, silt and clay:



Sand is fraction between 50 and 2000 μmeter;
Silt is fraction between 2 and 50 μmeter;
Clay is fraction smaller than 2 μmeter.
Textural classes depend on percentual content of sand, silt and clay as shown in
textural triangle (after: CEC, 1985; in Finke et al.):
Investigation of soil texture by finger testing – VSA technique
This checking is one of the basic soil investigations because soil texture belongs
to the most important indicators, which set up soil properties. When laboratory
analyses are not set-up, or it is necessary to assess texture directly in the field
the finger testing can be used. It is provided for moist soil, which is checking, by
kneading between fingers.
3
Evaluation rules of finger testing method (after Soil texture leaflet 895):
Textural
Category
Sand
Loamy sand
Sandy loam
Sandy silt loam
Silt loam
Clay loam
Sandy clay loam
Silty clay loam
Clay
Sandy clay
Silty clay
Finger testing evaluation
Feeling of hard grains of sand, soil is rough, it is not
remaining together but it is dividing into smaller parts even
single particles. Soil does not strain the fingers.
Soil is not predominantly rough and gritty, slightly feeling of
grains, soil strains the fingers. It is difficult to roll the soil
into a ball.
Soil is not predominantly rough, no feeling of grains, it is not
difficult to roll a ball. Soil does not feel smooth and silky as
well as gritty.
Soil is not predominantly rough, slight fl of grains. Soil feels
smooth and silky between fingers, as well as gritty.
Soil mould feels smooth and silky. It forms an easily
deformed ball.
Soil mould forms strong ball, which smears but which does
not take a polish.
Soil mould forms ball which does not take polish. Soil feels
between fingers also rough and gritty.
Soil mould forms ball which does not take polish. Soil feels
between fingers also smooth and silky.
Soil mould is like plasticine, polishes and feel very sticky
when wetter.
Soil mould is like plasticine, polishes and feel very sticky
when wetter. Soil is also rough and gritty.
Soil mould is like plasticine, polishes and feel very sticky
when wetter. Soil is also smooth and buttery.
Soil texture is basic soil property, which significantly and directly influences
physical soil properties and directly/indirectly soil chemical and biological
properties.
Direct influence of texture on soil properties, e.g.:




Porosity (higher amount of clay particles brings higher capillary and total
porosity; on the contrary with sand particles increase);
Water and air regime (higher amount of clay particles brings higher
amount of water content in soil profile in the expense of air; on the
contrary with sand particles);
Humus content increases with increase of clay content;
Soil consistency and stickiness increases with clay content increase
together with increase of difficulties in soil cultivation: minute soils –
proper time for cultivation is very short because depends strongly on
actual water content, which can change rapidly. Even slight rain influences
moisture content in clay soils dramatically and it can be harmful to
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cultivate them (e.g. ploughing) because of possible compaction due to
high water content (higher than field capacity).
Representative available water content (% of volume in 10cm depth)
according to textural categories (after Soil texture leaflet 895):
TEXTURAL
CATEGORY
TOPSOIL SUBSOIL
Sand
8
5
Loamy sand
12
9
Sandy loam
17
15
Sandy silt loam
19
17
Silt loam
22
21
Clay loam
18
15
Sandy clay loam
17
15
Silty clay loam
18
15
Clay
18
15
Sandy clay
17
15
Silty clay
18
15
Peats
35
35
Representation of Soil Phases according to Texture
Optimal Soil
Water
Air
Sandy Soil
Air
Water
Organic
matter
Organic
matter
Soil
Soil
5
Clay Soil
Water
Air
Organic
matter
Soil
1.2. The stone/gravel abundance
Stones (with sharp edges) and gravel (with rounded edges) are particles with
diameter > 2 mm. Stones and/or gravel abundance (abundance classes) is % of
their volume in soil matrix. Size is represented in diameter classes.
Evaluation of stone/gravel abundance
(according to FAO-guidelines, FAO, 1990a):
Abundance class
Description
None
0%
Very few
0 - 2%
Few
2-5%
Common
5 - 15 %
Many
15 - 40 %
Abundant*
40 – 80 (90) %
Dominant*
> 80 (90) %
*80% for surface stoniness
Size class
0
F
M
C
Description
Not applicable
2 - 6 mm
6 - 20 mm
> 20 mm
2. Soil Structure, Shape of Soil Aggregates
Soil structure is configuration of single particles and soil aggregates in the space.
Soil aggregates consist from 2 or more particles bounded together by cementing
elements. Soil aggregates formation depends directly on soil texture:



Sandy soils - no, or very unstable aggregates;
Loamy soils - wet aggregates are more stable then dry aggregates
Clay soils - the best developed aggregates but with high inclination to
deformation in wet conditions.
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Evaluation of the grade, size and type of structure (defined according to
FAO-guidelines, FAO 1990a).
Grade of
structure
N
Description
W
weak
M
moderate
S
strong
structureless
Size classes
V
F
M
C
X
very fine
fine
medium
coarse
very coarse
Type of
structure
P
R
C
A
S
G
B
M
N
W
no observable aggregation or no orderly
arrangement of natural planes of weakness
(massive or single grain)
soil with poorly formed indistinct peds, that are
barely observable in place even in dry soil, breaks
up into very few intact peds, many broken peds and
much apedal material
soil with well-formed distinct peds, durable and
evident in disturbed soil which produces many
entire peds, some broken peds and little apedal
material
soil with durable peds that are clearly evident in
undisturbed (dry) soil, which breaks up mainly into
entire peds
Ranges of size of structure elements (mm)
platy
prismatic/
(sub)angular granular
columnar
blocky
<1
< 10
<5
<1
1-2
10 - 20
5 - 10
1-2
2-5
20 - 50
10 - 20
2-5
5 - 10
50 - 100
20 - 50
5 - 10
> 10
> 100
> 50
> 10
crumb
<1
1-2
2-5
Description
platy
particles arranged around a generally horizontal
plane
prismatic
prisms with rounded upper end
columnar
prisms with rounded caps
angular blocky bounded by plains intersecting at largely sharp
angles
subangular
mixed rounded and plane faces with vertices
blocky
mostly rounded
granular
spheroidical or polyhedral, relatively non-porous
crumb
spheroidical or polyhedral, porous
massive
no structure
single grain
no structure, individual grains
wedge shaped structure in horizons with slickensides
See also Appendix
7
3. Degree of Clod Development
Clods development in agricultural soils depends on many factors:



Actual moisture content during ploughing; too dry soils (actual moisture
content below field capacity) tends to significant clods development;
The shear strength of soil matrix;
Soil structure quality.
Soils with degraded structure have significant clods development, which
increases with increasing of structure degradation.
Clods development increases costs of cultivation because for preparation of
appropriate seed bed more intensive cultivation is needed when after ploughing
extra treatment and time is needed for clods decomposition to proper size for
seed bed (see Appendix).
4. Consistence
Soil consistency in characterized by sticking of soil matrix to the other bodies,
e.g. plough body and between themselves.
Stickiness is evaluated in wet soil and plasticity is evaluated in moist soil.
Evaluation of soil stickiness:

No sticky: soils does not remain on fingers;

Slightly sticky: Soil remains on fingers, but soon falls away, no resistance
feeling when move fingers away;

Sticky: Soil remains on fingers, when moving them away we feel slight
resistance;

Very sticky: Soil remains very firmly on fingers, significant resistance by
moving them away.
Evaluation of plasticity:

No plastic: it is not possible to roll the soil into a ball;

Slightly plastic: it is possible to roll bigger balls (3 – 5mm of diameter);

Plastic: it is possible to roll balls of diameter 1 – 3mm;

Very plastic: it is possible to roll balls with diameter< 1mm.
Soils with high consistency are difficult in cultivation.
5. Soil porosity, bioporosity
In VSA soil macroporosity is evaluated according to number of pores per 1 cm 2 of
soil matrix according to folloving scheme:
8
Porosity
Weak
Medium
High
Count of pores
< 20
20 – 50
> 50
Soil porosity is important for air and water movement in soil profile. In generall,
soils with developed structure havehiher porosity than stuctureless soils. Sandy
soils have the lowest total porosity among all texural categories (about 38% of
volume). On the contrary clayey soils in normal state have the highest total
porosity (more than 48% of volume).
6. Crust and cracks formation
Crust and cracks formation is evaluated when soil is dry.
Crust formation occurs when soils structure is destroyed.
Driving forces in crust formation are:



Incorrect soil moisture during cultivation (e.g. heavy soils cultivated during
high actual moisture). This leads to soil aggregates destroying.
Excess of salts in soil profile;
Fe, Al movement.
Evaluation of cracks size:



Narrow: < 2mm
Medium: 2 – 10mm
Wide: >10mm.
Consequences:
Crust and cracks development influences significantly water and air regime of the
soil. Crust on the top of soil decreases infiltration of water and increases risk of
soil erosion on sloppy area.
Cracks are reason for preferential flow which decreases efficiency of irrigation or
rain water and increases risk of possible soil contamination in deep soil profile or
ground water.
7. Soil Color
Soil color depends on many factors:
 Soil matrix composition;
 Soil texture: in general, sandy soils have the lowest humus content
because of high mineralization of organic matter and clayey soils have the
highest humus content. Exceptions in case of clayey soils are caused
mainly due to soil degradation.
 Soil water and air regime: grey color in conditions of lack of oxygen
 Quality and amount of organic matter or humus content
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For VSA soil color is good indicator of changes in humus content of soil which
can be caused by improper cultivation.
Evaluation of soil color according to humus content (Němeček et al.):
Evaluation of humus
content
Low
Slight
Medium
High
Very high
Humus
content (%)
<1
1-2
2-3
3-5
>5
Soil color (moist state)
Light brown, light grey
Brown, grey
Dark brown, dark grey
Black and brown, black and grey
Black (soils in lowlands)
Grey-brown (soils in hilly lands)
All changes in soil color from original one to lighter tone indicate loss of humus
content and improper cultivation as well as soil degradation, e.g. erosion of top
layer.
8. Number and color of soil mottles
In VSA soil mottles are evaluated according to number, size and color (see
Appendix). They are good indicators of soil aeration, which can be decreased,
e.g. by loss of soil structure, especially macropores. Oxygen demand is
represented by orange mottles, high and long time demand by grey mottles. High
amount of grey mottles is evidence of water logging in soil profile.
9. Soil depth and thickness of humus horizon
Soil depth is specified from soil surface to parent material layer.
Soil Depth (cm)
Evaluation
till 18
18 – 30
30 – 60
60 – 100
> 100
Very shallow
Shallow
Medium deep
Deep
Very deep
Evaluation of the thickness of humus horizon
Evaluation
Thickness of humus horizon (cm)
Shallow
Till 18
Medium shallow
18 - 24
Deep
Very deep
24 - 30 (50)*
>30 50*
Note: * is for accumulated soil subtypes
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10. Number of earthworms
Evaluation of the number of earthworms (according to Shepherd) per 20cm cube
of soil:
NUMBER OF
EARTHWORMS IN
ARABLE LAND
NUMBER OF
EARTHWORMS IN
PASTURES
VISUAL
SCORE
>8
>20
2
4–8
10 – 20
1
<4
< 10
0
(VS)
Explanation: VS 2: good conditions
VS 1: moderate conditions
VS 0: bad conditions
11. Tillage pan presence, thickness and depth
Tillage pan arises due to improper cultivation, mainly:



High moisture content during ploughing and passing the soil by heavy
machinery especially in case of clayey (minute) soils;
Ploughing to the same depth for many years;
Improper crop rotation: excess of root crops.
Evaluation of soil firmness (hardness): Evaluate and compare upper and
lower part of topsoil (0 or 0.5cm and 20, 25cm in dependence of depth of
ploughing). Knife test is for dry soil.
SOIL
FIRMNESS
EVALUATION
Loose
Soil mass is not sticked together
Friable
Soil mass break after slight pressure
Hard
Knife penetrate to soil by applying stronger pressure
Very hard
Soil mass break only due to high pressure, knife penetrates in
soil only to he depth of 1 – 2cm
Compacted
Knife does not penetrate into soil
Note: see also Appendix
Consequences:

Compaction is significant degradation factor;
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
Significant decrease of crops, even soil fertility;

Decrease of water infiltration into soil profile and aeration of soil;

Decrease mainly of macroposes volume;

Degradation of soil strucure;

Plough pan is cumulative process from bottom to the top;

Changes in chemical and biological properties;

Harmful for environment as a whole.
12. Presence and percentage of soil wetting, water stagnation
on soil surface
For stagnation of water on soil surface are several reasons, mainly:

Improper cultivation – compaction mainly in case of heavy (clayey)
textured soils
 Crust formation
 Differences in soil temperature on the surface and in deeper parts of soil
profile (after winter when deeper parts are still frozen).
See Appendix.
Consequences:




Difficulties in timing of cultivation (too moist soil profile);
Anaerobic condition with changes in chemical and biological properties
with posible toxic reactions on plants roots (hydrogen and ferrous sulphide
production);
Degradation of soil structure;
Crust formation.
13. Presence and degree of soil erosion
Soil erosion: water and wind is serious soil degradation factor, which influences
all soil functions. Susceptibility of soils to erosion depends on many factors:

Cultivation practises and crop rotation;

Soil texture:
 Sandy soils are susceptible to both water and wind erosion, fine sands
are more susceptible to wind than to water erosion
 Loamy soils with high amount of silt are more susceptible to wind
erosion; their agregates are more stable in wet conditions;
 Clay soils with degraded structure are susceptible more to water
erosion because in case of crust formation the roughness of soil surface is
low;
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
Slope (in case of water erosion): even in case of small slope: 3 – 7
degrees

Lenght of slope

Wind velocity
Evaluation of soil erosion/deposition (according to FAO – guidelines, FAO
1990a):
Code
N
S
R
G
T
P
Description of erosion
no visible evidence of erosion
sheet erosion
rill erosion
gully erosion
tunnel erosion
deposition by water
Code
W
L
A
D
Z
C
Description of deposition
water and wind erosion
wind wind deposition
wind erosion and deposition
shifting sand
salt deposition
karst erosion
The area affected by erosion; according to ISRIC – UNEP, 1988 (in Manual):
Code
1
2
3
4
5
Area (%)
0-5%
5 - 10 %
10 - 25 %
25 - 50 %
≥50 %
The intensity of soil erosion according to FAO – guidelines (FAO, 1990a; in
Manual):
Code
S
M
V
E
Description
slight
Some evidence of loss of surface horizons. Original
biofunctions largely intact
moderate
Clear evidence of removal or coverage of surface horizons.
Original biofunctions partly destroyed.
severe
Surface horions completely removed (with subsurface
horizons exposed) or covered up by sedimentation of material
from upslope. Original biofunctions largely destroyed.
extreme
Substantial removal of deeper subsurface horizons (badlands).
Complete destruction of original biofunctions.
VSA criteria in soil erosion evaluation:



Visual observations during season;
Size o dust plume during cultivation;
Checking possible changes in thickness of topsoil between in the different
positions on the slope field: top, middle and in footslope.
Soil erosion decreases significantly soil fertility and is harmful for all soil functions
and environment.
13
14. Soil Hydrophobicity – Water Repellency
Soil is hydrophobic when water drops do not penetrate into soil environment
directly but after some time. This time is investigated in the WDPT test (Water
Drop Penetration Time) by adding water drop onto studied soil surface.
Evaluation of the test (after Dekker):
WDPT (sec)
<5
5 – 60
60 – 600
600 – 3 600
> 3 600
Evaluation
Not hydrophobic
Slightly hydrophobic
Strongly hydrophobic
Very strongly hydrophobic
Extremely hydrophobic
Main reasons of soil hydrophobicity:

Actual soil moisture; moisture close to wilting point is already reason for
hydrophobicity

High humus content; especially films of humus acids on soil particles or
aggregates

Surface area of soil particles; soil particles with low surface area are more
susceptible to hydrophobicity, e.g. sandy soils.
Consequences:

Water infiltration into soil profile is not homogeneous;

In hydrophobic soil preferential flow occur. For this high speed of water
movement is typical, so possible movement of pollutants into deep soil
horizons or ground water table is easy, contact time with soil matrix is
short ;
Decreased efficiency of irrigation.

15. Depth of groundwater table
Depth of groundwater table influences soil water regime and aeration.
Evaluation of Ground Water Table (after Kutilek) and consequences:
Depth of Ground Water Table (m)
0 –0,10 (0,40)*
0,10 (0,40)* - 0,55
0,55 (0,9)** (1,0)*** and more
Influence on soil
Soil is fully saturated with water; not fertile
Soil and roots system are adequately
supplied with water
Groundwater has no influence on water
supply of roots system
14
Note:
* for heavy soils only
**for sandy-loamy soils only
*** for loamy, clay-loamy soils and clays
16. Presence of carbonates
For this testing we need hydrochloric acid (HCI, diluted 1:3), which we will drop
onto investigated soil matrix.
Evaluation on the carbonate test:



Soil matrix does not effervesces: carbonates are not present;
Soil matrix effervesces slightly and short: 0.3 – 3% of CaCO3 is
present, soil with low amount of carbonates;
Soil matrix effervesces strongly and long: > 3% of CaCO 3 is present,
soil with high amount of carbonates.
Consequences:


Soils without carbonates have usually week aggregates with high
susceptibility to deformation or break-up. Very often low and very low pH
(< 4) occurs with negative influence on soil fertility;
Soils with very high amount of carbonates have also high pH (above 8.5)
which can influence negatively most agricultural cops.
Cited literature
1. Čurlík, J. and Šurina, B. 1998. Príručka terénneho prieskumu a
mapovania pôd. (Handbook of soil terrain investigation and mapping). Soil
Science and Conservation Research Institute, (SSCRI). Bratislava.
Slovakia.
2. Dekker, L.W. 1998: Moisture variability resulting from water repellency in
Dutch soils. PhD Thesis. Agricultural University, Wageningen, 240pp.
3. Finke, P. et al. 2001. Manual of Procedures. Georeferenced Soil database
for Europe, Versios 1.1. EC/JRC/IES/ESB. Italy
4. Kutílek, M. 1978. Vodohospodářská
SNTL/ALFA, Praha-Bratislava, 296pp.
pedologie
(Hydropedology),
5. Ministry of Agriculture, Fisheries and Food, Soil Survey of England and
Wales. 1994. Soil texture. Leaflet 895.
6. Němeček, J. et al. 1966. Prieskum poľnohospodáskych pôd ČSSR
(Survey of agricultural soils in Czechoslovakia).
7. Shepherd, G. 2000. Visual Soil Assessment. ISBN 1-8772214-92-9. New
Zeeland.
15
APPENDIX
16
Soil Porosity Evaluation; internal porosity evaluatio
(after Hogston, 1978)
17
Tillage Pan: Presence and thickness, roots development
18
19
Presence and percentage of soil wetting
20