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Optimal soil structure for plant growth: field evaluations and management
guidelines for improved soil quality
Bruce C Ball,
SAC Edinburgh, Scotland
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Talk structure
• Soil quality and soil structure
• Soil compaction
• Visual evaluation of soil structure
• Optimum soil structure
• Remediation of compacted soils
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Soil quality
• Soil quality involves the ability of the soil to maintain an
appropriate productivity, while simultaneously reducing the
effect on the environment and contributing to human health
Schjonning et al. 2004
• Most important quality: soil structure (?)
• Main agronomic threats to soil structure are compaction,
loss of organic matter and waterlogging
• Agronomic control of threats is by soil management
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Soil and water management challenges:
•Increased food production
•Soil protection
•Lower input production
•Alternative fuels
•Climate change
•Flood and pollution control
•Decreasing water resources
•Decreasing labour resources,
especially soils specialists
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Soil Structure
Structure is the arrangement of
particles and pores that allows:
• roots to anchor the plant
• water to drain through
pores and cracks
• water retention
• air to roots for favourable
gas exchange
• mineralisation of nutrients
and release to crop roots
• biodiversity of microbes
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Soil compaction
• Compaction: increased soil bulk density (compactness) and decreased porosity due to application of loads over short times
• Occurs under loading by wheels or animal hooves where soil is wet, loose, weakly structured
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Excessive compaction and crop
growth
Reduced porosity: reduces drainage and increases
the chances of waterlogging and losses of N as
nitrous oxide, reduces water storage
Increased strength: restricts root growth and uptake
of nutrients
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Soil structure: the importance of macropores
• Macropores and cracks allow water infiltration and drainage
• Macropores keep the soil aerated reducing nitrous oxide loss by denitrification
• Macropores increase water uptake and crop yield
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Visual evaluation of soil structure
• Soil structure affects root penetration and water, oxygen and nutrient availability for the crop
• Good, uniform soil structure helps ensure sustainable crop growth with minimum environmental problems
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Visual soil structural quality assessmentSpade test – quick and cheap and gives a measure
of field variability
1. Extract a spadeful of soil
2. Break up the spadeful
3. Assign a score between 1 (good) and 5 (poor) – compare with pictures in a key
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Visual soil structure quality
analysis: equipment required
• Spade – flat and square ended preferred
• Knife to cut vegetation
• Tray to contain the soil
• VSSQA test card
• Optional extras:
camera
stool or table to raise the soil blocks off the ground
scoring sheet and scoring spreadsheet
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Properties used in the assessment
• Ease of break up of the soil
• Size and appearance of aggregates
• Porosity
• Root appearance and location
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Soil structures at SCRI tillage
experiment
Sq 2
Normal
ploughing
Sq 2
No-till
Sq 3-4
Normal
Ploughing +
compaction
Sq 3-4
Minimum
tillage
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Visual evaluation can detect layers
of contrasting structure
• Can guide further
diagnostic soil
measurements
• Can indicate suitability
for minimum tillage or
need for subsoiling
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Soil structure, soil strength and wheat yields(Danish data)
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1
2
3
4
5
6
7
8
Normal
plough
Min
tillage
No-
tillage
Sq soilstructure
Penetrationresistance(Mpa)
Wheat yield(t/ha)
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Peerlkamp structure vs grain yields
1.0
0.8
0.6
0.4
0.2
0.03 5 6 7 84
D 141
E 417 D 6
D 87
D 177
E 219
D 42
D 132E 418
E 220
D 51
Rela
tive y
ield
yrel = - 0.79 + 0.225 Peerlkamp
r2 = 0.63*, SE = 0.25
M1 (Peerlkamp note of topsoil 0-25 cm)
D site and plot numberWinter wheat and corn dominated rotationMean 2002-2006
E site and plot numberPermanent corn, Mean 2002-2005
Yield increased
300-350 kg/ha per
unit of original
Peerlkamp score
Mueller et al., 2009
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Visual structure and crop yield
• Soil properties most closely associated with
visual soil structure and with grain yield were
soil density (compactness)and
macroporosity BUT correlations were site
specific
• Aggregate characteristics are more reliable
indicators of soil structure than biological
characteristics
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Optimum soil structure for crop growth is
related to soil compactness and wetness
From: Hakansson, 2005 Compaction of arable soils
Soils can be too loose
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0.1
0.15
0.2
0.25
0.3
0.35
0.4
19-Mar 08-Apr 28-Apr 18-May 07-Jun 27-Jun 17-Jul 06-Aug
Date
Vo
lum
etr
ic w
ate
r c
on
ten
t
Soil structure influences the soil water contents for
best crop growth – the Least Limiting Water Range
• root growth optimal between the horizontal lines
• width between lines can be altered by management
mechanical impedance hypoxia
Image: B. McKenzie
2008
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Optimum soil structure?
• Crumb and rounded, porous aggregates, weak enough to allow roots to
grow and adsorb water and nutrients, strong enough to resist
compaction (Sq 1-3)
• Surface important. Seedbed demands fine aggregates firmed together
BUT some larger aggregates are needed to prevent surface collapse
during heavy rainfall
• Local compact layers at the base of the topsoil can protect the subsoil
from compaction but need enough cracks and pores to allow water and
root movement through them
• Structural requirements vary with crop and location
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Remediation of soil structure
• Scores 1-3 are satisfactory
• Scores 3-5 need changes in tillage or
cropping to sustain productivity e.g. loosen
a compact zone
• Aim to reduce structural variability to
increase crop consistency
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Importance of roots
• A well-aggregated soil increases root proliferation and structural stability
• Perennial crops may penetrate compact layers, but main effect of rooting is to dry the soil
Image: B McKenzie
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Compaction remediation
• Surface layer compaction: need to re-open the macropores between structural units. Soil aggregates should be displaced enough not to return to their original position after subsequent traffic
• Subsurface compacted layers (pans): these can protect the subsoil from surface loads. Make fissures through the layer with minimal break up and soil re-arrangement. This keeps the support capacity of the compacted layer while creating pathways for drainage and root movement through to the layer below
Images: I Dickson, B McKenzie
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Compaction remediation
Severe wheel rutting after harvest: make fissures across the ruts (e.g.
with tines to 30-35 cm depth) to allow water to drain into the
adjacent uncompacted soil
Image: I Dickson
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Conclusions
• Compaction status is important for crop growth
• Visual soil evaluation can help identify:
1) the ‘right’ structure for the crop and how to achieve it – reduce variability in crop growth
2) if minimum tillage is possible and if subsoiling is required
3) Need for further diagnostic soil measurements (e.g. LLWR) and their depths
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Acknowledgements:
• Tom Batey, University of
Aberdeen
• Lars Munkholm, Arhus
University, Denmark
• Mandy Liesch, University of
River Falls, Wisconsin
• Paul Hallett and Blair
McKenzie, Scottish Crop
Research Institute, Dundee