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1 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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1

Optimal soil structure for plant growth: field evaluations and management

guidelines for improved soil quality

Bruce C Ball,

SAC Edinburgh, Scotland

2

Talk structure

• Soil quality and soil structure

• Soil compaction

• Visual evaluation of soil structure

• Optimum soil structure

• Remediation of compacted soils

3

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

4

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

5

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

6

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

8

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

9

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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11

from G. Shepherd, 2000

Visual scoring (VS) of

production costs

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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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16

Aggregates in Sq2, Sq3

and Sq4

Sq2

Sq3 Sq4

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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)

0

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

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