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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

  • 7

    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

  • 10

  • 11

    from G. Shepherd, 2000

    Visual scoring (VS) of

    production costs

  • 12

    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

  • 13

    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

  • 14

    Properties used in the assessment

    Ease of break up of the soil

    Size and appearance of aggregates

    Porosity

    Root appearance and location

  • 15

  • 16

    Aggregates in Sq2, Sq3

    and Sq4

    Sq2

    Sq3 Sq4

  • 17

    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

  • 18

    Visual evaluation can detect layers

    of contrasting structure

    Can guide further

    diagnostic soil

    measurements

    Can indicate suitability

    for minimum tillage or

    need for subsoiling

  • 19

    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)

  • 20

    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

  • 21

    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

  • 22

    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

  • 23

    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

  • 24

    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

  • 25

    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

  • 26

    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

  • 27

    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

  • 28

    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

  • 29

    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

  • 30

    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

  • 31

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