Soil structure: the importance of space in soil

Brian Murphy Visiting Scientist Office of Environment and Heritage

Visiting Fellow, Australian National University

Definition of Soil Structure

• Arrangement of soil particles – Formation of pores in the soil – Formation of shaped structural units in the soil

• Crumb, lenticular, columnar, blocky

– Response to force and stress – • wetting - sodicity, tillage - friability

– Resilience and reformation of structural units and pores after the application of stress.

• Pile of bricks v house.

Functions of soil structure • Physical fertility

– Water flow – Air flow – Air porosity – Soil strength – Friability / cloddiness – response to tillage – Surface crusting – Surface ponding

• Determines the volume of soil available for water and nutrient supply to the plant (root distribution)

Structure affects volume of soil to obtain water and nutrients and provide anchorage

See Letey

Red Ferrosol on mafic rocks Wellington

Red Kandosol – wind blown parna Wagga

Good structure

Poor structure

Columnar structure – sodicity Yellow Sodosol, Gulgong Granites

Columnar structure unit, roots on outside of clod.

Where in the profile is soil structure important

100 cm

50 cm

Surface

Surface soil Infiltration, emergence

Subsurface / tilled layer

Subsurface layer – erosion and loosely tilled soil

Direct drill, untilled

Traditional tillage, soil in a loose tilled condition

Texture and soil type for managing soil structure

• Soil organic matter primary influence on soil structure – Sandy loams to fine sandy loams

• Both soil organic matter and exchangeable actions influence soil structure – Loams and clay loams

• Exchangeable cations (ESP, self-mulching) main influence on soil structure, soil organic matter minor but important effect (SOM still necessary for nutrient cycling) – Clays

Managing soil structure • Soil organic matter • Clay content and cations (ESP) • Tillage of soil –

– destroy aggregates – Importance of moisture content, tillage at appropriate

moisture content, less damage

• Surface cover to protect soil from raindrop impact and erosion – Stubble retention – Pasture growth

Managing soil structure

• Plant growth – Roots can make macropres – Provides organic material to form aggregates – Provides food for microorganisms including fungi ,

bacterial and macrofauna

• Maintain or increase soil organic matter – SOC > 1.0g/100g – getting there, but ??? – SOC > 2.0 g/100g – soil structure getting better

Soil organic matter and soil properties Clay loam

Sandy loam

Management of soil organic matter - tillage and bare ground result in structure

degradation

Management of soil structure Stubble retention, minimal soil disturbance

Managing soil structure Good plant growth, rotations

Management of soil structure- Plant growth and root systems

Surface soil sodicity Exchangeable sodium 3 to 10%

Sodicity Interaction of ESP and EC

Graph to predict potential instability to wetting in soils based on Exchangeable Sodium

Percentage and Electrical Conductivity(1:5)

50 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02 0.025 0.03 0.035 0.04 0.05

40 0.003 0.005 0.008 0.01 0.013 0.015 0.018 0.02 0.023 0.025 0.031 0.038 0.044 0.05 0.063

25 0.004 0.008 0.012 0.016 0.020 0.024 0.028 0.032 0.036 0.040 0.050 0.060 0.070 0.080 0.100

Exchangeable 22.5 0.004 0.009 0.013 0.018 0.022 0.027 0.031 0.036 0.040 0.044 0.056 0.067 0.078 0.089 0.111

20 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045 0.050 0.063 0.075 0.088 0.100 0.125

17.5 0.006 0.011 0.017 0.023 0.029 0.034 0.040 0.046 0.051 0.057 0.071 0.086 0.100 0.114 0.143

15 0.007 0.013 0.020 0.027 0.033 0.040 0.047 0.053 0.060 0.067 0.083 0.100 0.117 0.133 0.167

Sodium 12.5 0.008 0.016 0.024 0.032 0.040 0.048 0.056 0.064 0.072 0.080 0.100 0.120 0.140 0.160 0.200

Percentage (ESP) 10 0.010 0.020 0.030 0.040 0.050 0.060 0.070 0.080 0.090 0.100 0.125 0.150 0.175 0.200 0.250

% 9 0.011 0.022 0.033 0.044 0.056 0.067 0.078 0.089 0.100 0.111 0.139 0.167 0.194 0.222 0.278

8 0.013 0.025 0.038 0.050 0.063 0.075 0.088 0.100 0.113 0.125 0.156 0.188 0.219 0.250 0.313

7 0.014 0.029 0.043 0.057 0.071 0.086 0.100 0.114 0.129 0.143 0.179 0.214 0.250 0.286 0.357

6 0.017 0.033 0.050 0.067 0.083 0.100 0.117 0.133 0.150 0.167 0.208 0.250 0.292 0.333 0.417

5 0.020 0.040 0.060 0.080 0.100 0.120 0.140 0.160 0.180 0.200 0.250 0.300 0.350 0.400 0.500

4 0.025 0.050 0.075 0.100 0.125 0.150 0.175 0.200 0.225 0.250 0.313 0.375 0.438 0.500 0.625

3 0.033 0.067 0.100 0.133 0.167 0.200 0.233 0.267 0.300 0.333 0.417 0.500 0.583 0.667 0.833

2 0.050 0.100 0.150 0.200 0.250 0.300 0.350 0.400 0.450 0.500 0.625 0.750 0.875 1.000 1.250

1 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000 1.250 1.500 1.750 2.000 2.500

0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.25 1.50 1.75 2.00 2.50

Electrical Conductivity 1:5 (EC) dS/m

Sodicity

Electrochemical stability index (ESI) = EC1:5 (dS/m) / ESP(%)

Expected dispersion behaviour

Potentially very unstable ESI < or = 0.02

Instantaneous dispersion on wetting

Potentially unstable ESI > 0.02 and < 0.05

Strong dispersion after working or raindrop impact

Relatively stable ESI > 0.05 Generally stable or slight dispersion after working or raindrop impact

Salinity (EC) beginning to affect plant growth

Usually stable to dispersion but plant grow th affected by salinity (Na salts affect plant rgow th more than Ca salts)

Managing structure of sodic surface soils

• Know the soil – What are texture and exchangeable sodium levels?

• Sodicity in clay loams and clays – Maintain ground cover to protect soils from raindrop

impact – Strategic tillage may be needed for germination and

emergence – Minimise tillage – Use of gypsum - Temporary and needs further management

inputs

– Biomass input - Cropping can be useful.

Subsoil sodicity Gully erosion

Conclusions

• Soil structure is the physical fertility of the soil and a suitable soil structure is required for a wide range of critical soil functions

• Soil structure determines the volume of soil available for water and nutrient supply to the plant (root distribution)

Conclusions

• Managing soil structure requires – Knowledge of the soil type being managed

• Clay content, sodicity, stability, resilience – Maintaining or increasing soil organic matter

levels – Plant growth is critical

• Biomass input • Surface cover

– Minimise tillage – Management of sodicity if present

Managing soil structure

• Raindrop impact protection – stubble retention – pasture growth

• Maintain or increase soil organic matter – SOC > 1.0g/100g – SOC > 2.0 g/100g

• Rotations • Maintain biomass input - nutrition

Managing soil structure

• Self-mulching soils subject to water erosion

Red Chromosol – deep root growth

Yellow Sodosol – subsoil restricts root growth

Subsurface tilled layer