Soil structure: the importance of space in · PDF fileDefinition of Soil Structure •...
Transcript of Soil structure: the importance of space in · PDF fileDefinition of Soil Structure •...
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