Compaction and Soil Moisture
Bob Sojka
Far WestAgribusiness Association32nd AnnualFertilizer & ChemicalConferenceJackpot, NV January 10-12, 2005
Good News & Bad NewsGood News & Bad News
• The Good News:– We know quite a bit about soil moisture– We know quite a bit about soil compaction– We know quite a bit about their effects in
many crops
• The Bad News:– Alfalfa is not one of the crops we have a lot of
specific information on for compaction
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What We’ll Cover
• Basic 3-phase Soil Physical Model
• General plant responses to compaction
• Effects of compaction on soil bulk density, strength, moisture, and aeration
• Effects of soil moisture on compaction
• Some data from Alfalfa
• Some management tips
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Three Phase Soil Model
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Sojka, 1999
Three Phase Soil Model
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Sojka, 1999
HARDNESS
Soil Strength
Mechanical Impedance
Penetration Resistance
Cone Index
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Mirreh and Ketcheson, 1972
Soil Gets Harder:
When more compact
When dryer
Fundamental Soil Strength RelationshipsDerived from In-Situ Measurements
Portneuf Silt Loam
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Subsoil 12-18”
Plow Layer 6-12”
Sojka et al. 2001
Corn Root GrowthCorn Root GrowthThrough Subsoiled PortionThrough Subsoiled Portion
of Hardpanof HardpanInto Soft B HorizonInto Soft B Horizon
Norfolk loamy sandNorfolk loamy sand
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Courtesy of Bob Campbell
When roots are forced to curve, branching Is Induced.
Hard soil resists root penetration, bending the root, inducing branching, further reducing penetration
LateralMeristem
Russell and Goss, 1974
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BD 1.2BD 1.2
BD 1.4BD 1.4
BD 1.6BD 1.6
BD 1.8BD 1.8
PEA Voorhees, 1975
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COTTON
Amarillo Fine Sandy Loam
Taylor and Gardner, 1963.
Three Phase Soil ModelSOIL AERATION:
Compacting soil decreases the proportion and total amount of voids.
The gases are pushed out.
The water remains.
Water fills a larger proportion of the remaining voids and the water films become thicker.
O2 diffuses in through water only one ten-thousanth as easily as through soil air
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COTTON
Tackett & Pearson1964
Poorly Aerated Roots Close Stomata
Sojka, 1988
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Low Root O2 Cuts PhotosynthesisOosterhuise et al., 1986
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Soil Oxygen in Alfalfa as Affected by Irrigation
Meek et al., 1986
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Three Phase Soil ModelWATER RETENTION:
Compacting soil decreases the average size of voids.
Less water is held at field capacity.
Water is more subject to capillary action, so more water is available at greater suction (lower water potentials). If root growth isn’t restricted water can be available over a longer drying period… but roots have usually already been restricted
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Compaction Squeezes Water Out of Big Pores
But Holds Water Tighter in Small Pores
This changes water holding properties and soil aeration properties
Hillel, 1971
Three Phase Soil Model
WATER CONDUCTIVITY:
Compacting soil decreases the average size of voids.
Smaller pores, Less water flow at saturation.
Water is more subject to capillary action, so more water flows in the dryer range (up to a point).
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Carleton, 1971
As Compaction Increases:
Water flow through large pores decreases, but flow through small pores increases.
Saturated Conductivity Decreases
Unsaturated Conductivity Increases (to a point)
High BD
Low BD
Flocker et al. 1958 SSSAP 22:181-186
Compaction decreases the numbers of interconnected large pores, so infiltration rate decreases.
SOFTNESS
Plastic Limit:
Wet enough for
Permanent Deformation
(Lower Plastic Limit)
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SOFTNESS
Liquid Limit:
Wet enough to flow
(Upper Plastic Limit)
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Soils Compact More Easily as Water Content Increases
Sandier Soil Compacts to Higher Bulk Density than Loamy or Clay Soils
Hovanesian and Buchele, 1959
Compaction Footprint
Underground Is Wider
Than the Tire
Vanden Berg et al., 1957
Depth of Compaction Increases With Load
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Dr. Al Trouse
Traffic/Tillage PansNorfolk loamy sand
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Meek et al., 1988
Soil Compaction in Alfalfa As Affected by Amount of Traffic
2 ft1 ft
6 inch
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Determinate Southern Soybean
Subsoiled at PlantingSubsoiled at Planting
Not SubsoiledNot Subsoiled
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Mr. Bob CampbellMr. Bob Campbell
CORN
Sojka et al., 1991
Norfolk loamy sand
Pratt Fine Sandy Loam
Fertilized
Unfertilized
SUGARBEETRoot Weights
kg
Mapfumo et al., 1998
Mapfumo et al., 1998
Mapfumo et al., 1998
Anti-Compaction Tips
AVOID FIELD OPERATIONS ON WET SOIL
Conserve Soil Organic Matter
Control Wheel Traffic Patterns
Minimize Operations and Traffic
Keep Axle Loads to a Minimum
Match-up Tractors, Implements, Tires, Trailers etc.
Preserve Organic Matter and Soil Structure
Break up Compacted Soil if Needed
AVOID FIELD OPERATIONS ON WET SOILAVOID FIELD OPERATIONS ON WET SOILNWISRLNWISRLKimberly, IDKimberly, ID
Structured vs Massive Soil
• Soil without structure is more easily compacted
• Structure provides voids for gas and water exchange and potential root channels
• Soil Structure tends to be enhanced and stabilized by Soil Organic Matter and a mixture of particle sizes
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Wet Soil:
1. Weakens structure (easy to compact)
2. Lowers O2 availability to plant roots
3. Favors microbial activity if warm
4. Can promote disease if prolonged
5. Can leach nutrients / agrichemicals
6. Can cause reducing conditions (Gaseous N-loss)
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Sojka and Bjorneberg, Current ResearchSojka and Bjorneberg, Current Research
Subsoiling Plus Polymers
Mixing fines into sandy soils can worsen the soil strength of a compaction prone soil by achieving the “Minimum Void Ratio.”
It takes 100s of tons of silt and clay per acre to convert a sandy soil to a loam (surface 6”).
Only a few tons of manure per acre can change the tilth of the of a sandy soil, by promoting inter-aggregate structure, allowing the soil to perform more like a loam