NWISRL

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

NWISRLNWISRLKimberly, IDKimberly, ID

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


Three Phase Soil Model


Sojka, 1999

Three Phase Soil Model


Sojka, 1999

HARDNESS

Soil StrengthMechanical ImpedancePenetration Resistance

Cone IndexNWISRLNWISRLKimberly, IDKimberly, ID

Mirreh and Ketcheson, 1972

Soil Gets Harder:

When more compact

When dryer

Fundamental Soil Strength RelationshipsDerived from In-Situ Measurements

Portneuf Silt Loam


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


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


BD 1.2BD 1.2

BD 1.4BD 1.4

BD 1.6BD 1.6BD 1.8BD 1.8

PEA Voorhees, 1975


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


COTTON

Tackett & Pearson1964

Poorly Aerated Roots Close Stomata

Sojka, 1988


Low Root O2 Cuts PhotosynthesisOosterhuise et al., 1986


Soil Oxygen in Alfalfa as Affected by Irrigation

Meek et al., 1986


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


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 ModelWATER 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).


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)NWISRLNWISRLKimberly, IDKimberly, ID

SOFTNESS

Liquid Limit:

Wet enough to flow

(Upper Plastic Limit)


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


Dr. Al Trouse

Traffic/Tillage PansNorfolk loamy sand


Meek et al., 1988

Soil Compaction in Alfalfa As Affected by Amount of Traffic

2 ft1 ft

6 inch


Determinate Southern Soybean

Subsoiled at PlantingSubsoiled at Planting

Not SubsoiledNot Subsoiled


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



Anti-Compaction TipsAVOID FIELD OPERATIONS ON WET SOIL

Conserve Soil Organic MatterControl 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


Wet Soil:1. Weakens structure (easy to compact) 2. Lowers O2 availability to plant roots 3. Favors microbial activity if warm4. Can promote disease if prolonged5. Can leach nutrients / agrichemicals6. Can cause reducing conditions

(Gaseous N-loss)NWISRLNWISRLKimberly, IDKimberly, ID

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

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