Soil Compaction in Alfalfa Fields
Ronald T. Schuler, Professor
Extension Agricultural Engineer
Biological Systems Engineering
University of WisconsinMadison
Soil Compaction
• Causes include combinations of:
Wheel Traffic
Tillage
Precipitation
• Types
Surface
Subsoil
Loam soil by volume
Non-compacted
Compacted
Soil
Water
Air
Compaction Issues
• Tires
 Size
 Inflation pressure
 Tire design, radial vs. bias
• Machinery




Avoid wet soil
Wheel alignment
Machine selection
Machine weight
Compaction Issues
• Tires
 Size
 Inflation pressure
 Tire design, radial vs. bias
• Machinery




Avoid wet soil
Wheel alignment
Machine selection
Machine weight
Contact Area-Size-Diameter
(Firestone)
Tire Size
Radial Tires
18.4R34
18.4R38
18.4R42
18.4R46
Bias Tires
18.4-34
18.4-38
65.1
69.0
73.0
77.0
Area(in2)
Flat
3”Sinkage
320
720
340
740
350
760
365
780
65.4
69.3
284
300
Tire Dia.
653
675
Contact Area-Size-Width
(Firestone)
Tire Size
Radial Tires
16.9R38
18.4R38
20.8R38
Bias Tires
16.9-38
18.4-38
16.9
18.4
20.8
Area(in2)
Flat
3”Sinkage
255
660
340
740
390
870
16.9
18.4
252
300
Tire Width
629
675
Compaction Issues
• Tires
 Size
 Inflation pressure
 Tire design, radial vs. bias
• Machinery




Avoid wet soil
Wheel alignment
Machine selection
Machine weight
Impact of Tire Pressure
18 psi
6 psi
Inflation pressure-slight tire bulge with
the proper pressure on radial ply tires
6 psi
18 psi
Bulge
Top View: Measured soil-tire interface
stress (psi) distribution of an 18.4R38 tire
Compaction Issues
• Tires
 Size
 Inflation pressure
 Tire design, radial vs. bias ply
• Machinery




Avoid wet soil
Wheel alignment
Machine selection
Machine weight
Radial vs. Bias Ply Tires
Contact
Area
At Proper Inflation Pressure, Radials
have greater Contact Area
Tractor Manufacturer Recommendations
Tire Size – 18.4-34 Pressures-psi
Axle Load (lbs)
5500
6000
6500
7000
7500
8000
8500
9000
9500
10000
Radial
6
7
7
8
9
10
12
13
14
15
Bias
16
16
16
16
16
16
16
16
16
16
Compaction Issues
• Tires
 Size
 Inflation pressure
 Tire design, radial vs. bias
• Machinery




Avoid wet soil
Wheel alignment
Machine selection
Machine weight
Impact of soil moisture on compaction
(depth and sinkage)
0
4
Depth,
In.
8
12
16
20
24
28
Hard dry soil
Normal
Wet soil
Compaction Issues
• Tires
 Size
 Inflation pressure
 Tire design, radial vs.
bias
• Machinery




Avoid wet soil
Wheel alignment
Machine selection
Machine weight
Wheel alignment-harvester and wagon
Wheels should follow in same track
Wheel alignmenttractor and wagon
Small Square Baler-Bottom Feed
Wheels aligned-tractor, baler and wagon
Mid-Size Rectangular Baler
Wheels aligned-tractor and baler
Compaction Issues
• Tires
 Size
 Inflation pressure
 Tire design, radial vs.
bias
• Machinery




Avoid wet soil
Wheel alignment
Machine selection
Machine weight
Sickle Cutterbar-Roll
Conditioner
14-foot width
Self-propelled mower-conditioner
Front self unloading wagon
Wheel Alignment –harvester and wagon
Dual rake merging two
windrows
Merger - Picks up one windrow
and places it on the adjacent one
Merging three windrows into one
System Comparisons-three cuttings per year
Type1
Width2 (ft)
Rake3
Pull
13
None
Pull
16
None
Pull
13
Rake
Pull
16
Rake
Self-Prop
16
Rake
Self-Prop
16
Merge
Pull
13
Merge
Self-Prop
13
Merge(Dbl)
Self-Prop
16
Merge(Dbl)
Area4
4.0
3.4
3.2
2.7
3.1
3.0
3.4
2.8
2.3
1 Mower-conditioner-pull or self-propelled
2 Mower conditioner width, feet
3 Rake-to rake, merge or do not rake two windrows together
4 Area covered by wheel traffic if uniformly distributed
5 T-M/A – ton miles per acre
T-M/A5
50
44
35
33
32
32
37
27
32
Compaction Issues
• Tires
 Size
 Inflation pressure
 Tire design, radial vs. bias
• Machinery




Avoid wet soil
Wheel alignment
Machine selection
Machine weight
Tractor ballasting criteria
Towed Load wt. vs.
Towing machine wt.
1 to 1
Max.Speed
mph
20
2 to 1
10
More than 2 to 1
Do not tow
Concluding Remarks
 Select appropriate tire
 Use proper tire pressure
 Align wheels
 Reduce traffic of large equipment
 Use safe towing machine weight
The End