Documentation:Soil Compaction - Penetrometer Resistance

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What is Soil Compaction?

Soil compaction from tire traffic

Soil compaction can occur naturally or can be caused by management, it is caused when a force is applied to a body of soil that increases soil bulk density and therefore reduces soil porosity. This reduced the amount of macropores, and can impact the soil drainage. The level of compaction can inform the physical, biological, and chemical processes in the soil which have implications for plant growth, water movement, water storage, erosion, soil biodiversity, and other soil properties (McKenzie, 2010).

Factors Affecting Soil Compaction

There are many factors that can affect the level of compaction in the soil. Natural compaction can occur in some soils high in salts, such as areas near saline water or in Solonetzic soils. This is caused by sodium interacting with the clay to form a hard columnar structure in the B horizon. Soils with a high clay content and in climatic zones with high rainfall are also more susceptible to soil compaction (Lickacz, 1993). In general, compaction can occur due to management decisions that increase the bulk density of soil. This can include:

  • Increased animal stocking rates
  • Increased tractor traffic across a field
  • Rain drop impaction
  • Not implementing cover crops
  • Heavy tillage
  • Low organic matter

How to Measure Soil Compaction?

Soil penetrometer with pressure gauge readout

There are several different ways to measure soil compaction. Bulk density can be used a relative measure of the level of soil compaction in the soil. Another less invasive method that can be used is a soil penetrometer. A penetrometer typically consists of a rod that is able to measure the resistance of the soil to penetration. The rod is meant to mimic the action of a plant root growing in the soil. There are different types of penetrometers that can be used to estimate soil compaction or root penetrability, each with different merits (Kees, 2005):

  1. Cone Penetrometer - Utilizes a cone tip and is inserted vertically down into the soil profile. The resistance measured on the gauge or digital readout is then recorded as a measure of the compaction in the soil.
    1. Digital Display - can record the resistance at every depth along the profile. Some can monitor insertion speed and are GPS enabled. These units are typically more expensive, but allow for more precise measurement compared to pressure gauge and pocket penetrometers, as there is typically a measurement of speed and pressure, and is not invasive.
    2. Pressure Gauge Display - utilizes a pressure gauge to indicate the amount of resistance in the soil, and the depth must be measured manually. The cost of these units is typically low, it is non-invasive, but these units are subject to user error.
  2. Pocket Penetrometer - Utilizes a rod that is inserted horizontally into the soil profile. The depth that the rod penetrates the soil is recorded as a measure of the level of compaction in the soil. Typically cost is low and the unit is very portable, however it is more invasive and is subject to user error.
Meyco needle penetrometer (pocket penetrometer)

Dickey-John Soil Compaction Tester

This is a Dickey-John Soil Compaction Tester or soil penetrometer with a top view of the gauge.

The Dickey-John soil compaction tester is a cone penetrometer consisting of an interchangeable cone tip with a diameter of 3/4" (for soft soil) or 1/2" (for hard soil), a rod marked at 3" intervals, and a pressure gauge (with units in psi) attached to the handle (Dickey-John, 2017). There are two different ways to use the penetrometer to characterize the compaction in the soil. However, there are several considerations you should make regardless of the method you choose:

  • Soil Moisture - can influence how easily the penetrometer can move through the soil, it is recommended to use the penetrometer when the soil is at field capacity (approximately 24 hours after rainfall). This will reduce inconsistencies between measurement dates, and ideally provide a good representation of the level of resistance plant roots would experience in the soil (Duicker, 2003). It is also recommended to collect soil moisture data concurrently with the penetrometer data. For example, from soil samples or a time domain reflectometer inserted at different depths.
  • User Error - the rate that the penetrometer is inserted and the amount of pressure applied can influence the reading accuracy. Since this penetrometer is manually operated, it is important to practice before beginning to record your measurements.
  • Rocks or Artifacts - if a rock or other artifact (such as a piece of wood etc.) are encountered, you should discard your measurement for that location and pick a nearby spot to redo the measurement.
  • Surface Litter - plant matter at the surface (either forest litter, crop residue, etc.) should be displaced so that the probe is inserted at the surface of the soil.

Depth-Interval Method

This method provides information about the level of compaction at different intervals in the soil profile and can characterize at a profile level, where soil compaction may be an issue. This can give information about what depth the plant roots are able to grow and where water infiltration may be an issue.

  1. Depending on the level of compaction in the soil, you can switch between the two cones (1/2" for harder soil, and 3/4" for softer soil). There is a small metal rod that can be used to screw and unscrew the cone. Depending on the cone you choose, there are two different readings on the pressure gauge. The inner readings are for the 1/2" cone, and the outer readings are for the 3/4" cone.
  2. Starting at the soil surface, insert the probe applying a firm pressure and at a consistent speed. Record the pressure gauge reading at each of the depth intervals in the area you are interesting in examining (3", 6", 12", 15", 18", 21", 24").
  3. It is ok to stop during measurements to record the pressure gauge readings, but it ideal to work with one person recording and the other using the penetrometer.
  4. Depending on your sampling design, multiple readings can be taken in a location to get an average reading for that point.File:Soil Penetrometer Data Sheet.pdf

Penetration Resistance Method

This method provides information to characterize a hard pan (or plough pan) layer in the soil.

  1. Depending on the level of compaction in the soil, you can switch between the two cones (1/2" for harder soil, and 3/4" for softer soil). There is a small metal rod that can be used to screw and unscrew the cone. Depending on the cone you choose, there are two different readings on the pressure gauge. The inner readings are for the 1/2" cone, and the outer readings are for the 3/4" cone.
  2. Starting at the soil surface, insert the probe applying a firm pressure and at a consistent speed. Once the reading on the pressure gauge hits 300 psi, record the depth using the probe.
  3. Keep pushing the probe into the soil and record when the probe is through the hard pan. This will be noticeable when the pressure on the gauge starts to fall.
  4. It is ok to stop during measurements to record the pressure gauge readings, but it ideal to work with one person recording and the other using the penetrometer.
  5. Depending on your sampling design, multiple readings can be taken in a location to get an average reading for that point.

References

  1. Dickey-John. (2017). Soil Compaction Tester. Available at: http://www.dickey-john.com/_media/11001-1296%20Rev%20B%20Soil%20Compaction%20Tester.pdf
  2. Duicker, S.W. (2002). Diagnosing Soil Compaction Using a Soil Penetrometer (Soil Compaction Tester). Penn State Extension. Available at: https://extension.psu.edu/diagnosing-soil-compaction-using-a-penetrometer-soil-compaction-tester#:~:text=A%20penetrometer%20consists%20of%20a,%C2%BD)%20inches%20for%20hard%20soils.
  3. Kees, G. (2005). Hand-held electric cone penetrometers for measuring soil strength. USDA. Available at: fs.fed.us/t-d/pubs/pdfpubs/pdf05242837/pdf05242837dpi300.pdf
  4. Lickacz, J. (1993). Management of Solonetzic Soils. Government of Alberta. Available at: https://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex166/$file/518-8.pdf?OpenElement
  5. McKenzie, R.H. (2010). Agricultural Soil Compaction: Causes and Management. Government of Alberta. Available at: https://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex13331/$file/510-1.pdf?OpenElement