How to Test Soil Percolation

A soil percolation test determines soil's capacity for holding and draining water. The percolation test is chiefly utilized for finding proper septic tank and septic drain line locations. Another purpose of the test is to evaluate growing sites for plants that require well-drained soil. For accurate results, the air temperature must be greater than 40 degrees F. Various agencies throughout the country may have specific details pertaining to your area and soil types. Contact your local agricultural extension service or county health agency for specific percolation test details.

Posthole digger
Knife
Gravel
Tape measure
Bucket
Hammer
Nail
Pencil
Paper

Instructions

- 1
  Dig at least six holes, 6 to 10 inches in diameter, across the test site with the posthole diggers. The depth of the hole must equal the depth of the system. In other words, if the septic system leach-lines are buried 3 feet deep, that must be the overall depth of the test hole.
- 2
  Scratch the sides of the hole with the knife to remove all smooth surfaces or smearing of the soil in the hole. Remove all loose soil from the bottom of the test hole.
- 3
  Fill the hole with 2 inches of clean, coarse-sized gravel (between 1/4 inch to 3/4 inch).
- 4
  Pre-soak the hole by adding 12 inches of water into the test hole over the gravel. Allow the water to stand overnight.
- 5
  Hammer a nail 6 inches above the bed of gravel the following day. This is the 6-inch reference point.
- 6
  Fill the hole with water to the 6-inch reference nail, prior to testing. Let the water stand for 30 minutes.
- 7
  Adjust the water level to the 6-inch nail reference point. Let it stand for 30 minutes.
- 8
  Measure the distance from the top of the water column to the 6-inch nail reference point. Record that measurement.
- 9
  Fill the hole again with water to the reference point. Wait 30 minutes and then measure the distance. Repeat the process until you have eight separate measurement readings for each percolation test hole. The final measurements should all be within 10 percent or 1/4 inch of each other.
- 10
  Calculate the percolation rate for each hole. The percolation rate is expressed in minutes per inch of water drop. As an example, if the water dropped 3 inches in the 30-minute time period, the percolation rate is 30 minutes divided by 3 inches. The percolation rate is equal to 10 minutes per inch (MPI). Perform this calculation for all recorded measurements and for all test holes.
- 11
  Average the percolation rates for each test hole. Combine and average the individual test hole rates together. This is the final percolation rate for the test site.
- 12
  Contact your local health department for your area’s soil types and proper percolation rates. All percolation rates vary based on soil type and the size of septic system required.

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