Testing Soil’s Ability to Drain

Goal: Determine a site’s suitability for a rain garden by testing how well the soil percolates and what type of soil is present.Objectives: Students will . . .

  • Measure and dig a cylindrical hole
  • Follow procedure to conduct an accurate percolation test
  • Record data collected in field
  • Calculate rates and averages from data collected
  • Apply calculations
  • Determine soil type by following dichotomous key
  • Observe sun availability

Materials: 1-2 digging shovels (square tip is ideal), rulers, Rain Garden Handout, 2-3 laminated copies of Soil Texture Feel Test Key, 2 gallons of water, clipboards, pencils, watch

Advance Preparation: See step 2 in the procedure: you may need to break this activity up into two days to allow for the presoaking and draining of the hole. Or you could have your first class dig and soak all of the holes so they are ready for classes later in the day.

Timing Estimate: part of 2 classes
Day 1
5 min Intro & get out to site
15 min Explain perc test, dig hole, add water, record data
10 min Soil test by feel
Check drainage before going in
Day 2
15 min Share data, calculations, apply results

After fleshing out ideas for locations of rain gardens on the school campus, your class will need to determine if those sites have soil that will drain quickly enough to be effective as rain garden sites. Decide how you want to divvy up the sites you are evaluating. If you are working with multiple classes, you may want to have each class focus on one site, while also pitching in on other classes’ data collection efforts.

Conduct a Percolation Test Procedure:

  1. Dig a circular hole 8” in diameter and 8” deep (the size matters, so measure carefully!). Set the soil aside for the next activity.
  2. Fill the hole with water and let it drain (this may take awhile and you may want to do it the day before). If it has just rained and the ground is already wet, you can skip this step.
  3. Fill the hole with water again and poke a stick or pencil into the side of the hole to mark the water level (length of stick should be resting on top of water). Record the time.
    *In terms of instructional planning, move on to the Soil Type by Feel Test Activity to be conducted after you start the perc test. The steps below explain perc test data collection and calculations.
  4. Check the hole periodically and measure how far the water has sunk each time. Ask students to record the data in the appropriate table in the Rain Garden Handout. You may need to have your classes collaborate or make other special arrangements to allow students to monitor the hole periodically until the water drains, which could take a day or more, depending on the soil.
    Sample data:3
    storm_table1
  5. Calculate the percolation rate in inches per hour using one of the following methods:
    storm-formula2

For extra math practice, you could have students use the second calculation method to determine the inches drained per hour for each time period and then calculate an average rate over the entire time period.

Applying the Results of the Percolation Test

If your soil drains all 8 inches in 24 hours, then your rain garden could be 8” deep. If it is a slower draining soil, you will need to make a shallower rain garden. If the hole drained less than 4” in 24 hours, pick different site! The ideal drainage rate for rain gardens is 0.5 to 1” per hour.

Determining Soil Type

While you have soil excavated, use the Soil Texture Feel Test Key sheet to figure out what kind of soil is under your site. Explain to students that you will be using a dichotomous key to figure out what kind of soil you have. A dichotomous key asks the user to make a series of choices based on their observations of something they are trying to identify. They are often used to identify plants or animals.

The key is self-explanatory, but make sure students have a lump less than the size of a golf ball for easier handling. They should wet it in small increments so it is moldable but not a slimy mess. Work through the flow chart, asking them the questions and asking them to respond with words or a show of hands. The most difficult part is the ribbon test and you can ask them to do this several times until they get the hang of it. Have them measure the ribbons so they get an accurate sense of which length range they are in.

Sandier soils tend to drain quickly while clay soils tend to be the slowest to drain. How do your infiltration and soil texture results compare to this generalization? Is there a difference in drainage between the different sites you evaluated? Use your results to select the best site for your rain garden or bioswale.

Observing Sunlight

While you are outside throughout the day, ask students to make observations about the sun/shade conditions at the rain garden site. The plants you ultimately choose for the garden will be limited by how much sun is available at the site. Think about how the sun sits in the sky at different times of year and whether shadows cast from buildings or trees will affect the garden. You should take notes on sunlight patterns throughout the day that you can share with your classes. From a plant perspective, here are the categories of sunlight availability:

  • Full Sun – at least 6 hours of direct sunlight
  • Part Sun or Part Shade – 3 – 6 hours of direct sunlight (part shade may not like exposure to midday sun)
  • Shade – less than 3 hours of direct sunlight

Click here to download PDF file.