Soil Infiltration Test Instructions

(taken from The Blue Thumb Guide to Raingardens, page 12)

To determine how deep to make your raingarden, you need to figure out how quickly or slowly water will soak into the soil. A simple method for testing this includes the following steps:

1. Dig one or two holes in the proposed garden area. Make them about 8 inches in diameter and 8 inches deep.
2. Clean the loose dirt out of the hole, and then fill it to the top with water.
3. Allow the water to soak into the soil for a minimum of one to two hours to saturate the soil.
4. Fill the hole back up with water so that the water level is about one inch from the top. Mark the starting water level with a toothpick or stick. Check your watch and record the time.
5. Measure how far the water level drops with a rule or tape measure. If you have sandy soils and the water goes down quickly, you might record the water level after 15 minutes, 30 minutes, and one hour. If you have clay soils and the water goes down very slowly, you may have to record the water levels after one, two, and four hours.
6. Based on the infiltration rate you measure, calculate how many inches of water will soak into the soil over 24 hours. The number of inches that soak into the ground should ultimately be your raingarden depth. If your water level dropped ½ inch after 2 hours and 1 inch after 4 hours, your calculation would look like this:
   
   (1 inch   /   4 hours)   x   24 hours   =   6 inches/day 

*Even if the infiltration test indicated that more than 12 inches of water will soak into the ground in 24 hours, the maximum raingarden depth should be 12 inches. This will help prevent mosquitoes from breeding and allow you to choose from a large palette of plants for your garden.