Yesterday I gave a class at the Theodore Payne Foundation on Easy Care for a Native Plant Garden. (As promised I am posting the powerpoint in the presentation section of my website.) Soil drainage was a topic covered in the class. In preparation I performed a standard soil drainage test in my yard. I dug several holes, approximately 10 inches deep and 8 inches wide, in several locations. I filled the holes with water, allowed them to drain, and repeated measuring the water level at the start and then in fifteen minutes intervals. My soil drained very quickly – approximately 2 inches in 15 minutes.
Originally I gave numbers to define, slow, medium, and fast-draining soil in the presentation. I had checked several websites and books for the numbers and came up with the following.
- less than 6/hr = slow
- 1 – 6 in/hr = medium
- greater than 6 in/hr = very fast
Although I had never thought of my soil as draining excessively fast, at 8 inches/hr. it was considered by most as such. It does drain well, but it is quite loamy and stays wet for a long time.
I decided to check more websites for numbers. They come out all over the place.
Guidelines for Soil Drainage Test from the Web
Slow – Less than ½ in./hr
Medium – 1 to 4 in./hr
Fast – Greater than 4 in./hr
Poorly drained – Less than ½ in./hr
Moderately well drained – ½ to 1 in./hr
Well drained – Greater than 4 in./hr
Too slow – Less than 1 in./hr
Well-draining garden soil– 1 to 2 in./hr
Too fast – Greater than 4-8 in./hr
Poor – Less than 4 in./hr
Moderate – 4 to 8 in./hr
Excessive – Greater than 8 in./hr
Poor – Less than 1 in./hr (over 8 hours for hole to drain – poor soil drainage)
Well-drained – 1 in./hr
Excessive – Greater than 1 in./hr
Poor – 3 in. or less/day (24 hrs.)
Adequate drainage– 6 in./day
Very good drainage – 12 in./day
It is true that most called for holes that are 12 inches deep, and one, the last one, called for a 2 ft. deep hole. Still the numbers are quite scattered.
Never being a stickler for exact numbers when it comes to gardening I modified the guidelines on my presentation to be the following:
Dig a hole, fill it once, fill it again and measure level change in fifteen minute increments.
- takes all day to drain = slow
- less than a day, more than minutes= medium
- drains in minutes = very fast
I’m happy with this, but thought I’d throw it out to all of you very knowledgeable gardeners.
How do you define slow, medium, and fast soil drainage?