Soil Drainage

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.

DRAINAGE GUIDELINES

• 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
http://www.ehow.com/how_4543171_test-soil-drainage.html
Slow – Less than ½ in./hr
Medium – 1 to 4 in./hr
Fast – Greater than 4 in./hr

http://www.onlinegardener.com/care/Soil%20drainage.pdf
Poorly drained – Less than ½ in./hr
Moderately well drained – ½ to 1 in./hr
Well drained – Greater than 4 in./hr

https://www.learn2grow.com/gardeningguides/soilmulchcompost/application/PercolatingSoil.aspx
Too slow – Less than 1 in./hr
Well-draining garden soil– 1 to 2 in./hr
Too fast – Greater than 4-8 in./hr

http://www.hort.cornell.edu/gardening/landscape/assessment_chapters/step_7.pdf
Poor – Less than 4 in./hr
Moderate – 4 to 8 in./hr
Excessive – Greater than 8 in./hr

http://www.blogdivvy.com/growing-vegetables/soil-drainage-test.htm
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

http://www.managemyhome.com/mmh/authored/articles/How+to+Evaluate+Soil+Drainage
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:

DRAINAGE GUIDELINES

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?