Raised bed on top of very sandy soil

Asked September 17, 2020, 1:25 PM EDT

Hello,

I am just a home vegetable grower, not a farmer. Can you help me decide how to fill a new raised bed? I did "mason jar" soil texture tests on the soil I have available, but not on the City of Ann Arbor compost which I will also be using.

Here are the details:
  • Bed made of 4 foot diameter by 1.5 foot high galvanized fire ring.
  • The soil beneath it is sandy loam, with .02% clay, 61% snd and 34% silt. This soil type is at least 4 feet deep in this neighborhood.
  • It has been my experience that it is difficult to keep this soil moist enough to grow much other than sand-loving natives.
  • I have access to some soil of 66.4% clay + 33.5% sand, .1% silt, but would have to haul it in 5 gallon buckets. And I am old!
  • I understand that the soil in the top foot or so of the raised bed needs to be " fluffy". I will probably use mostly Ann Arbor City compost for that.

Questions:
  • Should I fill the bottom few inches with the 64% clay/34% sand ? How deep?
  • Other suggestions for holding enough moisture in the top foot of new growing soil?
  • Any suggestions for whether to use all or nearly all compost as the growing medium?
  • Any thoughts about putting lots of sticks, logs, etc below the compost?
Thank you for any information you can give me.

Janet Osborn


Washtenaw County Michigan

1 Response

Hello Janet,

Thank you for using the Ask an Expert service!

I will answer your questions in bold below.

Questions:

  • Should I fill the bottom few inches with the 64% clay/34% sand ? How deep? No. Do not add the clayey soil to the bottom. You still want good drainage and don't want to trap the water at the bottom of the "container" around and in that clayey layer. Actually, having sandy soil at the bottom should already slow the movement of the water, because water does not move as well from more finely textured soil to more coarsely textured soil. See The Myth of Drainage Material in Container Plantings from Washington State U.
  • Other suggestions for holding enough moisture in the top foot of new growing soil? Besides adding the compost, water shallowly, but more frequently (less water applied per watering). Consider installing drip irrigation, if possible. Also fertilize more frequently, with smaller amounts of fertilizer each time. Consider adding a layer of mulch, such as dried leaf mulch, on top of the growing media to reduce evaporation. Watering the vegetable garden from U. of Minnesota Extension; Mulches for the Home Vegetable Garden from Virginia Cooperative Extension
  • Any suggestions for whether to use all or nearly all compost as the growing medium? Try using half compost, mixed well with the sandy, native soil. Please see my colleague's article on Starting a raised bed garden (from MSU Extension). Or even 1/3 compost and 2/3 native soil. Also see, Soil to Fill Raised Beds from the U. of Maryland Extension.
  • Any thoughts about putting lots of sticks, logs, etc below the compost? Do NOT do it. Linda Chalker-Scott from Washington State U. wrote an article, which basically suggests to me NOT to use Hugelkultur (where sticks, logs and branches are placed at the bottom of the mound). See “Hugelkultur: What is it, and should it be used in home gardens?
I hope that helps a bit! Plus, consider getting a soil test of your final soil mix. homesoiltest.msu.edu

Regards, Irene