Best options for filling a tall raised bed
I built some large raised beds (2.5' X 8') in my yard and I'm trying to find a way to fill them economically. I could order a delivery of planting mix but that would be $300-400. I know that I don't need 2 feet of planting mix to grow vegetables, so I was looking for ways to fill the beds partially before putting planting mix on top or amending the top layer of soil. I could get top soil delivered for about half the price and then amend it but I'm concern about it being contaminated (all suppliers list their top soil as being sourced from "building sites"). I've also considered bark chips, which I could get free from arborists but I wouldn't be able to control what type of wood I'm getting and I know that some trees could make the soil excessively acidic. What do you think my best option would be?
Multnomah County Oregon
Thanks for your question and congratulations on your decision to build raised beds.
You are right to recognize how important soil is for good productivity. But don't think of your raised bed as a box sitting on the ground needing to be filled. That ground contains soil that can be/should be incorporated into your raised bed. At a minimum, loosen the "ground" with a tiller or spade for a few inches so plant roots can penetrate it. At a maximum, improve it with organic matter and amendments, and plant directly in it.
For the best results, don't think of the soil in raised beds as layers of materials. The best soil will be fully incorporated vertically throughout the bed.
You don't say how many raised beds you have. A suggestion, especially for this first year, is to stage your planting, one raised bed at a time. Prepare the soil and plant one bed for early vegetables. Then, begin preparing the soil in the second bed for later crops and so forth. In this way, you are getting results from the garden as you are building the capability for future results.
Note: not all crops need the same soil depth. No one says your raised beds need to be filled to the same level or to the top.
All of the above suggestions are offered to help you stretch out the initial cost of "filling the box". In successive years, you can continue to improve the soil by adding your own compost and planting cover crops during the winter. Gardening is a process. Building good soil is also a process.
Oregon State University Extension has a publication that provides guidance on raised beds. Soil is discussed mainly on page 1 and 5. You can view it at this link.
That publication provides links to additional sources you may want to access.
One further thought: have you tested your "ground" soil? You can test it yourself with a kit available at most garden centers. This week-end, the Clackamas County Master Gardeners are providing free soil pH analysis at the Spring Garden Fair in Canby. Guidelines for taking a soil sample are provided at this link.
Best of luck.