The following excerpt is from Compost Science for Gardeners: Simple Methods for Nutrient-Rich Soil (New Society Publishers, January 2023) by Robert Pavlis.
Keyhole gardening is both a composting method and a gardening method. I am going to describe the more traditional design of this method so you understand the whole concept, and then I’ll describe some variations that are easier to use.
The whole thing starts by building a special keyhole garden which is a circular raised bed that contains a center composting area. One side of the circle has a pie-slice cut out of it so that you can reach the center. I guess the term “keyhole” comes from this shape, but I don’t really see a keyhole?
If you Google keyhole gardens, you will find many different designs for such a garden. The raised bed can be any height, and the wall can be made out of any material, including rocks, bricks or even wood. It does not even have to be a circle. It could be a square with one side indented to allow access. The garden can also be domed with the center higher than the walls.
The center composting area is some type of cylinder or cage that can hold material. I have seen people use wire mesh, steel drums, and plastic buckets. The requirement here is that the bottom is left open so the compost sits on soil, and the sides should have lots of holes to allow soil organisms access to the organic matter.
This design is essentially a garden built around a composting cage. New organic matter is added into the top of the cage as you collect it. It slowly moves down as older material decomposes. Nutrients leach out the side into the garden, and macroorganisms can move from the soil into the compost for a snack. The cage itself is either never emptied or only emptied every few years.
This composting process is quite simple, but making the garden is more work. There are ways to simplify the system and still retain most of its benefits. The raised bed can be only a few inches high and made with a single row of bricks. The center compost pit would need to be lowered so composting happens below grade.
Keyhole gardening will also happen even if the garden bed is not raised, or raised a few inches with no wall around it. The main point is to have a garden all the way around the composting pit so that the whole garden benefits from leached nutrients.
This is an interesting idea, but I think it has some fundamental flaws. The claim is that nutrients flow from inside the cage into the surrounding soil. Although this will happen to some extent, most nutrients move down in soil, not sideways. Plant roots live mostly in the top 6 inches of soil. That means plants won’t get most of the nutrients.
Reports that I have seen show plants doing equally well over the whole garden, but you would expect the ones near the center to do much better if they were getting nutrients from the compost, so I suspect the plants are getting their nutrients mostly from the original soil and not from the compost.
Raised beds dry out faster and therefore require more watering.
If you would like to build one of these gardens, consider building it very close to grade and don’t raise it up more than 6 inches. The compost cage in the center should be placed at grade so most of the material is above the garden, and it should have a solid bottom. These changes will reduce watering requirements and increase the nutrients that make it to plants.