It is not a job for a shovel, to replace the first 5-6 feet of compacted soil and rocks with rotting wood and manure.
My husband, Bob, has a front end loader with a backhoe attachment. We've been tackling the garden area in sections, digging one pit at a time. On average, the pits have been about five feet deep, and around 12x12 feet across. Some pits were 6 feet deep, and a couple were shallower; they were dug side by side, essentially joining on the edges.
The process has been to remove the soil and rocks from a section, piling it off to the side. Bob removes the large boulders, and I hand-sort the cobble and small boulders from the soil, setting interesting rocks aside for future above-ground projects. Once the majority of the rocks are removed, the soil is available to put back in the pit, layered in among large pieces of rotting wood, manure, leaves, and whatever other organic matter I can get my hands on.
Big thanks to Bob for all the digging, and for teaching our daughter to operate the backhoe in the process too!
The above-ground hugelkultur piles built during 2012 were a great source of woody organic matter for the new beds. One by one, they were pulled into the pits with the backhoe. The above-ground piles also had pieces of biochar in them -- partially burned wood that has a strong ability to hold water, helps reduce leaching of nutrients, and fosters the uptake of nutrients by plants.
The first few pits were filled with aspen, thanks to our friend, Lee Johnson, and our neighbor, Betty. I had read that aspen is a great species of wood to use for hugelkultur, so I sought it out. Aspen groves have a propensity to thrive and die in cycles, and around here, it is easy to find landowners who would like it out of the way after it dies. I was able to make real progress on this project because Lee decided to help by bringing wood by the truck- and trailer-load -- one of the most amazing gifts ever! Thank you, Lee!
The Willy's truck is loaded with manure from our neighbors, Chuck and Kathy, who own alpacas. The manure from their alpacas was amazingly rich and supported a lot of robust veggies the following growing season! This pit started off with long sections of aspen, which were arranged in rows, with alpaca manure...
...and leaves! I was able to use a lot of leaves thanks to the generosity of local people like Lenore and Paul Bouchard, Brianne Rowe Smith, and others.
Here is the same shot facing East instead of West.
The second layer was made up of shorter lengths of aspen, being nudged into place here with the backhoe. Soil was also spread over each layer.
Into the pit goes one of my best producing above-ground hugelkultur beds. It was quite small and would not have produced well for as long as this new, larger bed will.
Dark, rich, rotten wood, going in... The advantage to using existing hugelkultur beds in these pits is that they had already started breaking down, and their functionality was ready to peak.
Spreading soil over the wood, manure and leaves ~ looking West, November 2012
Looking South across the first two pits, September 2013
Harvesting fava beans from the first underground wood-for-food bed (Sept. 2013)
Big, beautiful onions from the aspen & alpaca manure pits (October 2013)
Don't have a backhoe? No problem!
It is not necessary to dig deep pits in order to reap the benefits of rotting wood for growing food. Our design came into place because of the fact that we have a backhoe. If you are working with a shovel, you can dig a trench only as deep as you feel like digging, and build your hugelkultur mainly above ground -- or don't dig at all and grow everything on the surface.
There are pros and cons to every approach, but you can be successful regardless of what angle you take. I am excited about the pit system we are implementing, because it allows me to experiment with rainwater collection with the surface contours, and because of the mass that can be developed for water retention and for generating heat to extend the growing season.
Benefits to above ground beds include quicker solar heating in the spring, and the ability to take advantage of solar exposure (South vs. North facing sides). Also, you can create microclimates depending on the shape and arrangement of the beds.
The sky is the limit! Eventually, I hope to have a combination of below and above ground wood-for-food beds in our garden.