Hugelbed and Biochar bed, Part 1

I've been working on a Hugelbed, otherwise known as a woody bed - the basic idea is to put a layer of wood underneath the soil which leads to a carbon rich, aerated, microbial-rich, and hydrophilic garden bed. Traditionally the beds were built with fallen woods that were too soft or punky for other uses. The beds are given a woody core with soil piled on top - with the bed traditionally reaching as much as 6-8 feet high at its center.

These beds have recently become more popular in the United States - with a common variation of first excavating out some soil so that part of the wood is below grade and the final bed only a couple of feet high. The bed I've created is a mixture of the two - I excavated roughly 1-2.5 feet below grade but am building the bed up relatively high so that its center will be 3-5 feet above grade when complete.

This shows the site after excavation but prior to starting construction. Some of the materials (the excavated dirt, wood pile and a good start on the wood chips) are stacked on the upslope

This hugelbed is constructed on contour, meaning the West lip of the bed is a good 12 inches lower than the East lip. The bed is 8-10' wide and about 65 feet long. This hugelbed has the following layers - first, the area below grade is filled with wood chips to the lip of the lower (West) edge. Second, the wood core is stacked on top of the wood chips. Third, a layer of sod is piled on top of the wood. Fourth a layer of old cow manure, old hay and straw, and more wood chips are spread over the sod. Fifth, a layer of hot (fresh) cow manure. And lastly, the soil that was originally excavated from the bed is piled on top.

Here you see the excavated site on the left side oft he picture with the materials piled up on the right side of the picture

Here you see my faithful golden retriever eager to help - for scale, the lip of the West side of the trench is roughly 30 inches high.

Some of the features of the bed are: First, the wood-chips and decaying wood will act as a sponge for water eliminating the need to irrigate the bed even in a dry summer. Since the bed is on a decent slope and is constructed on contour it should also capture some of the water that would otherwise run off the pasture. Second, as the wood decomposes, it will naturally aerate the soil as this wood is less than 1/2 the density of soil - this eliminates the need to till the soil. Third, the wood is a natural host of Mycorrhizal activity. From Wikipedia: Mycorrhizas form a mutualistic relationship with the roots of most plant species. While only a small proportion of all species has been examined, 95% of those plant families are predominantly mycorrhizal.^[3] They are named after their presence in the plant's rhizosphere (root system). More on mycorrhizal benefits below.

Before woodchips...

And after woodchips - we got about 70 yards (2000 cubic feet) of wood mulch from nearby municipality. They delivered it to my driveway for $200. This is a truly monstrous pile of woodchips - we used about 800-1000 cubic feet of them to fill in the base of the bed.

Sugar-water/mineral exchange 

This mutualistic association provides the fungus with relatively constant and direct access to carbohydrates, such as glucose and sucrose. The carbohydrates are translocated from their source (usually leaves) to root tissue and on to the plant's fungal partners. In return, the plant gains the benefits of the mycelium's higher absorptive capacity for water and mineral nutrients due to the comparatively large surface area of mycelium: root ratio, thus improving the plant's mineral absorption capabilities.

Plant roots alone may be incapable of taking up phosphate ions that are demineralized in soils with a basic pH. The mycelium of the mycorrhizal fungus can, however, access these phosphorus sources, and make them available to the plants they colonize. Nature, according to C.Michael Hogan, has adapted to this critical role of phosphate, by allowing many plants to recycle phosphate, without using soil as an intermediary. For example, in some dystrophic forests large amounts of phosphate are taken up by mycorrhizal hyphae acting directly on leaf litter, bypassing the need for soil uptake. Inga alley cropping, proposed as an alternative to slash and burn rainforest destruction, relies upon Mycorrhiza within the Inga Tree root system to prevent the rain from washing phosphorus out of the soil. In some cases, the transport of water, carbon, and nutrients could be done directly from plant to plant trough mycorrhizal networks that are underground hyphal networks created by mycorrhizal fungi that connect individual plants together.

Suillus tomentosus, a fungus, produces specialized structures, known as tuberculate ectomycorrhizae, with its plant host lodgepole pine (Pinus contorta var. latifolia). These structures have in turn been shown to host nitrogen fixing bacteria which contribute a significant amount of nitrogen and allow the pines to colonize nutrient-poor sites.

View of the Hugelbed from the north, after wood-chips. At the far end of the bed we have stacked wood on top of pallets to start a bonfire - we are going to try  to turn the south-most 12 feet of the bed into "biochar."

View of the south end of the bed, with the wood stacked and ready to burn.

View from the South end of the bed looking North.

View of the Hugelbed from the West (downslope) after the wooden core is stacked in the center of the bed.

Now for the fun...

...particularly if you are a pyromaniac

After starting to smother the biochar. We made a crucial mistake of starting to smother it with wood-chips and THEN dirt. That, combined with the fire laying on another pile of wood-chips led to a rather significant problem...

On day 5 the fire is still smoldering. As the burning wood collapses, it creates large air holes, making it very difficult to keep the fire smothered.

By day 7, it is clear the smoldering fire will consume my entire bed - it has already spread an additional 5 feet north of my original plan. I am forced to dig out all of the woodchips and fill it with dirt to create a fire break.

Digging down 2.5 feet next to a smoldering fire is smelly and uncomfortable work. I'm glad when I'm done.

Fire seems to finally be out on day 16. We have now also covered the entire core of the bed with  sod, hay, and straw. View from the South looking North. Note the firebreak about 15-20 feet down the bed.

Now, the bed has been covered by a good layer of old cow manure with lots of hay and straw mixed in which we got from our neighbor.

The next stage will be to add a thick layer (8-12'') of fresh cow manure (from last winter) gathered from our neighbors barn. I will also begin to plant fruit trees into the north side of the bed.

I'm hoping that the Nitrogen from the cow manure will soak into the wood core and wood-chips so that we get even more Nitrogen stored up for future years. I don't have much hope for plantings this summer - but we'll try some vine crops, peas, and otherwise will just do a cover crop like Hairy Vetch.