A mini-digger picking up piles of dead trees, brash and leaves and dumping them in a huge hole in the ground is hardly most people's idea of permaculture - a practice more commonly associated with low tech solutions like chicken tractors, chicken greenhouses and no-dig gardening. But reading the Permaculture Association UK's latest newsletter, I came across an account of Hugelkultur - the practice of burying huge amounts of undecomposed organic matter in the subsoil underneath raised beds. While the initial energy input may be huge, the end result is said to yield healthy, fertile soil for years and years to come - with minimum need for external inputs. Read on for more info.
The original article by Dom Marsh doesn't seem to be available online, but it recounts how the Cornerstone Housing Co-op in Leeds, UK, hired a digger to build these raised beds - using huge amounts of biomass like logs, brash, and grass and leaves buried in 4'x18' trenches - and then piling subsoil and topsoil back ontop. The author is clearly all too aware of the irony of using fossil fuels and mechanized diggers to build a low impact garden - but argues that it is still a minimum input, maximum output system that will ensure "years and years of fertility with little need to top dress the soil." The system is also said to soak up moisture, minimizing the need for irrigation, and as the wood decomposes over time it not only releases heat and nutrients, but it also creates air pockets that encourage a thriving and diverse soil life.
Of course, anyone with a rudimentary knowledge of soil science will know that decomposing biomass will initially "rob" the surrounding soil of nitrogen - and a quick search online has yet to offer an answer on why this isn't a problem in Hugelkultur. It could be that the depth at which the material is buried means that nitrogen robbing is less of an issue - or that the mix of different types of biomass means that there is some immediate release of nitrogen to counteract the robbing process. Rich Soil has an interesting article on Hugelkultur, explaining how it is associated with Austrian farmer Sepp Holzer - a man who has also pioneered experiments in everything from aquaculture to perennial grain to irrigation-free farming.
I'd love to hear from anyone with practical experience in Hugelkultur - or any scientists who may be able to shed light on how and why this works (or why they believe it doesn't!). You know where the comments box is.