Hugelkultur in a grow bag: why aged wood, not fresh
At the bottom of every Nabhi bag, beneath the soil, stand four or five pieces of old wood. You will never see them, but they work for the whole life of the bag.
As the wood slowly breaks down, it soaks up water like a sponge and releases it during a hot spell, and it feeds the soil a little at a time, for a long time. This is hugelkultur, and it is the pillar that gives a Nabhi bag its long-term stability.
What hugelkultur is
Hügelkultur is German for "mound culture": a garden bed built over buried wood. The practice is said to go back centuries in German and Eastern European gardens. The name was first published in a 1962 German gardening booklet by Herrman Andrä, who was inspired by the plants thriving on a pile of woody debris in his grandmother's garden. The Austrian farmer Sepp Holzer later developed it further, and it is now widely used in permaculture (Wikipedia).
In a field, hugelkultur means a long mound of logs covered with soil. In a grow bag, it means something much smaller: a few upright sticks at the base, with chopped dry leaves and grasses over them. The principle is the same.
Why aged wood, never fresh
The wood has to be old, and this matters more than anything else in the hugel layer.
Wood is very rich in carbon and poor in nitrogen. As fresh wood begins to rot, the microbes doing the work take nitrogen from the soil around them to fuel themselves. In the short term, that leaves less for the plant (Wikipedia; Oregon State University Extension). This is the classic first-year hugelkultur problem: hungry, yellowing plants.
In a small grow bag, fresh wood can cause a second problem. Active decomposition gives off heat, and a young plant's roots, or a seed just germinating, sit very close to it.
Wood that has already weathered for a season or more is past that hungry, hot stage. It has started to soften and break down, so it holds water readily and gives back nutrients instead of taking them.
Where our wood comes from
We do not cut a single tree for our bags.
Every monsoon, farmers around Gudibande have to guard their crops from wild animals coming out of the forest. That means staying in the fields, day and night, through rain, sun and cold. With no shelter nearby, they build small makeshift huts from local wood each year. When the season ends, the huts come down, and the old wood usually goes home as firewood.
We found a better use for it. By then, the wood has stood through a full season of weather, and termites have already begun breaking it down. It is exactly the kind of aged wood hugelkultur needs.
The trees themselves are native species that grow naturally on common land set aside for grazing cattle. Locally they are known as pachre, palavareni, tangadi, pulugu, vodda, kaanuga and vyapa. None of them is planted or cut for us; we use only what the season has already finished with.
The grass that crowned those huts becomes the mulch on top of our bags. The hut, in a sense, moves to your balcony.
What about termites?
It is a fair question. If the wood has been partly eaten by termites, could termites come to your terrace with the bag?
We make sure they do not. When we chop the hut wood into lengths of 6 to 8 inches, we drop the pieces on the ground, leave them out in the sun and shake them. The termites leave the wood at this stage, before any of it goes near a bag. What goes into your bag is the softened wood they leave behind, not the termites themselves.
How long it lasts
A log 2 to 3 inches thick holds its structure for a long time, breaking down gradually rather than all at once. It goes on holding water and feeding the soil well beyond the life of a single crop, which is why a Nabhi bag can be refreshed and replanted instead of rebuilt.
Hugelkultur is one of four pillars in a Nabhi bag, alongside the olla, hot compost and forest microorganisms, with a crown of mulch on top. Read how they work together in the Nabhi method, or choose a plant built on all of them.
Sources
- Hügelkultur, Wikipedia
- Wood chips as mulch, Oregon State University Extension