The Permaculture Principle Observe and Interact gives me a great excuse to bring out the ‘mad proffessor that seems to direct much of what I do. I love to read about a new technique, or in the case of this post an old technique, and then ‘play with it’. Yesterday I did just that with an experiment in Soil Fertility and burning.
Biochar, Burning Wood and Terra Preta
I have done a lot of reading on Traditional Agricultural practises. One technique that is or has been used all over the world is the burning of wood prior to planting crops. This is normally associated with swidden (slash and burn) agriculture. We have an image of indigenous people destroying forest to grow crops, depleting the soil of nutrients, then moving on. The reality is that this is a sophisticated way of farming that uses a period of building soil fertility under trees, and then using that fertiltiy to grow crops. In many ways it resembles the old English Two field rotation. One year crop, one year fallow. In fact it isn’t that long since the burning of stubble in grain fields was made illegal here. What I’ve also read is that in India the wood was slow burned, or charred, rather than burnt fiercely. This throws up some interesting possibilities to observe and interact .
The discovery of rich, black, man made soils in the Amazon, has led to many suggestions of how they could have been created. The black colour is made up of charcoal. Some people suggest that biochar was made separately, but I have also read a book that suggested that a slow charring of wood in the fields may have been responsible.
Wood Ash and Soil Fertility
I have written quite a bit about wood ash as a soil amendment. The ash is about 50% lime, and it also contains all of the minerals that were in the wood, less Nitrogen and Sulphur. So a burning of wood gathered away from the growing area would help to build up minerals and replace lime, which is continually being leached from the soil. Combining the minerals and lime from wood ash, and adding charred wood to help build soil structure seems like a very sophisticated way of improving soil fertility, that uses nothing more than labour, simple hand tools , and fire. Time to observe and interact.
Observe and Interact
Reading about this stuff is fine but we need to try it out to see how it works, and how it might be used to help creating a more sustainable way of growing food. That to me is the heart of Observe and Interact. The tinkering and experimentation with ideas.
I am hoping to put up a polytunnel later this year, and have had the area where it will be sited covered with carpet to suppress weeds. Over the last week or so I have been cutting back a small section of my hedges. The wood is used for fuel in the house, but contains a lot of small wood, of which we have a plenty. The wood has a high proportion of thorn trees, mainly hawthorn, which are difficult to shred for mulch, so I thought that it would be interesting to use some of this wood to char over the site of my polytunnel.
What I did was to process my firewood in the normal way, cutting the small stuff to burn now, the intermediate stuff to use for our pizza oven, or bonfires in the summer, and leaving the thicker lengths to burn indoors.
I burnt the small stuff slowly, using a metal rake to move the burning section gradually forward (downwind), and chopping the charred wood and ash into the soil to stop it all smouldering or burning away.
It was quite slow, as I was cutting the wood as I went. If I had prepared the wood in advance it would have been much quicker. Using seasoned wood would have speeded things up, but the green wood may have increased the amount of char left behind, which was part of the point. The pictures throughout the post should give you a sense of what I did.
Wood Char Results
By moving the fire slowly forward, and ‘chopping’ and raking the charcoal in to the soil, I was able to work the fire over a section of the (future) polytunnel bed. The slightly out of focus picture below shows a strip of ground ‘worked over’ by a slow burning fire. The boards allowed me to move the fire forward without compacting the soil.
The picture below shows an area that has been worked over by the fire, and then lightly raked in.
There was a lot of char left. Mixing it with the soil seemed to help prevent all of the wood from burning. It also mixed the ash into the soil. The picture below shows the first stage in mixing the char and ash into the soil. The larger lumps of char were easy to break up with a rake.
The ground was wet and moderately compacted at the beginning. The fire dried the surface of the soil. The depth depended on how hot, and for how long the fire remained above it. This suggests that you could actually use the fire to regulate how deep it worked. A quickly moving slow burn penetrated less than two inches, whereas a longer period had an effect up to four inches down. This could be used with species that are propagated by root cuttings, or sucker. The plants would regrow from undamaged roots a few inches below ground. Comfrey comes to mind as an example. What was left on the surface was a mixture of fine crumbly soil, and charcoal of differing sizes. I could envisage using this to clear an area of weeds and seeds, especially grass seeds. The best result was compltely unexpected, and unplanned, a dramatic improvement to tilth.
If you are a regular reader of my blog, you may have read that we have a very heavy clay soil. Like all clays it is difficult to obtain a decent tilth for sowing seeds. You have to avoid doing anything with the soil if it is too wet, and it changes from too wet, to too dry in a very short time. If the soil is wet when you need to sow seeds, there is nothing that you can do except wait, and hope. Under the burning wood, the soil dried out and become really crumbly. I was able to create a really good tilth in seconds, with the only lumpy bits being charcoal. I was really impressed by this. The ability to create a good seedbed with surface weed seeds burnt off, with nothing more than a rake and a match was awesome. This bed would have been perfect for sowing seeds, and with the addition of lime, minerals and charred wood to the soil from the burning, it should be possible to give a good start to seedlings whatever the soil conditions are.
Practical Uses of Charring Wood
The most obvious advantage of doing this is the preparation of soil prior to planting. The tilth, lack of weeds, and weed seeds, would make it really easy to broadcast seeds and rake them in, especially larger seeds like grains and legumes. Now I wouldn’t want to see a large scale return to burning stubble in fields, but I could see a much smaller scale use in gardens, and perhaps allotments. Here, if allowed, it would be possible to utilise junk mail, hedge clippings, weeds, brambles, etc to help create a good seedbed, and perhaps improve soil fertility over time.
On a larger scale it should be possible to combine the feeding of prunings to livestock, with the collection and burning of the remnants to help build soil fertility for small areas, perhaps concentrated in the areas where the most intensive growing of food takes place. Combining the char, wood ash, manure (animal and human) and compost should create a very productive growing area, which would aggregate fertility, rather than lose it.
I did a bit more of this today and had a chance to think of all of the benefits that I had gained from this observe and interact experiment.
- Piles of firewood ready to use in the house and for our wood fired pizza oven.
- The quick disposal of thorny twigs not needed for other purposes.
- Mineral rich wood ash and biochar directly over the site of my future polytunnel.
- The reduction of weeds and weed seeds in the upper layer of the soil.
- An undertanding of how fire can be used as part of a sustainable agricultural system.
- Some ideas for how I can create good conditions for planting in clay when the weather has not been helpful.
- Fun. Two days of playing with fire on cold, damp days.
- A deeper respect for traditional knowledge.
Not bad at all