Clay soil burning

jsbtNovember 26, 2013

Hello :-)

I have a small piece of land. A small 4 meter x 4 meter part of it is of the best clay soil, perhaps the best in the world for making pottery: hard as a rock when dry and like chewing gum when wet. When dry that soil laughs at my pickaxe and when wet it glues my pickaxe to the soil. Not even grass can grow there. However, I am not interested in pottery industry, but growing my plants there.

I read a lot about clay soils and about composting them. Recently I found an article in an very old magazine here . At the end of the article, it mentions that there is an explanation on how to burn clay: "For the process of burning clay, see Horticultural vol. ii. p. 442, and vol. iii. p. 184. Ed."

However I cannot find the mentioned article on clay burning. Seems like when clay is burnt it will become a bit more porous and will not aggregate any more. This would facilitate draining and also adding compost to it. However, there should be the cons which perhaps were not taken into consideration 130 years ago: burning the clay would perhaps destroy nutrients? And if I burn clay and then add compost to 'refill' nutrients into the burnt clay? It would be bad if from clay that piece of soil becomes as sandy as the Sahara.

I made research, contacted the owner of the website that hosts the above book, nobody knows about clay burning. It seems clay burning is kind of a subject lost in history. I never found anything on that and there is no explanation on how to do it. As my piece of land is small, I would be willing to try clay burning and adding compost, provided it will not destroy the soil.

All your ideas, suggestions and information on the clay burning subject are welcome.

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Clay soil particles are inert minerals and by themselves would not support combustion, something that would support combustion would need to be added to that soil and then enough to create sufficient heat to cause the soil particles to expand. It would be far easier to do that in your oven, but then that would stink up the house for a fairly long time.
Heating that soil to a temperature that might cause the clay particles to expand would kill off any part of the Soil Food Web that might be there.
The link you provided did not work, hopefully this one will, but it does not really address the question of how to "burn clay soils".
Make your life much easier and simply add enough organic matter to that soil.

Here is a link that might be useful: 1851 advice about clay soils

    Bookmark   November 26, 2013 at 6:28AM
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seysonn(8a WA/HZ 1)

It is interesting:
Isn't that how bricks are made from red clay by firing ?
Now, instead of forming the clay into a shape, make it into smaller pieces or even particle and fire. Those small particle would be like if you crush a brick or a clay pot. They will not stick togetther anymore and will have porosity to retain moisture.
In effect that is how things like Turface and Floor-Dri are made.

But I suppose it might not be economical to do it in a large scale due to high energy costs.

    Bookmark   November 26, 2013 at 8:04AM
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Hello Kimmsr, thank you for the link, this is exactly the article mentioned in my opening post. You are right,composting will work in the long run. However, I found very interesting this old technique from 1851, which is forgotten now, perhaps because as Seysonn explains, it is not economical. In my case, I am cleaning my piece of land, I have plenty of old wood good for nothing except to burn. As the clay soil is restricted to a small 4x4meter area, I would like to try the clay burning process and then post the results here. But first, I need to find how to do it. Back in 1851, perhaps farmers just cut down trees to open a new area for cultivation and burned the logs there in place? Please, share your thoughts and ideas.

    Bookmark   November 26, 2013 at 10:06AM
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toxcrusadr(Zone 6a - MO)

I have not heard of this as a soil treatment but it makes a certain amount of sense. I suppose they may have just built a fire over it, but it would likely only heat the first few inches to a very high temp.

It's not so much burning of the soil itself, but more like kilning. To do it efficiently, it seems to me you would need to find a way to make sure the clay is broken up while being heated. Otherwise you end up with enormous clumps like bricks which then have to be broken up. They do have million-dollar machines that can heat soil while tumbling it down a rotating drum - similar to a cement kiln. I've seen these in action. Not feasible for a home garden.

What I would do, is build a raised bed with imported soil over top of this stuff. It sounds so bad that you'll be there for years trying to fix it. And this is coming from someone who has spent years fixing clay soil, so I'm not a wuss in this department.

    Bookmark   November 26, 2013 at 10:53AM
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I will never fully understand how Google algorythms work. I spent a couple of weeks looking for the Horticulturist books online and never found anything. Now, in a couple of clicks, I found volume II. And there is how to burn clay soil. They say it is easy and I believe so, for small volumes, and resolves quickly the issue. I will do it next February or March and post here the results. Here you have the text, which has no copyright:
* A simple mode of burning: clay in the kitchen garden is the following: make a circle eight or ten feet in diameter, by raising a wall of sods a couple of feet high. Place a few large sticks loosely crosswise in the bottom, and upon those pile faggots or brush, and set fire to the whole. As soon as it is well lighted, commence throwing on lumps of clay, putting on as much at a time as may be without quite smothering the fire. As soon as the fire breaks through a little, add more brush, and then cover with more clay, till the heap is raised as high as it can be conveniently managed. After lying till the whole is cold, or nearly so, the heap should be broken down and any remaining lumps pulverised, and the whole spread over the surface and well dug in ' As an example." says Loudon, " of the strong clayey soil of a garden having been improved by burning, we may refer to that of Willersly Castle, near Mattock, which the gardener there, Mr. Stafford, has rendered equal in friability and fertility to any garden soil in the country. "When I first came to this place,' says Mr. Stafford, "the garden was. for the most part, a strong clay, and that within nine inches of the surface; even the most common article would not live on it; no weather appeared to suit it; at one time being covered by water; at another time rendered impenetrable by being too dry. Having previously witnessed the good effects of burning clods, I commenced the process, and produced, in a few days, a composition three feet deep, and equal, if not superior, to any soil in the country.' "

Hope to have helped gardeners with the same issue as me.

    Bookmark   November 26, 2013 at 11:03AM
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I would not say it is easy or makes any sense .

If you are able to dig out a clump of clay to throw it in the fire, it means you are able to shovel the whole area and amend it with organic matter and solve your problem with heavy clay.

    Bookmark   November 26, 2013 at 11:50AM
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I agree with avgusta. My garden is 100 feet by 125 feet of red Georgia clay. I have added sandy soil and compost and it does well. I do raised rows a no till method and if I turn it at all I turn it with a fork and never walk on my rows also.

    Bookmark   November 26, 2013 at 2:18PM
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Pure clay will not burn, but it will absorb a lot of heat before it is completely dried out. I am sure that there are locales with "clay soil" that contains significant organic material, and that stuff will support combustion. Our local soil, here in Madison, is fairly high in clay, but also has considerable organic component. I am able to dig up and break up the clay clods, and work in compost made mostly from leaves. Our garden soil is now easily workable, but it took several years, and many annual additions of compost, to achieve this.

    Bookmark   November 26, 2013 at 4:12PM
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That whole book (actually a collection of articles) is a trip.

I've never heard of "clay burning" myself, but it seems to be a way to harden off small aggregates of clay improving air movement in the soil and lessening water holding capacity in the clay.

Basically, when clay is cooked to a high enough temperature (especially those high in iron/iron oxides and aluminum/aluminum oxides)...the clay can harden up and become dehydrated (more accurately, de-hydroxylated (removal of OH- groups attached to the clay)). This makes parts of the clay hydrophobic (water repelling) which keeps it from getting "sticky" and improving air movement through the soil. It's not permanent, but it can be rather long lasting (years/decades).

That said, this is a very difficult and overkill way to go about improving soil. It might be best to leave this one in the 1850s.

This post was edited by nc-crn on Tue, Nov 26, 13 at 20:14

    Bookmark   November 26, 2013 at 8:02PM
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toxcrusadr(Zone 6a - MO)

OTOH, if no one here has actually tried it, how do we really know?

From the description, the author made a 3-ft thick deposit of clay and biochar. I gotta wonder how much of the improvement was also due to the charcoal.

I have a rural property with some terrible clay, some of which is in piles as we speak, and I also have brush piles in need of burning. Hmm.

    Bookmark   November 27, 2013 at 11:47AM
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@nc-cm, thank you for the explanation, very interesting. @hammerga, compost is sure a solution and will solve the clay issue as you say after some years of work. I was reading another page of the book and the author says exactly that, compost will work in the long run, clay burning is an instant solution. I was worried clay burning would destroy the soil, but the author says the contrary, as it faciliates penetration of compost and organic matter. @toxcrusadr charcoal may play a part, but as nc-cm says, clay burning causes a chemical reaction removing OH. I will burn the clay on March, if you also do so, please post here the results. On my case, I will not build a a wall, but burn the layers of clay and brush inside the same hole in the ground I retired them. I am glad to have revived this long forgotten solution. Let's see if it effectively works! :-)

    Bookmark   November 28, 2013 at 5:57AM
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Putting any pollution concerns aside this most likely was not a too popular thing to do because in reality it did not do what was expected. Since the kilns that the now extruded and dried clay is placed to fire into bricks need to be in the 900 to 1200 degree range I doubt that burning in the filed would produce anything similar.
Adding organic matter to clays is much easier then would be trying to prepare that same clay for burning, I would think.

    Bookmark   November 28, 2013 at 6:34AM
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As promised I am back there to post the results of clay burning. I did a test with a small quantity of clay soil. Here is what I did: I dug a hole around 100 cm diameter and 70 cm depth. The non-burnt clay soil colour is generally tan (#d1b26f) but going to cobalt green in some parts. It is humid, sticky like chewing gum, even after 3 weeks without rain. I tried to smash the soil into smaller particles, but not much success doing this. Meanwhile, I had a fire going on inside the hole. I followed layers of dry plant branches with layers of the pieces of clay I had excavated from the hole. After the hole was filled in, I let the fire extinguish. End of day, after cooling, I excavated the layers of carchoal and soil. From tan, the soil colour had changed into dark rust colour, very close to the colour we see in commercially sold bricks. I took one piece of burned clay, it was hard to break it but eventually I managed to turn it into grains. I took another piece of burned clay and dissolved it easily in water, it was not sticky anymore. Clay burning works, however, in my case it is not feasible because I would need great quantities of wood because of the size of my piece of land. I agree with kimmsr that the best way in my case would be to add organic matter to clay soils. However, if you have a small backyard and want to improve soil in small areas, like for instance make a small elevated bed for your flowers or legumes, the clay burning may work very well and, as the original article from the 1851 says, will give you good results after some intensive work.

    Bookmark   April 14, 2014 at 11:09AM
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wayne_5 zone 6a Central Indiana

Thank you jsbt for reporting back. I have to wonder how long the change would last out there in the open garden.

I like the idea of changing the texture and structure by adding large amounts of sand and peat moss and lots of other organic matter to clayey soil. It works nicely for me.

    Bookmark   April 14, 2014 at 11:45AM
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Keep in mind too that some things people believed in the 1850's today we know is nonsense.

    Bookmark   April 15, 2014 at 7:05AM
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The good thing is that we should never stop learning and checking things. The original article in 1851 was written at the Horticulturist because of a demand from a reader: "that articles which may be obtained of the dry salters are much more accessible to me than wood ashes, peat, etc, which every farmer in the country can easily obtain, but which are less abundant here than guano. Very respect-fully yours, E. R. Boston, April 28,1851." His problem was that peat moss and other organic matter was difficult to obtain. The articulist then made reference to an alternative process, clay burning - and he mentioned the name of the person and the city where that person worked during some years burning clay in his backyard and obtained a very good soil for legumes, the articulist say. It was then a bit more than a mere belief, it was tested by someone before. And according to my test, it works, therefore it is not nonsense - if you do not have other solutions at hand. In my case it is easier to purchase compost. I did it once in in the lower half of my piece of land and incorporated 4 tons of compost using a rented tractor, it worked very well as wayne_5 says and after 3 years I still see how the compost ingrained itself into the clay soil and the earthworms doing their job very well - I am amazed how they manage to dig their tunnels into that sticky soil, sticky enough to stop pickaxe like glue. But this clay burning works too, only it is much more time/energy consuming. But tell you what: I will burn more clay and make a special bed and plant some vegetables there, just for fun and to test how they will grow on this burned clay soil.

This post was edited by jsbt on Tue, Apr 15, 14 at 8:33

    Bookmark   April 15, 2014 at 8:27AM
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toxcrusadr(Zone 6a - MO)

I would like to see some pics of how this works. I don't quite see how wood can burn when down in a hole layered with soil. How does it get enough air? Or are you only placing a few chunks of clay amongst the sticks and branches?

    Bookmark   April 15, 2014 at 10:58AM
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Something else to keep in mind is that burning will drive off organic acids and cause a large increase in soil pH.

    Bookmark   April 15, 2014 at 8:34PM
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wayne_5 zone 6a Central Indiana

Again, how long does the result last?

    Bookmark   April 16, 2014 at 10:43AM
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@toxcrusadr: at the present stage of the soil, as sticky as it is, I did not manage to grind it into small pieces but only take chunks with a pickaxe, there was plenty of free space for air to come in. The burning pine tree branches also provided space for air.
@wayne_5: it depends if clay burning leads to a physical or chemical reaction. A physical reaction could be reverted. A chemical reaction mostly not, afaik (I can be wrong). I believe it happened a chemical reaction, as the soil colour changed from light tan into reddish or rust depending on the chunk of soil and when I dissolved it in water it turned into a chocolate colour. Here you have a picture of a small quantity of burnt clay I mixed with water. The big difference is that it dissolved in water while the non-burnt clay soil is sticky like epoxy and hardly dissolves in water. Notice the colour: not tan anymore, but chocolate. When dry is reddish colour like those commercial bricks sold everywhere. If this clay burning was a chemical reaction as perhaps nc_crn explained better above, then it will last a long time (perhaps forever).
@slimy_okra: yes you are right, whatever little organic matter in that "epoxy-like" soil, it was burnt. The minerals are there though, even after burning.
PS:Clay burning is very time consuming, but I am thinking just out of curiosity to burn more clay and try to setup a small test bed for legumes and see what happens. If and-or when I do that I will post here the results with pictures.

    Bookmark   April 16, 2014 at 11:47AM
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toxcrusadr(Zone 6a - MO)

"Organic acids" is probably not the most correct term here. Carbonates (calcium and magnesium, mostly) decompose to give off CO2, and when they are rehydrated, they form hydroxides which are very alkaline. This is why ash is alkaline anyway, and I suspect ash will control the final pH if it is mixed into the soil, at least temporarily until it's leached out. I had not thought about what happens to the clay though. Depending on the type of clay it may go through a similar process. I would think, although don't have specific knowledge, that the clay would recover also after it's exposed to the environment. Did the original book discuss this?

    Bookmark   April 16, 2014 at 1:58PM
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wayne_5 zone 6a Central Indiana

I saw part of a soil health video where chunks of soil were dropped into water. Unlike what you would think, it was the ones that did not melt down quickly that were considered of better structure. I don't remember the why, but might guess that too fine of melting and merging for clay loam soils might let them recongeal into hard lumps when they dry. Now if the texture is coarse enough, that would not be a problem.

Deflocculated soils make hard lumps. Sodium and magnesium deflocculte, but calcium flocculates soil and makes soft and fluffy lumps.

This post was edited by wayne_5 on Wed, Apr 16, 14 at 15:26

    Bookmark   April 16, 2014 at 3:21PM
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