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More on pot sherds
Hello, I wrote this while my internet connection was down, and forgot that there was another thread going down below on the pot sherds. However, that thread has nothing on the physical aspect and has had no activity for a few days, so forgive me for starting a new thread....especially with my first post on the forum.
Anyway, here goes....
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Hello everyone, I'm very, very new to all this terra preta stuff - only finding out about it a few days ago - but I've been reading up over the last couple of evenings, and last night I watched the BBC Horizon 'documentary'. It wasn't in the least bit helpful and I would have better spent the 45 minutes reading this forum, but it did have one redeeming feature - actual footage of terra preta.
What immediately struck me was the amount of pot-sherds, though they glossed over that in the film. It was not just a few randomly strewn bits as I had believed, but was absolutely full of them. Something struck me that made me immediately think that they must be added on purpose: they're all horizontal. This is something I've not seen written about anywhere (bear in mind I've really not read much, so apologies if you've already covered it here), but it strikes me as the most obvious characteristic of the soil. It's hard to find pictures of terra preta on the net, but the best one I've found is here:
!!!!!!!!!!!! it turns out I'm not allowed to post links - you'll have to search google for: 04_00_39.html terra preta; It should be the only result. It's a soil profile at the smithsonian museum of natural history - title is "dig it" !!!!!!!!!!!!!!!!
Now, as you can see in the photo, pretty much all the terracotta is horizontal. I'll get onto why I think that's important in a minute, but firstly, lets ask an obvious question - how did it get there? It doesn't just move through the soil with the actions of worms, since if it did it wouldn't be horizontally layered, it would be any old angle. The only way it could have got there, horizontal and layered, is if the soil was built from scratch starting at the lowest level. In other words, the soil wasn't just amended with carbon and terracotta in the top layer that moved down over time - they built it in layers. So, was it built slowly over a period of decades, centuries even? Possibly. I can imagine they may have used the pot sherds and charcoal as a mulch and it built up over time, but I actually tend to believe it was all built in one go, again, because of the horizontal nature of the terracotta. If it were, in fact, built up over time in layers, the terracotta wouldn't be flat because of tilling. Even if they used no-till agriculture, I find it hard to believe that over the course of a growing season - after cropping and 6 months worth of general movement - it could remain flat. Moreover, it seems obvious that the top layer in that photo HAS been tilled, due to the random angles of the pieces.
However it was built, one thing was clear in my head after seeing that picture and the documentary last night: the terracotta was laid flat, in layers, intentionally. So why? Well, it's plausible it does nothing, despite being laid on purpose. Equally, the other theory I read about is possible, that it harbours certain micro organisms. However, I think it more likely that the intent and the effect was mechanical. This is where it gets a bit hypothetical, since I'm no soil scientist, but the obvious problem with soils in areas of extreme weather (including my own soil) is that when it rains all the nutrients get flushed straight out. As is well established, the charcoal counteracts this on a chemical and biological level. I think that the pot sherds may counteract it on a mechanical level.
This is certainly a gross simplification of how water moves through soil, but look at the picture on that website - specifically the band of terracotta in the middle - and imagine water flowing through it like a marble in a bagatelle game. The water has to go around the impermeable terracotta, so is in contact with far more charcoal. It seems to me the pot sherds are acting to effectively increase the time and amount of contact between the water and the charcoal, so that more nutrients can be absorbed. Effectively, they are increasing the surface area of charcoal and soil exposed to water, meaning that by the time it flows out the bottom, the rain water is carrying far less minerals and organic matter than it would do if the terracotta were not there.
Is this plausible? I would love to hear anyone's input on this.
Keep up the good work
Gaudencio
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