[Terrapreta] Depth of char incorporation and mixing?

[rukurt at westnet.com.au]

code suidae wrote:
> Articles I've read about Amazonian Terra Preta mention that the char 
> enhanced areas can extend anywhere from a foot to as much as 5 feet 
> below the surface. I suppose ideal depth for a given crop depends 
> mostly on how deeply the crop roots, but it also seems that there 
> might be a component of how deeply the soil microbes keep up their 
> activity. Is there a point of diminishing returns where lack of 
> microbial and fungal activity reduce the effectiveness of the char, or 
> where water that would otherwise have been held where crops would use 
> it ends up draining down through the charcoal too deeply to be of use 
> to the crops?
>
According to AD Karve, the main microbial soil activity occurs in the 
top 10cm or so of soil. This is where plants mainly get their nutrients. 
The deeper roots are mainly to get water.

Of course, some plants, alfalfa and oats for instance go quite deep to 
get nutrients, but I think those are mainly nutrients that have been 
leached down into the subsoil and that is why farmers often plant them.

The Indios might very likely not have known this and dug the charcoal in 
deeper. On the other hand, there are other things that can incorporate 
surface material into the lower soil profiles. Worms for instance, or 
rotted out deep root that provide "shafts" for surface material to fall 
into the soil. Or soil, when it dries often cracks to considerable 
depth, again giving access to lower areas. I remember on the Queensland 
Darling Downs someone losing a watch down one of these crack. They dug 
down some four feet to get it back. Then there are floods and siltation 
and the collapse of earth banks. Over 6500 years a lot can happen.

Kurt