Tomatoville® Gardening Forums - View Single Post - Biochar enhances yield and quality of tomato under reduced irriga

bower · April 25, 2015

In my experience wood ashes are best sifted to remove any charred wood - it didn't benefit and had a negative effect on growth in my garden (worse than unburned wood chips, which take forever to break down in this cold, wet environment and low pH soil - charred wood takes forever, and forever,... and a day).

In the article cited in the OP, it seems likely that benefits to the tomatoes were due to structural reasons - water retention by the coarse particles - which improved yield under reduced irrigation.

Reading further about this, it seems biochar is being promoted more so for turning agricultural soil into a carbon sink (and 'carbon neutral' activity) because of the low rate of breakdown. It may have some benefit as a coarse material that traps moisture in specific soils or situations and very slowly returns nutrients to the soil (eg burnt forest regeneration), but it's hard to imagine any other benefit to crops, which require a high turnover of nutrients for rapid growth. And there is a risk that toxic PAH's in diet may be increased by the practice.

I had concerns after seeing the toxic effect of charred wood on plants in my garden, which agrees with the fact that PAH's - byproducts of incomplete combustion of wood - are toxic to many, if not all plants, as well as animals - some crops do take up and accumulate them.

See this article by Quilliam. Is biochar a source or sink for polycyclic aromatic
hydrocarbon (PAH) compounds in agricultural soils?

A few quotes:

from the CDC: http://www.atsdr.cdc.gov/PHS/PHS.asp?id=120&tid=25

"The PAH content of plants and animals living on the land or in water can be many times higher than the content of PAHs in soil or water. PAHs can break down to longer-lasting products by reacting with sunlight and other chemicals in the air, generally over a period of days to weeks. Breakdown in soil and water generally takes weeks to months and is caused primarily by the actions of microorganisms."

MotherEarthNews: http://www.motherearthnews.com/organ....aspx?PageId=2

"In addition to gathering ashes (and keeping them in a dry metal can until you’re ready to use them as a phosphorus-rich soil amendment, applied in light dustings), make a habit of gathering the charred remains of logs. Take them to your garden, give them a good smack with the back of a shovel and you have biochar."

..."This charcoal releases its carbon 10 to 100 times slower than rotting organic matter."

Charring, traditionally used to harden and preserve fence posts:
http://www.permies.com/t/22394/timbe...d-preservation

lentils and wheat are examples of crops that may take up PAH's from soil:
http://www.toxipedia.org/display/tox...c+Hydrocarbons

Personally, I'll be keeping charred wood debris away from my garden, and from any watercourse or groundwater catchments.

April 25, 2015	#10
bower Tomatovillian™ Join Date: Feb 2012 Location: Newfoundland, Canada Posts: 6,793	In my experience wood ashes are best sifted to remove any charred wood - it didn't benefit and had a negative effect on growth in my garden (worse than unburned wood chips, which take forever to break down in this cold, wet environment and low pH soil - charred wood takes forever, and forever,... and a day). In the article cited in the OP, it seems likely that benefits to the tomatoes were due to structural reasons - water retention by the coarse particles - which improved yield under reduced irrigation. Reading further about this, it seems biochar is being promoted more so for turning agricultural soil into a carbon sink (and 'carbon neutral' activity) because of the low rate of breakdown. It may have some benefit as a coarse material that traps moisture in specific soils or situations and very slowly returns nutrients to the soil (eg burnt forest regeneration), but it's hard to imagine any other benefit to crops, which require a high turnover of nutrients for rapid growth. And there is a risk that toxic PAH's in diet may be increased by the practice. I had concerns after seeing the toxic effect of charred wood on plants in my garden, which agrees with the fact that PAH's - byproducts of incomplete combustion of wood - are toxic to many, if not all plants, as well as animals - some crops do take up and accumulate them. See this article by Quilliam. Is biochar a source or sink for polycyclic aromatic hydrocarbon (PAH) compounds in agricultural soils? A few quotes: from the CDC: http://www.atsdr.cdc.gov/PHS/PHS.asp?id=120&tid=25 "The PAH content of plants and animals living on the land or in water can be many times higher than the content of PAHs in soil or water. PAHs can break down to longer-lasting products by reacting with sunlight and other chemicals in the air, generally over a period of days to weeks. Breakdown in soil and water generally takes weeks to months and is caused primarily by the actions of microorganisms." MotherEarthNews: http://www.motherearthnews.com/organ....aspx?PageId=2 "In addition to gathering ashes (and keeping them in a dry metal can until you’re ready to use them as a phosphorus-rich soil amendment, applied in light dustings), make a habit of gathering the charred remains of logs. Take them to your garden, give them a good smack with the back of a shovel and you have biochar." ..."This charcoal releases its carbon 10 to 100 times slower than rotting organic matter." Charring, traditionally used to harden and preserve fence posts: http://www.permies.com/t/22394/timbe...d-preservation lentils and wheat are examples of crops that may take up PAH's from soil: http://www.toxipedia.org/display/tox...c+Hydrocarbons Personally, I'll be keeping charred wood debris away from my garden, and from any watercourse or groundwater catchments.