How much bio char is too much

Anyone ever added so much bio char to their garden that it has a negative result?

Why do you ask?

For me, adding char is one component to building robust, ongoing soil fertility. Compost, aeration, and thoughtful management are other components. It's the plants and microbes and fungi and bugs that do the real work -- my job is to give them better tools to work with. It's not like there is an instant "pow" -- but compare my soil to abused/depleted soil a few years down the road, and I can assure there is definitely some "wow."

Tom Pivac wrote:Anyone ever added so much bio char to their garden that it has a negative result?

1. I've heard that if you add a lot of biochar that hasn't been "activated" with compost tea or similar ways of getting the good microbes into freshly charred biochar, that it can reduce soil fertility while the char sucks up the nutrients.
2. I've heard "10%" wafted around a lot.

That said, if I added biochar to a 1 ft cube of soil today, worms and other critters would move that around, so by next spring, theoretically, I could add more. Theoretically, as the surface soil improves, we can dig deeper and get char into lower levels of soil. Terra pretta in South America is apparently 3 ft deep in places. What proportion of Terra preta is "biochar" I don't know.

Douglas Alpenstock wrote:Why do you ask?  

Built my first charcoal retort over the weekend and just wondering what sort of quantities I should be aiming for.

I approach it the otherway round. My biochar making is my primary method of getting rid of woody material that won't compost. We generate a LOT of it from approx 3 acres of fairly shrubby/woody garden. I've never felt even close to having too much biochar. That said, I don't go out of my way to bring in extra material to burn.

Tom, I'll try and get my hands on the US Forestry study that says this, but what I recall is that you're best off spreading it thinly to all your land and then building it up as you make more because, while their is benefit to more, the rate of benefit decreases.  What I remember in this study is that 5 tons per acre (about a quart/ft2) gave a significant boost to tree growth rates, 10 tons per acre resulted in significant growth rate increases over 5 tons, but it did not double the rates.  Dropping it on the ground in your forest garden and covering it in leaves so that the leaf litter breaks down around the biochar is a way to mimic how this system works in nature in the years following a wild fire.

I have the same question. I have been adding biochar to my garden for three years, now it is a large garden and the amount I have put in is negligible at this point. The thing though is I make char during the winter, use it to filter water in my fish farm, the fines I either throw into the compost pile or feed to my ducks which the bedding goes into garden. So I guess my question is when is it detrimental to the soil? 10%? Or are we fine up to 50%?

The OP has a retort, which makes great biochar but tends to not produce as much.  Unless you have a postage stamp garden, I wouldn't worry about it until you've got something like 10% at a two foot depth or so.   Or if you notice a decline.  You are sequestering carbon, which is outstanding for the planet, even if it's neutral for your gardening space.  

John S
PDX OR

To summarize quickly, there was a study done on biochar innoculation into soil

can't find it off the top of my head but I remember it well, the graph shows an increasing production yield up to the point they stopped adding more and testing it (20%)

the more "economical" range for application is 8-10% by volume (so let's say you want 12 inches down to be "ammended" to 10%, you would need 1.2 inches of crushed/powdered biochar (keep in mind charcoal is 98% air voids 2% carbon, once you crush it volume shrinks so it's a lot if you are doing a large area) spread over the area you wish to amend to 12 inches deep

if you wanted 24 inches deep, you would need 2.4inches of powdered charcoal laid on top then mixed in roughly to get to 10%, additional biochar above 10% will increase production still but around 13% is when the gains start dropping off (still on a positive slope though)

so you can't really "add too much", it was said earlier that raw charcoal will suck up nutrients until it is in synch with the soil, that can be up to 5 years in poor soil with little organic matter...adding raw charcoal and then amending manure really helps as manure is rather loaded and water soluable so a good amount of manure on a field with raw charcoal applied would "lessen" the time for the field to get back into growing condition.

Hope that helps.

Great info, Ryan.
John S
PDX OR