Co-composting with biochar

There are quite a few research publications of the positive effects of biochar co-compoated with organic waste to improve soil structure and increase crop productivity. In the past winter, I have routinely added charcoal to food waste and manure liquid and composted with chopped leaves. Although I didn't have a non-biochar control side by side, my anecdotal experience is that biochar does speed up the decomposition and improve the quality of compost.

I guess the benefits come from several aspects:
1) the small pieces of biochar have large total surface area and the food particles adhered them are more available to microbes
2) bigger pieces of biochar reduce matting, compaction, and add air-filled porosity to increase oxygen level in the interior
3) the high absorbance nature helps keeping a constant moisture level in the compost pile. I observe less soggy bottom and the hot center is less likely to dry out.
4) biochar provides shelter for microbes
5) in the beginning of composting, pH value will drop then increase to near neutral when finished. Since my chopped leaves are mostly oak, which is acidic, the modestly alkaline biochar might increase the pH in the mixture so it won't go too low as to inhibit microbes. ( I just got some test strips. Will monitor pH in my next batch.)
6) small amount of ash in biochar adds to the fertility of final product.

Ratio wise, usually 20 to 25% biochar was used but I just add about 15% due to higher amount of ash compared to industrial grade char.

I am quite happy with biochar compost and the plants show healthy growth too. From now on, all my composts will be brown+ green+ black.

Please share your thoughts on composting with biochar. Happy burning!

Good analysis and looks like you're getting great results. Just adding char to compost is a great way to charge it, if not the fastest. I would expect the biochar also helps absorb some nutrients that may other have leached out of the decomposing compost, either into the earth or air.

Your system makes perfect sense to me. I do something similar.

But I am deeply envious of your natural acidic input to the system -- around here I am always cautious about pushing my soil even more basic than it is naturally.

Mixing char with compost to make biochar is a tried and true process. Many on this forum use it.  I am currently using a liquid inoculation, as I believe it is faster, and I can more precisely measure where I am putting the biochar in the driplines of the trees and bushes.

I may use this method later, after I have completed the map of all of the locations where I feel I need to get biochar into.  Then I will just be generally adding biochar because i want to, because it is a generally good thing,  rather than needing it in that area.

John S
PDX OR

John, I'd agree that your liquid inoculation approach is probably both faster and more precise.

Adding the char to compost is a it more of "general approach" - it takes time to inoculate, you don't know your exact application rates of biochar, etc.

One nice thing you can do if you have animals is add biochar to the livestock bedding, then compost the spent bedding with the char before applying. Stacks an extra function onto the process. I do that with my chicken coop.

Freshly prepared biochar can sequester nutrients making them not available for plants. I tried charging charcoal in compost tea for hours prior to making potting mix. The plants with biochar added actually were smaller than control without biochar. That makes me wonder how to effectively charge the charcoal so it is not inhibitory.

May Lotito wrote:Freshly prepared biochar can sequester nutrients making them not available for plants. I tried charging charcoal in compost tea for hours prior to making potting mix. The plants with biochar added actually were smaller than control without biochar. That makes me wonder how to effectively charge the charcoal so it is not inhibitory.

Mary, from my limited experience, I think you have the approach correct, just not the timing . Most times when I hear about people charging with compost tea or other liquid inoculants, they do it for a couple of weeks, not a few hours.

Not charging or insufficient charging has been shown to temporarily decrease the benefit of biochar.   One problem with liquid inoculation is that if you put the char under liquid for any length of time, it becomes anaerobic.  Now you are developing all of the microbes that lead to disease.

One solution to this problem that I have found is to limit the time under liquid. I will generally soak the char for a few hours under the liquid to start on the first day.  Then I will simply drench them, once a day, for about a week.  It takes about one minute.  Then they have time to inhabit and occupy all of the little nooks in the biochar, and it hasn't gone anaerobic.  It's a little more work, but I haven't seen the gaps of 1-2 years after I install it, until it helps growth. I have seen great growth on many of my plants the first year.  

I don't use compost tea in biochar. I like compost tea and I use it in the garden, but not for biochar.  It's too time consuming to make, and by the time I'm done with my drenching, it has lost its microbial edge.  I use a mixture of whole wheat flour, ag lime, old wood mycelium, rotten fruit, seaweed, compost, worm compost, and urine.  Those are nutritious items I have around or can get very inexpensively. If I had animals other than worms, I'd use their poop too. The drenching doesn't just keep the biochar in an aerobic state. The pouring and crashing through the biochar keeps the liquid inoculation aerobic for the week that I'm using it.  

John S
PDX OR

I've seen people use biochar above ground for trees etc but doesn't the sun kill beneficial microbes etc?

I would have to dig up my sources, but I recall from amateur research that 5% is the recommended limit for biochar being beneficial in a compost pile (not slowing it down by locking up nitrogen). Have used this rate in Johnson Su compost, with an additional cap of 2” of char at the top to reduce off gassing. It made excellent compost. Finished compost can be mixed 1:1, but going towards that limit does seem to slow nitrogen availability, which has mixed +- effects according to researchers (I believe these were studies referred to me by Kelpie Wilson who sells the Ring of Fire kiln we use). I have seen good results with 1 part char mixed into 2parts finished compost on my young trees so far.

For liquid inoculation, I used compost extract with a bit of fish based organic liquid fertilizer for the final quench of the char once it was mostly cooled. If soaking, I would use extract instead of tea for greater diversity and simplicity, and bubble the mixture vigorously as it soaked, effectively making it a tea by the end. I might go heavier on the fish than I normally do for tea sprays with this approach, as the char will adsorb a lot of N.

The ratio will depend on the size aka availabe surface area of the biochar and the nitrogen level of feedstock. Several of the researchsvI read were done in developing countries poultry manure are mixed with charred rice husks. So the husks don't need further processing and are uniform in size. Maybe the suggestion for lower percentage of biochar requires finer particles like pulverized powders?

Kenneth Martin wrote:I've seen people use biochar above ground for trees etc but doesn't the sun kill beneficial microbes etc?

When it gets hot and dry, without a food source, living organisms will die or go dormant.   They are more likely to dwell on the shielded side touching the ground.

Yes, it is recommended to cover charged biochar with mulch or soil so that it doesn't dry out.

Great info! Gonna have to try this. I compost LOTS of Oak leaves.

May Lotito wrote:Freshly prepared biochar can sequester nutrients making them not available for plants. I tried charging charcoal in compost tea for hours prior to making potting mix. The plants with biochar added actually were smaller than control without biochar. That makes me wonder how to effectively charge the charcoal so it is not inhibitory.

I've read that this inoculation period needs to be a few months long.

It's been years, but, I recall reading that the South American terra pretta is believed to have included biochar and very well composted manure (possibly human).  

I did some experimenting with growing herbs in my own attempt at terra pretta in planter boxes.  The well composted manure I used came from a horse corral.  
The "Fresh" stuff under-performed the control.  The control was soil from the garden.
The year old batch produced plants that were noticeably larger and more robust.  I don't remember the numbers, but, I'm thinking I was able to harvest about 1/3 more herbs from the year old terra pretta than the garden soil.  My unsophisticated palate couldn't taste the difference.