I have been experimenting (casually) with biochar and have become a believer. So now I am hooked. I am wondering if there is a consensus on how much biochar is optimal for agriculture. Is it 100%, 50%, 5% in the top 12 inches? I have read a few things, but seems to be personal opinion rather than demonstrable results. Assuming that the char is in small particles, is there an optimal amount; or a point of diminishing return?
Most sites seem to recommend around 5% (some say 5 to 20 % but 20 seems high) bio-char in a volume of soil. I would keep this in the top foot for sure, as that"s where your humus is going to be most formed. For my purposes, I only incorporate bio-char into the soil itself when I build a bed and I'm not scientific about my volume, but it's not as much as 5%, probably more like 2%. Later, I incorporate bio-char when I add compost or under mulch. This will slowly incorporate more char with organic amendments into the soil matrix, feeding the system from above, as nature does. This allows me to use it in a no-till system (after the beds are built).
I hope to do some experiments of my own with volumes, when I have a bigger system to make more char.
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From my own experience in one humble season in my tiny garden, and also watching my neighbors starting out their own new gardens at the same time without adding biochar, I would say (drumroll, please...) it depends.
I added 8% by volume to the soil mixture for my raised beds along with lots of other wonderful stuff and I'm getting very good but not perfect results. Some of my neighbors with their heavy, rocky clay soil knew what they were doing, worked nice stuff into the soil too (no biochar) and also got great results. Totally unscientific. Others had more problems but generally results were surprisingly good considering how depleted the soil looked at the outset. I'm happy with my perect soil mix including 8% biochar, but not totally convinced I should have passed up using the clay soil already in my garden plot. I inoculated it with EMs and other creepy crawlies, sea minerals, sheep dung and molasses, but didn't have the luxury of letting it relax and chill out for a long while before I mixed up the rest of my soil and stuck my growies in it.
If you have newish and fertile clay soil (e.g. not ancient, washed-out Amazon-type clay) it may be hanging onto a lot of nutrients and minerals for you already without the benefit of biochar. That seems to be the case in our community gardens here. Biochar may help lighten up the soil a bit along with tons of organic matter, and of course you're doing your thing to sequester a bit of carbon for a millenium or two, but really, the added benefit as far as garden yield may not be huge, and in some cases may be negative.
OTOH, if you have sandy or other low-nutrient soil (e.g. Amazon) I imagine you would get a much more visible benefit as you'd be significantly increasing the CEC of your soil.
I think our science on general benefits of biochar to growies and soil quality is still in its infancy and we really have a long way to go and a lot of science to do before it's all figured out. It seems to really help some plants even in the first year, while to others it does the opposite (general balance being positive up to a point). Also, I think the way it's prepared/seasoned/inoculated beforehand may really, really, really affect its effects in the first year especially. You don't want it to be filling up its little microcavities with nutrients by sucking all the nutrients out of the surrounding soil to the detriment of your plants, you want to get that done beforehand. If you do that really effectively, I'd bet you can increase the precentage of biochar in your soil by a lot. Some folks out in Hawaii seem to experiment with this a lot and prepare many different brews, with some stories & recipes included. Have to let it sit for a good long time though.
Some folks in Canada and in Germany, among others, have done some interesting studies, but still far from complete in my book. I'd love to see tests of the Hawaiian stuff on different types of soil, controlled tests over 5 or 10 years. A maddening number of field trials I've seen stop releasing data after two years, just when things are getting interesting and we could learn something.
Tiny garden in the green Basque Country
Doe, a deer, a female deer. Ray, a pockeful of sun. Me, a name, I call my tiny ad ...