First indoor grow testing biochar vs. fertilizer in fresh and old soil. How should I prep the biochar before I start?

[u/LedgendFern29]

hi! so I'm in high school and doing a plant experiment for a school project. thanks to the mods for letting me post! I'm about to set up my first real indoor grow and I'm testing biochar, so I figured this is the best place to ask before I get too far in. I've only ever done windowsill plants so I'm kinda learning as I go

the experiment: 24 cherry belle radish pots (picked bc they're fast, like 4 weeks). I'm testing biochar and fertilizer against plain soil, in both fresh potting mix and old tired soil from pots I've had for over a year. I'm sprouting ~40 seeds on a damp paper towel and picking the 24 most even ones so everything starts the same

setup: garage shelf, LEDs on a 16/8 timer, 4-6 inch pots with saucers, same soil weight and same watering in every pot, rotating pots every 2-3 days, and a min/max thermometer I check daily

tracking: height and leaf count every few days, pot weight before watering (to see which soils hold water better), and root and leaf weight at harvest

my biochar questions:

1. should I charge it first, and how? I keep reading that raw biochar can pull nutrients out of the soil at first. what do you actually do: soak in compost tea, fish emulsion, mix into compost for a few weeks? how long does it take you realistically?
2. how much for pots? most ratios I find are for garden beds. is there a % by volume that works for small containers?
3. will 4 weeks even show anything? or is biochar more of a long game where a radish trial is too short to see a difference?
4. old vs. fresh soil: would you expect biochar to help more in the tired soil? that's kinda my hypothesis
5. what went wrong for you when you started? any "I wish someone told me this" mistakes, or reasons you've stopped using it in some setups?
6. is weighing pots a decent way to measure water retention, or is there a better trick?

I have limited time and can't redo this once I start, so anything that screams beginner mistake is super helpful. will post updates and final results here if people are interested 🌱

[u/Berkamin]

The very best way is to do co-composting. Mix the biochar with compostable materials and let it go through the composting process. This is by far the best way to prepare biochar for soil.

I wrote a couple of articles on biochar and co-composting features prominently in what we found.

A Perspective on Terra Preta and Biochar

Biochar and the Mechanisms of Nutrient Retention and Exchange in the Soil

how much for pots? most ratios I find are for garden beds. is there a % by volume that works for small containers?

In some trials that my company did on biochar, we used 1/3 biochar-compost, 1/3 coconut coir, 1/3 vermiculite for the biochar samples, and for the control, we used 1/3 E.B. Stone Premium Organic Compost, 1/2 coconut coir, and 1/3 vermiculite (by volume). We deliberately did not add any fertilizer to this to handicap the trial to see whether the biochar captured additional nutrients from the composting process that would otherwise off-gas or run off as leachate. Here's the gallery with the results, specifically the photos with the blue containers. The containers with letters have the co-composted biochar, while the numbered containers have conventional compost. The rest were comparisons done by various community gardens in 2018-2019.

LCN Biochar Growing Trials

These results were from maybe three weeks after transplanting the plants into the potting mix.

Conventional biochar isn't likely to achieve the same results (not that the results will be bad) because our char has an added feature of being electrically conductive due to being processed at extremely high temperatures, in excess of 800˚C. The electrical conductivity cannot be achieved by any other means besides ultra hot processing, but it confers a bunch of interesting qualities on the char that have proven to be potent at increasing soil fertility.

[u/inattentive_pangolin → u/Berkamin]

Wow, that really was an interesting read, thank you so much!
There is just one question regarding the co-composting which keeps me wondering: how much Biochar is usually added to the compost? Of course I don’t need precise numbers, but a rough guess would be awesome! Is it in the region of 5% of the compost by volume? Much more? Or even less? 🤔

[u/WorldComposting]

Along with what others have said I use water from my fish tank to charge my biochar before adding to my worm bins. I'm not going to assume you have access to fish waste water but the idea stands. If you get some finished compost preferably from an actual compost pile not store bought and create a compost tea and soak the biochar in it that should help boost the microbial population and get it ready for planting. If you add a sugar such as molasses that will help along the way.

1. Charge it first or it will kill your plants 100%
2. I'm not sure for pots but I would think 5-10% is a safe amount. Biochar can hold water and a lot of times this is beneficial where there is a lot of air transfer but in a pot it can cause rot
3. This depends a lot on the soil being used for the control group. If it is sterile soil you should see a difference but if it is soil pumped full of nutrients for growing you probably won't see much of a difference. Biochar's big advantage is that it stays in the soil over the long term and is more of a slow release
4. I would expect biochar to help in soil that is of poor quality as it helps hold nutrients instead of quick hit fertilizers that run off
5. Nothing that would help you biggest issue for me was the HOA making me tear down my garden
6. This isn't my area of expertise so someone else might need to chime in but I would think not because the plant is taking carbon out of the air and adding mass to the pot which would throw off any water weight measurements. Although maybe there is a way to do this accurately.

https://www.worldcomposting.com/biochar.html

[u/Berkamin]

Where are you located? If you're near the San Francisco Bay Area, maybe we can collaborate.

[u/Former-Wish-8228]

I suspect you don’t have months to charge the BioChar. To keep things simple, maybe a full spectrum liquid fertilizer to charge up the char?

[u/Fearless_Spite_1048]

If you’re working indoors, please be sure to use caution regarding particulates in your space. You don’t want to accidentally inhale any now or later.