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Homemade soil amendment: charging biochar in hot bed compost

Kyle Putnam
Mostly-finished compost from a hotbed filled in February 2024.
Mostly-finished compost from a hotbed filled in February 2024.

Back in February, inspired by Jack First, we built two hot beds. The value of hot beds deserves an extended-edition post but the gist is this: you build a hot compost pile with a cold frame over the top to keep the heat in. You add a planting substrate and then can direct seed or transplant into it. The heat from the compost protects the plants from freezing temperatures while spurring on quick growth in late winter months.


At the end of the growing season, you are left with a large amount of nearly-finished compost, which can be used elsewhere in the garden or as the growing substrate for next year's hot beds. In February, I was concerned about the level of sawdust in the original compost but the plants did not experience any nitrogen tie-up and the compost has broken down into a beautiful fluffy substrate an the redworms have started to move in. Not one to leave well enough alone, I amended it further for next year.


A plastic container holding homemade biochar
Homemade biochar

The CEC (cation exchange capacity) of my sandy soil is abysmally low, something around 7.8. CEC can be thought of generally as a soil's ability to hold onto important nutrients like calcium. What this means is that, particularly in areas with high precipitation and a high water table, nutrients are prone to leaching right out of the soil and into the water supply. This is both expensive and can quickly unintentionally turn a garden or farm into a source of water pollution.


Biochar has been offered up as one way to increase soil CEC. The research behind biochar is all over the place with different types of biochar having a wide range of CECs depending on the feedstock and how it was made. Generally, though, biochar's extreme surface area allows for improved nutrient retention while acting as a hotel for microbes. Because biochar can bind with nutrients, it must be charged prior to application. If you apply fresh biochar to your soil, do not be surprised to have poor results in the short-term.


Layers of compost, biochar, and homemade fertilizer in a covered compost bin
Layers of compost, biochar, and homemade fertilizer in a covered compost bin

We made our biochar in a barrel with excess douglas fir firewood. To crush it, we put it in a couple of contractors garbage bags and rolled over it with a big Dodge truck. A handful of passes does the trick. As long as most of the pieces are small-ish, I don't get too myopic about the size. To this, I then add a pared-down version of a complete organic fertilizer. I add soybean meal for nitrogen, lime because our soils are perpetually acidic and calcium-poor and azomite and rock minerals for micronutrients. Because I use composted manures, I assume I have an excess of phosphorus and have been leaving it out of the mix. Then, I just layer everything: compost, biochar, fertilizer, compost, biochar, fertilizer and so on.


Kept covered, this will sit out of the rain over the winter. The redworms present will continue to break the compost down into vermicast. The compost and fertilizer will charge and innoculate the biochar. Late spring, when soil temperatures warm enough to support plant growth, this will be a perfect amendment. A somewhat similar "professional potting soil" costs $15 for two cubic yards. I expect to have 64 cubic yards, a value of about $480. Even considering it is not an identical product and there are some inputs invovled, this still creates significant value for the garden, especially since the compost from the hotbeds grew crops from February through October!






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