Making Biochar, Rain or Shine
My small scale biochar production cycle is weather-driven. When rain is infrequent, I make char while the sun shines and pour out the new biochar into a framed area dubbed my "biochar patch." Quite a bit can accumulate before rain comes to rinse the biochar and fill up its macropores, making it ready for the next step. I like to allow no more than a day to pass after a good rain before I crush the moistened biochar with a tamper and screen it through a salvaged patio tabletop into a bin. Dry biochar makes too much hazardous dust when crushing and sifting, so rain is welcome, though a couple of dry days are then needed for my feedstock to dry again. This gives me time to split more wood in preparation for the next round.
The admittedly tedious process of sifting biochar by hand has the advantage that any remaining torrefied pieces can be culled and sent back for more charring. Torrefied wood, i.e. pieces not charred completely through, inhibits plant growth, and does not belong in the final product. A less laborious (but more polluting) method of granulation is pouring damp biochar into a chipper/shredder. Occassionaly, I do this and then purge the machine by shredding leaves or sticks to prevent the residual biochar from corroding the metal and to avoid having a black dust cloud emerge when the machine is started up the next time. I've seen others use gas powered portable blower vacs to grind up biochar. (My electric blower may do the trick with dry biochar - something to try out.) The output from my shredder is a gelatinous concentration of biochar particles that needs to be mixed well with the soil or compost so that it becomes aerobic once again.
My biochar patch serves as a unique area of water retention for the garden that I am working to turn into something of a tropical paradise. Aside from getting free water to rinse and moisten the biochar, staying in rhythm with nature this way includes the benefit of a nitrogen charge whenever a new storm comes through. During lightning storms, some of the diatomic nitrogen (N2) that predominates in the atmosphere is ionized. These nitrogen-rich ions become entrained by raindrops that deposit it on the soil where it is mineralized into a form that plants can use and biochar can store as a slow-release fertilizer. If enough rain falls on biochar, I suppose it accumulates a sufficient charge of nitrogen to make it garden-ready.
Particles needn't be so fine as from a shredder when biochar is added to the soil. It helps earthworms to ingest them, but the larger (< 2 cm) particles that drop through the expanded metal screen used in a manual approach will break down from freeze-thaw cycles over time and the larger size doesn't reduce biochar's effectiveness in the meanwhile. Some situations are well suited to top-dressing with raw biochar in bulk form. Weathering eventually breaks up these unground particles, so they leach into the soil beneath. In the interim, they acquire nitrogen from above and pioneering microbes from below that make this labor-saving method of biochar application most economical. In my next post, I want to talk about one more reason top-dressing could be preferable to burying biochar.