Some of the author's finished and pulverized biochar.

By Kai Hoffman-Krull

An article in the January 2018 issue of Growing for Market references the author's biochar-based potting mix. Here's the recipe.

We and one other farm on Waldron Island, Washingon, called Nootka Rose, have been experimenting with using biochar in our seed-starting soil. We’re finding that it can make up to about a quarter of our mix. We got our mix from Nootka Rose, so here it is:

  • 15 (five gallon) buckets of worm castings, made up of 1/3 biochar
  • 3 buckets of sand
  • 1 bucket of biochar, crushed
  • 6 buckets of peat
  • 17 pounds of fertilizer

For the fertilizer, we use:

  • 6 lbs of blood meal
  • 3 lbs of pulverized lime
  • 3 lbs of bone meal
  • 3 lbs of kelp meal 
  • 2 lbs of 8-2-4 chicken manure-based fertilizer 

For the worm castings made up of 1/3 biochar, the char is added to the worm bin. So 1/3 of the material going into the bin is char. You need to keep supplying them with plenty of food, as they don't eat the char, but the char serves as a great medium for the worms to move around in, and keeps the worm castings from getting slimy.

Being able to make a quarter of our potting soil from our forest has been a financial game-changer for us, and is providing a truly quality product. And it only takes one burn to make all the charcoal you’d ever need for a spring planting season. The best part—you get to go hang out by a huge fire a few times each winter. A bonfire is not the worst way to spend a cold day.

Kai Hoffman-Krull runs a market garden with his wife Sarah on Waldron Island, located in the San Juans off the coast of Washington State. Starting with raw forested land four years ago, they integrate vegetable and fruit cultivation with wild foraging to supply their farmers market stand and restaurant accounts on neighboring islands. Kai studied soil science at the Yale Farm and Forestry School, and served as a manager of the Yale Farm from 2010-2012. He spends most of his current days developing the farmstead’s water system, building structures, and knowing he should care more about weeding.


For the link to an article about the author's participation in an on-farm trial of biochar, click here.

For the author's article about making and using biochar, see the January 2018 issue of Growing for Market.