In the past I’ve just used off-the-shelf mixes such as Miracle-Gro, and while it did grow some plants I also lost a lot of them before they even went outside. Swan Island recommends using a mix that doesn’t have a lot of added fertilizer, so for 2018 I did my own mix that’s not as strong. It worked out really well, so I’m using it again in 2019.

I use 4 parts coir (ground-up coconut husks) and 1 part Ocean Forest. The coir is basically just a loose filler that doesn’t have added nutrients, and the Ocean Forest is an off-the-shelf potting mix with things like earthworm castings, bat guano, and fish/crab meal.

potting soil supplies

The coir comes as a compressed block. You add water and it fluffs up a lot. The usual way involves mixing it with 5 gallons of water in a wheelbarrow, but that’s way more than I need at once so I just process it in pieces.

The block is difficult to cut – I’ve tried a large serrated knife and even an electric carving knife, and they barely made a dent. The best way to break it up seems to be hammering a screwdriver into the side of the block and then tearing the layers apart. Once you get a gap started, you can break off chunks pretty easily.

breaking apart coir brick

The pieces soak up water like a sponge and quickly expand into a loose pile:

coir test chunks dry
coir test chunks wet

I keep a few gallons of expanded coir in a plastic container and add more chunks and/or water as it gets low. I can then mix large scoops of the damp coir and Ocean Forest in another container as I need it, usually in batches large enough to do maybe 10-15 plants.

One nice thing about this mixture is that I see very few fungus gnats compared to when I was using off-the-shelf potting mixes.