The trickiest part of the whole project was to figure out what kind of soil to use for the planters. I spend two month on it and almost went crazy, while my seedlings where severely outgrowing their tiny plastic cups, waiting to get transplanted.
To support the capillary action of the SIPs one needs to use a material that wicks water well. Normal garden soil doesn't do it.
It could have been so easy and cheap if I would have done what most people that are using SIPs suggested: to use a largely peat based potting mix. But it turned out that peat is nearly as sustainable as fossil fuel, growing about 1 mm a year. Because it didn't make sense to me to finance the destruction of peat bogs so I could plant some vegetables, I had to find some sort of substitute. I had high hopes for coir, a by-/waste-product of the coconut industry. It wicks very well and breaks down very slowly, meaning that you can use it for a very long time. Its dried and compresses before shipping, so its very lightweight and doesn't take up much space. A brick-sized block of coir makes about 8 liters of substrat when soaked in water. People had good results with a 40% coir / 40% peat / 20% perlite mixture in SIPs, but nobody seemed to have tried only coir. The guys from Global buckets started experimenting with it, so did Heidi and Bruce from Green Roof Growers ( check out their blog and rooftop gardens !!! ). I as well set up some test SIPs with 75% coir, 25% perlite VS 75% peat, 25% Perlite. So far none of us had good results with coir. There seems to be something in the coir that stunts the growth of the plants.
For most of my planters I used 40% coir, 40% peat, 20% perlite, 1/2 cup of dolomitic lime ( for fruiting plants ), 3 cups of earthworm casting and 1/4 cup of organic dry fertilizer mixed in + 1 cup of organic dry fertilizer on top of the soil. The plants seem to like it.
Another material that seems very obvious to use is compost, especially because it is cheap, local, sustainable and it would close the food cycle. There are a lot of people saying that compost doesn't work in SIPs, that it doesn't wick well or stay to soggy and that it causes serious problems with fungi growth. Although the people from the Montreal Rooftop Garden Project seem to be successfully using it for years. I also set up some test SIPs with a compost mixture.
There still has to be some serious experimenting done with alternative substitutes for peat in the future.
The guys from Global Buckets are doing something very interesting at the moment, based on research from the Echo Organisation. They are trying to grow plants in trash. The only thing roots need is air, water and nutrients, one really doesn't need something that resembles soil. The Echo website has plenty of documentation on successfully growing plants in anything from soda cans, Styrofoam balls, old underwear, wood chips, carpets and grass clippings. I think they are really onto something and their documentation is very worth checking out. Note that Echo is a Christian mission, I myself am not religious and I don't believe that religion generally is a good thing. They are however doing an awesome job on their urban farming research.
I will post again throughout the year on how the plants are doing in the different soil mixtures that I'm testing this season. I will also post more about the peat controversy.
Next year there will be more experiments.