Mon, 24 Nov 2008 14:34:43 PST
Interesting to read about the decline and fall of Supersoil.

Some very green-thumbed growers here use commercial Sunshine Mix, peat 
based, but I prefer to mix my own starting with "Island's Finest Top Soil." 
This raw material is fairly cheap, typically $2 or $3 for a 30-liter bag. It 
appears to be excavated from an old lake bed, so it's a natural mixture of 
peaty materials and silt. Heretofore, there's always been enough rough 
twiggage in it to demand that it be screened before use, but this year no 
such problem.

My target is a mix that is close to the famous John Innes potting compost 
without getting involved in the arcane intricacies of Cornish silver sand, 
stacked turves, and hoof-and-horn meal.

Here's the exact recipe I use these days:

30 liters of soil
16 liters of horticultural perlite
150 g of organic 4-6-8 vegetable fertilizer
170 g of lime
7 g of fritted trace elements


1. If the perlite bag doesn't say "horticultural", it may contain soluble 
fluorides. These won't cause a problem if you are pouring the stuff into the 
wall cavities of your house as insulation, but your plants will take 
considerable exception. My original recipe (which I may have posted here 
before) only called for 6 liters of perlite, but I found the result was somewhat 
too peaty and airless for comfort. The increased quantity is simply based on 
"how the stuff feels" when you rub it between the palms of your hands.

2. I use an organic fertilizer in the belief that it will break down more slowly 
than a fertilizer compounded primarily from soluble salts.

3. Lime. I use ground limestone or agricultural lime, not dolomite. The 
purpose of the lime is to bring the pH of the finished product up to about 6.5; 
dolomite is too insoluble to do this. I recommend to those who also mix their 
own soils that they buy some pH testing paper and use it to verify the pH of 
their final product. The lake bed material I start with has a pH around 4, very 
acid, so requires a surprising amount of lime to bring it up to near neutrality.

4. Fritted trace elements. This takes care of any trace element deficiencies 
but is little enough it won't overdo any of them.

Rodger Whitlock
Victoria, British Columbia, Canada
Maritime Zone 8, a cool Mediterranean climate
on beautiful Vancouver Island…

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