I recently decided to give raising carnivorous plants another shot and found out quite a bit about the potting soil and other requirements. They do vary some between the different plant species but there are certain elements in them that are the same. I'm caring for Venus Fly Traps and a Sundew variety that should do OK in the same soil. So, that is the specific mix I went for.
In the course of learning the "best practices" way of doing this I spoke to peat producers, professional plant growers, a couple botanists, sales people and several enthusiastic and helpful hobby growers. Everyone had an "I killed my plants story." and everyone had something interesting to add.
Step 1: The Components and "tools"
Peat Moss- see below
Sand- See below
A large mixing bowl
A measuring cup of some sort- I used the bottom half of a glass soda bottle.
Peat moss, specifically sphagnum peat. Not to be confused with green moss, long fibered peat or carpet moss, all of which can be found in bags calling them "peat moss". There are differences and they usually have negative effects. Almost all are not immediately evident and will only show up after weeks or months of seemingly healthy growth though some chemicals that are found in certain products may kill your plants very quickly.
I used Uni-gro premium organic peat. It's distributed through L&L Nursery Products here in California. It doesn't state the source on the package so I called them. It's Canadian sphagnum peat that comes to them from a company in Alaska and is then packaged by L&L for resale. This is a good one for US growers. No chems, fertilzers, wetting agents, just peat moss. It was just a few dollars for a 4 quart bag.
The other component I used was a very clean chemically inert black cover sand from Mosser Lee. Again, no chems, additives, fertilizers or anything else. It's cleaner than the washed sand recommended by more sources than any other component and it's non-leaching so you won't need to worry about mineral content over time.
Distilled water. Yes, you need this to make the soil. Rain water will do but it is so easily contaminated you'll either need to control the harvesting process carefully or just go ahead and buy distilled water. This is not the same as bottled water and chlorine isn't the big offender, so setting a bucket of water out for 24 hours the way you might do for fish will not make it safe for your plants. Many bottled waters have salt added for taste. If you must, you could buy low sodium drinking water but it's a poor risk with distilled water easy to obtain. Soft water is not acceptable for the same reasons. While these things might not kill your plants right away, some will and all they will over time. Again, the salt in Bottled water added for flavor is negligible as a food additive but will accumulate over time and harm your plants.
Notes on other materials:
Perlite or pulverized lava rock may be used in place of sand if it is additive free. Just make sure it doesn't have a fertilizer component like miracle grow and maybe call the producer and bug them about salt content as well.
Coconut husk will work but requires several rounds of soaking to force out the small amounts of salt that are present in the coir. If you really want to use coconut fiber and have an extra month to process it there is information out there. Using it as is, out of the package, will likely kill your plants after a few months of watering when the salt finally starts to effect the roots.
I did not do a soil salinity test on any of the materials but did do a pH test on the final batch for the plants and based on direct information from the producers of the products was assured that there were no salt or chemical additives.
Step 2: The Mix
For Venus Fly Traps you probably want to use a 50/50 mix of peat to sand. It should be noted that this is 1/1 by volume not weight.
...it should also be noted that it isn't 1/1 right out of the bag either. To get a proper mix you need to wet the peat and then measure the amounts by volume. This isn't as simple as pouring in some water so on to the next step.
Step 3: Wetting the Peat
Peat is actually not that easy to wet. In his book Savage Garden, Peter D'Amato suggests a fairly workable method.
I added a dry cup of peat to mixing bowl and then an equal volume cup of distilled water. After mixing by hand and holding the peat under water, it tends to float, I ended up adding another measure of dry peat and gently mixed until the peat was damp.
I then pulled wet peat from the mixing bowl with my hands letting it drain a little but not wringing it out too much until I filled the measuring cup again.
it ended up being roughly two dry measures of peat to one dry measure of sand to get the suggested 1/1 soil mix as per several written sources including emails from a commercial grower and one phone conversation with a botanist.
The crux of this is that the information is out there but preparing peat is rarely a step spelled out in making the potting mix. It's assumed that you have already prepped it. Not a good assumption.
Since bags of peat don't have built in hydrometers it's best to just wet it and use that as the mixing benchmark.
Step 4: Mixing the Soil
This was as simple as it sounds. I just put a plastic bag in another mixing bowl and poured in the sand and wet peat and then mixed by hand until it was evenly distributed.
It looked pretty good but the black sand made doing Munsell color rating pretty pointless. While as noted earlier, I did not do a soil salinity test I did perform a pH test and came up with an appropriately acid soil, somewhere in the neighborhood of 5.5 and 5. Could have been a bit more acid but it should be fine.
There was actually no driving need to check salinity since I knew the source of my peat and that the sand was additive free and chemically inert.
Step 5: Next Steps
I'll try this mix on the plants I have an maybe a few new ones and try to remember to update this in a few months.
I'm also planning to make a second batch using some additive free Perlite I found and will update at some point if I manage that one as well. (Just ordered some Spoon Sundew seeds and should have enough for test plots. So, we'll see how it goes.)
Good luck with your CP's and make sure to check the best ratio for your species. Some need a more sandy mix than others.
Further reading I'd suggest:
The Savage Garden by Peter D'Amato or his website for California Carnivores.
You can also try the website for the International Carnivorous Plant Society