There are as many recipes of mixes of soil as there are recipes for meat loaf. A basic recipe calls for 3 parts sphagnum peat moss, 2 parts vermiculite and one part perlite. Many add other ingredients such as lime. My water has a very high ph so I do not add any lime.
I use a 3-2-1- mix for all my plants except streptocarpuses where I use 3 parts sphagnum peat moss, 2 parts vermiculite and 2 parts perlite. It is a very light mix but the streps are sensitive to being over watered and with the extra amount of perlite they seem to thrive on it.
Peters Sunshine Peat Moss is getting harder to find in the area where I live. I have been told by store owners that the brands they carry are from the same geographical area that Peters is produced in but I have been skeptical to try it. The old saying is "If it ain't broke don't fix it".
The vermiculite I like to use is course. Again ,it is hard to find but I tried the horticulture vermiculite and it was seemed much too fine and the soil seemed to stay too wet. The best time to find both the peat moss and the vermiculite is in the spring when all garden centers carry lots of supplies.
In the fall I make many bags of mixes and store them in the garage. I live in a very cold climate and peat moss freezes and I have been known to be chopping peat moss in a garage when it was -20 below. Now when I am short of a soil mix all I need to do is run out in the garage and grab a bag of the premixed.
When I bring a bag inside the house, I let it thaw, add water and mix and mix until I think my arms are going to fall off. I like to make sure the peat is evenly distributed and finely shredded. No matter how careful I mix it, the perlite seems to float to the top of the mix so I keep a small container and can add extra perlite if I think it needs it.
To me the real advantage of making my own mixes is that I know exactly what is in the mixes. There are no surprises added by an outside source. I am not sure if it saves any money doing it myself and it is time consuming but it is exactly the way that I like it.
Many thanks to Sharon Johnson for the article and pictures! Top photo is the basic soil mix ingredients, the middle photo shows the strep mix with 2 parts perlite, and the bottom photo is what a good mix should look like completed.