- Sphagnum Peat Moss: good for absorbing and retaining moisture, also slightly acidic (which most veggies like);
- Perlite: expanded volcanic rock, reduces compatibility, improves drainage and keeps soil mix light;
- Vermiculite: mica, expanded by heat, absorbs moisture, provides air spaces, contains nutrients potassium and magnesium;
- Zeolite: aids water retention, holds and releases nutrients, contains calcium, magnesium, sulphur, iron, and other minor trace elements, "helps split roots to create heavier healthy fine roots", and helps balance pH.
I paraphrased this from the bag, I don't actually know this from memory or what exactly it all means. I can definitely see a theme of "lite" ness, so perhaps a "heavy" soil is not so good for getting plants started.