vermiculite and small bark pieces are 2 other ingredients commonly used in mixes. Not sure what you mean when you ask for ingredients that don't change the characteristics of the mix as everything will change the characteristics.

Personally, I wouldn't use either of them. Bound to be some additional sources with better offerings around somewhere. Otherwise, I'd by the basics in bulk and make my own. The Container Gardening forum Anney linked you to offers many recipes for good container mixes. Dave

Surely in the vastness that is Los Angeles County there must be at least one houseplant nursery that sells soil expressly aimed at owners of these plants. Have you tried finding recipes for such soil in garden books and maybe making your own. If you have such large quantity of plants that you need to buy such in bulk, then it would pay you to find a recipe for what plants you have. Remember, some plants require special attention to what's in their soil. For the most part though, commercial potting mixes is the most efficient and I might add...safest way to go. It can provide a formulation that house plants prefer in that it provides all the nutrients they need for a specific time. Here is one recipe you might consider: blend equal amounts of coarse sand, loam or good garden topsoil and peat moss, leaf mold or fir bark. To each 2 quarts of this mix, add 1/2 cup each charcoal and perlite. Each of these ingredients has a purpose. The topsoil contains particles of clay that holds fertilizer and makes it availablefor plant roots. Sand, perlite and leaf mold hold air around the roots which is essential for good plant growth. Leaf mold also provides nutrients. Charcoal bits keep the soil 'sweet'. Peat moss gives good structure to the soil and holds moisture. Any soil you make yourself must be sterilized. The loam or topsoil may contain pests and/or disease and weed seeds. Sterilizing takes care of this. To sterilize soil: thoroughly dampen the mix with water. Spread it out no more than 4 inches deep in shallow ovenproof pans. Place in a 180ºF oven and bake for at least 2 hours. Prepare to have your kitchen stink. An alternate method is to cover the pan with kitchen foil or clear baking wrap and seal. Insert a cooking thermometer through the centre of of the covering into the soil....but not touching the pan. Place in oven and heat until the soil temperature reaches 180º. Keep the temperature at that reading--no higher--for 30 minutes. Remove from oven and let it air for a few days. Store in paper or plastic bags.

I'm not sure what you have in mind for using the potting soil exactly, but I have my own special mix made that I use for my rare cycads. With plants that are worth what these are worth, getting the best soil is well worth the extra money. You mentioned drainage, well, that is the most important thing in growing cycads. That, and having a mix that doesn't break down real fast and get mucky over time. Also, I don't want insects getting in my mix (ants and termites love pine bark)and I don't want my pH to go acid in a year. My mix is mainly Canadian peat, well point sand (very coarse sand, almost small rock)I do use cypress saw dust and also perlite. If you can't find anything good, I am 2 minutes off I-4 in Lakeland and have my soil professionally made for me by a place that has a computer mixing it and I buy 30 yards at a time. If you are just wanting to put up some things, I could spare 1/2 a yard or something like that.