There are about 7-1/2 gallons in a cu ft, so 4 cu ft gives you 30 gallons. If you need 1 gallon of peat and 1 gallon of perlite for every 5 gallon lot of bark, you divide the 30 gallon total by 5 gallons (in each lot) and come up with exactly 6 lots. @ 1 gallon of peat and perlite per each of 6 - 5 gallon lots, you need 6 gallons of each. 'Gallon' is always a measure of volume. Al

I thought I posted this on the gardenweb forum. Now it's houzz? Had to sign up for a new user name. Anyway soo - you can get the Naked Gardener Compost or Debco Premium Potting Mix from Masters (closing soon). Course Perlite is from The Perlite & Vermiculite Factory Myaree (much cheaper than anywhere else). Sand I got from a bulk landscape place (lawn mix) but you can get 10 or 20kg bags of builders sand at Bunnings. Zeochlor is in the pool section at Bunnings. Rock Dust I got from the manufacturer direct somewhere in Bibralake (called Natural PK) but I think but you can now get a similar product from Waldecks (in Myaree at least). Waldecks also sell bentonite clay. I still want to try the gritty mix but getting my hands on all the materials has proved very difficult when I tried last year. I found a Queensland company that makes course DE (for mopping up oil/chemicals) but they didn't have a distributor in Perth. Nobody I could find has pine bark in the right size - and even if they did I'd have to by a cement mixer and modify it with a fine wire to screen it. Honestly, if an agricultural company were to produce the gritty mix and sell it in bags I think they'd make bank - even considering how long it lasts.

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.

My experience is with Promix bought in bales, that seems to work fine. What are you trying to grow? Maybe you will want to just buy the components in bulk and mix up your own. You can grow a lot of different kinds of plants in bark mixed with sand, with slow release fertilizer added. Use a higher proportion of sand for those types wanted more drainage or aeration than usual.