Making my own potting mix

I need bulk potting mix because I am selling plants. Ready made is far too expensive $80 the m3.
Here are the ingredients I have at hand:
- hardwood sawdust
- horse manure without straw picked up from a paddock
- woodchips (pine or gumtree)
- sand which has some clay content/very poor sandy soil
Can I make up something suitable using the above ingredients?

That cubic meter of pre-made potting mix will fill 2,197 three inch pots. Seems like a sweet deal to me. That's about 4 cents per pot.

I make my own potting mix which is approximately 50% sandy garden soil, pasteurized to kill weed seeds, and 50% leaf mold compost. I wouldn't add any type of wood product to a potting mix because the microbes trying to digest it sap the nitrogen from the rest of the mix. I don't use horse manure at my place because of previous problems with it being full of weed seeds. I suppose it depends on how composted it is already.

I don't trust the local suppliers of composts and potting soils to provide products that are free of industrial wastes, and pesticides, and heavy metals, so I make my own... Early on, weed seeds germinating were my biggest problem. So I started pasteurizing the screened soil at about 140F for a couple hours. I also had problems because the leaf-mold was not fully composted. These days patience is the name of the game for me when making potting mixes. I'll pasteurize the soil, and add the well composted leaf mold. Then stir it for weeks, keeping it watered so any residual weed seeds germinate, and then do some germination tests in each lot to make sure that the plants grow properly.

I think that sand and well composted horse manure would make a great potting mix.

How do you pasteurize? With black plastic and sun? And I guess you only pasteurize the soil bit and the leave mould you add afterwards.
My experience with horse manure is a lot of weeds. I used it fresh which was not a good idea.
I mainly use pots with a diameter of 10cm or so. But, in the end I might be better off purchasing some, which is a matter of time as well.
The landscape supplier says it is the premium mix but I did not find it extraordinary.
Otherwise I have very good experiences using cocopeat. You can discuss weather it is sustainable or not, but it is definitively not cheap.
I mixed the cocopeat more or less with what I had like horse manure and soil.
I think I have to concentrate on making a lot of compost and fast.

I pasteurize dirt in the oven in a large roasting pan. I add a cup of water. I bake at 225 F for an hour (thermometer in soil reaches about 140 F) and then turn the heat off so that it cools down slowly. I add leaf mould later. I can only make it in small batches so time-wise it's a loosing proposition, but at least I know what went into it. I love coco-fiber! Too bad it's grown so far away.

^^^^ i've done that before ^^^ its kinda a pita, but it does nicely cook out all the funk/insects/insect eggs/weed seeds out of used recycled bag soil/soil/ or whatever...
when re using soil, or any soil thats outside...i too many times have made the mistake of planting in that only to have bugs in the soil, or maybe its from bug eggs in the soil...especially to start seeds inside...and then theres bugs inside...you know they came from the used soil/outside soil....

its much easier to do outside, if you can have a bonfire or a pit fire and then cook your soil/potting mix on that. even doing it right on top of the fire and letting it smolder and cook some, or using a big pot like contraption to hold the soil over the fire....

i also love the coco stuff, i think its some of the best additive type stuff you can buy and works well in potting mixes. if only it werent so expensive and not local.

even though its so simple and obvious...i have never pasturized soil with the black plastic covering and let it sit outside in the sun method...i should try that out someday...it seems like an awesome way to go...without having to actively do much...

It's now autumn here but we still have some fairy hot days. If you life in a really cold climate you probably could freeze some insects out as well, by spreading thinly.

Really good seed starting mix needs to do just a few things.
It needs to hold water (vermiculite is great for that and pretty cheep).
It needs to support the growing stem from below the cotyledons, sand or a very sandy loam, either mixed with vermiculite do a great job of this.
Seeds provide enough nutrients so they can get off to a good start, no fertilizers or extra nutrients needed. Nutrients become needed after the second set of true leaves come out.

The way I like to start plants is in commercial trays (75 starts to a tray, this is manageable).
I use a blend of sand and vermiculite to fill the trays, drop seeds into each divot and water.
Next I cover the tray with saran wrap, this keeps the moisture in.
The trays do get bottom heat (I use heating pads covered in plastic, these are cheep, I can regulate (with a thermometer) the soil temp enough to ensure sprouting.
Once the seeds sprout, the plastic wrap comes off and they are misted two or three times a day, depending on the weather.
When the sprouts are up and the second set of leaves are coming out I prep 4" pots with potting soil made up of 1 part sand, 1 part compost and 1 part leaf mold (mix very well before putting into pots.
Transplant seedlings to pots, water and watch them grow.

This method works well for both my garden and it works well for plants to be sold.
For selling, peat pots are ok, but I prefer the cheep plastic types, they are sturdy enough and easier for the buyer to transport home.

You can also get the commercial trays with 277 or so divots, but these work best if you have a seed dropper, otherwise they are tedious to use, and you need a commercial kitchen size roll of plastic wrap, which are somewhat bulky and more expensive on the front end.

With the mix I mention, you don't have to worry about weed seeds, don't have to sterilize (pasteurize) the medium and it holds lots of water with the vermiculite in it.