Could I make absolutely FREE quality potting mix from what's around?

I'm wondering if I could make quality potting mix from what's naturally around. The natural resources I have in my area are:

1. Lots of pine needles and rotted pine compost beneath them.
2. Homemade compost.
3. Worm castings (my worms don't produce much though).
4. Fallen deciduous leaves, and the leaf mold beneath them.

The following are a bit of a drive but doable:

5. Horse manure
6. Seaweed/ seagrass.
7. Beach sand. I'm trying to find river sand.

Could I make a decent potting mix out of all these? Or do I really have to go buy stuff?

to me potting soil is compost and all the above you listed mixed in to make the compost.....lol
I would be very careful of the pine. tends to be more acidic.

The store bought potting soil had been cooked. They do this so that it's sterile and won't infect your plants with fungal or other deleterious organisms. Or let volunteer seeds grow.
The bad thing about this is that it's sterile... it's dirt (dead), not soil (living).
Here's a video on making potting SOIL at home:

1 part compost to 2 parts coarse sand makes fine potting soil.

If you make biochar, it's a great addition. Also free.  

Most of my potting soil is compost made from chicken litter, wood chips, and biochar. If the wood chips aren't all the way broken down, I don't even add sand because the wood chips make it drain well.

With the addition of bark, gravel and charcoal, you probably have all the ingredients for a few types of potting mix: acid/alkaline loving plants, orchids, bulbs, seedlings, even cacti and bonsai.

It's just about using the correct recipes.

Manures should be composted, fresh manures in potting mix will likely kill plants. Though I have placed 1 or 2 dry nuggets of chook poo on the tops of pots just as slow release fertiliser.

Seaweed is a conditioner and should be laid on tops of gardens, not buried. Never heard of it being used in pots.

Beach sand, like seaweed, should be well washed before use, though it's often too fine for most mixes except perhaps seedlings.

hau Tim,  I think the key to your question is "your definition of Quality Potting Mix".

It is very possible to make a great potting mix with the components you listed, but you would possibly run into a soil drainage problem as in, not enough drainage, but that is really a rather small concern since containers hold small amounts of soil.
Pine needles will give more acidity than the rotted pine "compost"( detritus ), I'd hold back the needles to be used as a mulch to keep the containers from drying out too fast.
Worm castings are great, they will provide members of the microbiome we want in quantity and they provide minerals.
I would stick with the leaf mold and leave the fallen leaves behind so they can become leaf mold.
To all of the above, you will still need some soil, not a lot but you do want to have some since the microbiome lives in it.

Others have mentioned many great amendment items, of them all, the char is the one that would give you multiple benefits and your plants would like that too.

With the addition of some char along with a well rotted or well composted manure, you would have a mix that provides everything a plant wants to thrive.

Redhawk

i like using %20 local soil.
most of it i use has had lots of organic material in it over a period of a few years
and the underlying soil is fine sand with a little silt+clay.
sugar sand (large particle sand) like most beach sand, is excellent IMO, its a great size for roots.
i would make sure its washed if it came from a salt-water beach.

composted leaf mulch is excellent, feeds the plants, and adds lots of microorganisms...

i get 100lb of coffee grounds from starbucks every couple of weeks.
(sometimes they will save it for you, if you ask nice), or just stop by often.
used-coffee-grounds (UCG)  usually need to be dried, if not fully composted (best)
%20 max on the UCG (%10 to %15 is better IMO)

lava rock on the bottom of containers helps drainage... the roots will actually hold on to the rocks getting into the pores.
i use lava rock and leaf mulch on the top also to prevent water loss.