This is a really helpful thread. I’m very new to landrace gardening, but seed starting quickly became one of my biggest questions. Based on the overall landrace philosophy, I tend to agree with those of you advocating for native soil. I’ll be trying a few different things next spring and see how it goes.
Reading this thread I had the hypothesis that maybe seeds produced by plants that were direct seeded outdoors will do better than random seeds when planted indoors in native soil.
For example, this year I direct seeded about 80-100 tomato seeds this year and only 20 plants germinated, with only 4-5 that might produce seeds.
More generally, I see potential in having hybrid production systems, one side for adapted seed productions, and one side for crop production. Seeds produced under selective pressure (low fertility, shorter growing seasons, non standard starting soil, unweeded beds,…) could allow us to relax a bit more on our inputs when we are focusing on producing lots of food.
We decided to make our own starting mix too.
I’ve done it with a young farmer who wants to have a lot of start up plants next year. Me i’m just sick of buying stuff expensively of dubious quality and sometimes terrible even.
So our recipe is :
1 riversand,
2 year old cow manure that lay in the field,
3 soil from an acacia tree grove,
4 soil from a alder grove,
5 few month old alder wood chips.
1 riversand, 1 manur, 1 mix of the soils and woodchips until it started to look and feel nice.
We used a tractor mixer and a excavator to dump it in there.
I love this next level i get to experience with farmers. No petit mixer for cement. No gigantic stone crushing blades ready to rip arms off if not careful!
So how did it turn out? Quite poorly. The mix without woodchips got like a mad ball of clay just turning around in the mixer, the woodchips saved the day. It became superbeautiful mix. But… it hadn’t broken down enough.
It got warm soon from bacterial activity. I did an experiment, side by side i put pumpkin seeds in a balcony tray full of it and another one with my normal old mix of a third sand with gardencenter ‘‘biological’’ compost. Our new mix won, seeds sprouted quicker, but soon they stunted and got yellow. But the funny thing is that Robinia Pseudoacacia seeds started to sprout. Which i had tried before and failed. The long moving around roughly in the mixer must have scraped off this super tough outer layer the seeds have.
Next time better!
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this mixture seemed nice, but may be a bit too rich given the behavior of the seedlings. 
In my eyes, is missing an essential element: the local earth. Maybe incorporate until I find the right percentage but I would tend to already test with at least 50%.
Young seedlings do not need much fertility. But your mixture may be great for repotting on older plants.
Here I had great inverse results with old compost then the plants grow less and less and die because the mixture becomes more and more drying… I rectified with 50% local clay soil is it works well for seedlings 
in any case great initiative to become independent of the commercial product that contains peat which destroys these wild environments for extraction, not counting the city green waste compost full of fuel and other salt from road… 
A farmer had bought a great organic soil for his company, everything die slowly… he had it analyzed and there was as much salt as in the sea 
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I’m glad to see new activity in this thread. I’m continuing to shift towards local starting mixes with local ingredients and I think I still have learning and experiments ahead of me.
Typically now when starting seeds I’m using a blend of at 40-50% commercial mix with the rest local sand and soil. I have found this to be better than commercial products.
I’m expanding my small perennial plant nursery and I’d like to be mixing soil later this year at the scale of Hugo’s last photo. I normally don’t have tractors or other equipment, but I may have some opportunity coming up to work with an excavator or something similar.