I too have been working on creating a 100% locally sourced potting mix. Drainage is my single largest issue. I sourced some very coarse sand from a local commercial pit that is up to 1/8" and it works, but the resulting mix is physically too heavy in large pots.
The following is not 100% local because of the perlite, but O my, can it grow seedlings.
2 parts composted shredded hardwood arborists chips. See note 1 below.
2 parts high quality compost (mix of Johnson-Su, hot and vermicompost)
1 part coarse perlite. See note 2 below
Note 1: These wood chips were left in a wire cage for two or three seasons (winter snows and summer rains) until quite dark, then I put them through my chipper. The resulting product looks much like peat moss in texture. I've made MANY different kinds of compost, including Johnson-Su, but I've never seen any product (under my microscope) with such a healthy spectrum of soil biology.
Note 2: I'm working on replacing the perlite with biochar (well activated), but I've encountered two issues. First, too much char (10%) keeps it too wet... I think about 5% will be best. Second, two out of three attempts to effectively inoculate the raw char have failed, and thus the plants fail to grow. Seems the char is still inoculating and sucking all the nitrogen out of the soil to the detriment of the plants. I've tested this by transplanting stunted plants into biochar free soil and they thrive.
