The roots should be so fine (because microgreens) that it shouldn't take too long to become either composted or fragile/fragmented enough to not matter. One of the best things you can do is get your hands in there and break up the soil after you grow the micros in order to break up the mat of roots. The roots should be quite fine, but there should be a huge amount of them which may aggregate the growing medium into chunks/sheets. You don't have to break it up into a fine textured medium, but you want to get some of the larger chunks broken up.

I had essentially the same question, but with wheat grass mats. Look down a couple of threads for one titled "potting mix". Sadly, I don't have a personal answer... I haven't got my works yet!

Wow, I can already tell what a difference the kind of seed can make for microgreens. The 2 big trays I set up last week Thursday... just open on a rack in the South window, watered in well, left open, and misted daily till sprouts started popping. Then I kicked on one of the small grow lights a couple inches over sprouting tray.... The Asian mix I tossed together of tatsoi, chinese cabbage, and pak choi sprouted on Monday or Tues... and is already growing kind of plush. I can tell I didn't distribute the seed evenly over the whole tray, lol. But I'm thinking it's possible I might be adding some greens to dinner for Christmas. The commercial lettuce mix I used just started popping it's first seedlings today and went under the light this afternoon after I got home from work. No wonder why I've done so poorly with indoor winter greens in the past- apparently I was expecting the wrong thing out of lettuce seed. And rummaging through my seed.. it's that planning on seed orders time of year for me. I got a lot of radish seed on hand- enough to garden up and try a tray or few for sprouting. And got lots of different popcorn, including some baby white. Lots of parsley and dill, and basil in various colors. Kales- including about a half pound of some homegrown stuff from a couple years ago. I let the patch go to seed that I had planted in with a bunch of different kinds to try stuff out... and did the chore of harvesting all the pods and cleaning. A bunch of carrot seeds- I'm just not successful with them in the garden yet, but might they make for good microgreens? And I have some brassicas- broccoli, cauliflower, cabbage- to use up for now too. Won't have garden space for them for a year or two till I get the new gardens out back set up. I have almost no peas. What I have is seed stock to grow out next year. And I'm out of nasturtiums sadly enough. Ran through the last of my taste trials this year (4-6 kinds a year for 8 years) , and so it's time to re-order the winners to grow next year. And since looks like nasts can be microgreens too- Milk Maiden, Moonlight, Out of Africa, and Tip Top Apricot are the best tasting nast greens. Orchid Flame is worth growing out for it's mature leaves and blooms better. Pheonix is more ornamental, and it shows. Most of the spotted throat flowers have the nice general thing going on like nice general greens. A notable for a mix rather than a particular variety is Alaska mix- It's tasty from sprout, bloom, greens and seed, reliable, and many companies offer it.

Looks good! Some successes. The top pic has a fast grow and proper density of seed. The others look waterlogged and the chard is sooo slow growing and not enough seed. Too wet. let them dry out to almost light by weight trays. A small fan on low will help. I plant all seed on TOP of the soil, then tamp down to get good contact. Very different than gardening outside. Seems counterintuitive but very different growing conditions indoors. Your spicy mixes, like behind the clay pot in the clear 'shoe bin', looks good...tiny seed so you hit the proper density. The peas could all be in one tray, not three. They get woody fast if too far apart. Learning curve. Proper density they grow tight together and that density creates very tender stems. (less light hits the stems.) Top growth needs light, stem and bottom growth does not. like baby greens, mesclun, the top growth forms a canopy. The stems are very void of light. (tender stems) What is the grassy stuff in the clay pot? Looks like wheatgrass or corn. Only good in a blender drink. Way too tough for salads. Corn needs absolute darkness to be prime edible. Void of all light. Covered by a cardboard box. Commercial growers use a dark chamber with humidity controlled like a big non-cold fridge. Don't give up on ones that don't pan out. On the Johnny's site, click on the 'fast growers' button. Not to necessarily purchase but to get to know what 'grows fast'. ^ It is on the sidebar of the micro green page.