I had a look through the list of seeds that can be grown in botany pots from Mystical Agriculture, and saw that both Soulium seeds from the base mod, and the Flux-Infused Ingot/Flux-Infused Gem seeds are missing recipes.

I wanted to put an issue here for posterity's sake, but I'd be glad to put together a PR with the required recipes.

Thanks for letting me know about this. Which version of the game and mods are you using?

I can't speak for the other guy, but I noticed this exact problem in your version for 1.20.1. There are 180+ mods active on my game, but the only relevant ones are Mystical Agriculture, Mystical Agradditions, and Redstone Arsenal.

For thoroughness' sake, I made a new instance of 1.20.1 with just MA+Agradditions+Cucumber library, EnderIO (to test Soularium), and CoFH's Thermal/Redstone Arsenal mods/libraries. The issue is missing recipes, and sure enough it still exists: I can grow MA's EnterIO-compatible "Soularium" just fine, but not the main mod's "Soulium" or the Thermal-compatible Flux-Infused seeds. The seeds just won't go into the pot.

Thanks for letting me know about this. Which version of the game and mods are you using?

Same boat as @SonyStinks, 1.20.1 game version, and a long list of mods (playing the Fantasy Skies modpack), the only ones of import being Mystical Agriculture, Mystical Agradditions and Redstone Arsenal. I also noticed it by looking through the source for the MA recipes, and not seeing the recipes there, but there were some for every other material I checked (both core materials and modded ones, e.g Thermal alloys, EnderIO alloys).

A new update should be on CurseForge soon that adds support for these crops. Please try it out and let me know if it resolves your issue. 13.0.19. CurseForge is still processing the file so the link won't work for a few minutes.