Streetwind

Not sure that's the best way to go. Iron is (currently) endgame content, and generally takes people far longer to reach than obtaining high-output farmland. I've only just found my first cassiterite after 20 hours in my singleplayer world, and I have four 12-block terra preta farmland plots by now. As the purpose of fertilizer is primarily to help with lower-grade farmland, that means it should be available relatively early, not gated into the iron age. Primitive Technology also regularly works with wood ash using clay pots and stone age tools, zero metal of any kind, so there's no practical limitation here. In my personal opinion, there's enough other crafting mechanics that clay pots could do (like rendering fat into tallow) that it would be well worth Tyron's time to make the pots more flexible under the hood. I guess we can only cross our fingers and hope Gameplay-wise, I would much rather be able to work with an inferior fertilizer relatively early, than wait a long time to get something that is as good as something else that is also difficult and time-intensive to obtain. Why have both items fill the same niche? Vintage Story's mineral potash is a limited-access, technically nonrenewable resource. It should be the clear king of K fertilizers. Your white potash is an easily-accessed, infinitely renewable resource; it should be clearly inferior, even if real life perhaps disagrees. We're playing a game here, after all. So I'd rather see something like wood ash being 10% and white potash being 30% where mineral potash is 60%, than white potash being too similar to mineral potash for comfort and then gated behind some lategame mechanic or resource in order to keep it balanced. A low-grade fertilizer can still be quite effective. Consider flax, which requires K nutrient. Everyone will agree that it's a key resource. String and cloth do so many important things, and there are no real alternatives. So flax it is. The stone age player will likely plant it on medium farmland, as that can be reliably obtained where terra preta takes time and luck to collect in any useful quantities. And when you do plant flax on medium farmland, you'll find that the first seven or eight out of nine stages grow reasonably well, while the last one or two stages stall for days and days on end because the K nutrient concentration fell too low. It would just take a small bit extra to make it all the way. Even a low-performance K fertilizer will have a tangible impact on flax farming, especially if it is renewable and easily accessed. So it doesn't actually need to be competitive with mineral potash as a high-end solution in order to have its niche.

Out of curiosity, would it be possible to implement the lye cooking in a pot? It would feel more immersive than just effectively pouring the stuff into the fire. Of course, pots already have the meal code attached to them, so I don't know if you can even give them a recipe that doesn't follow the meal format... If it's possible, the output item could be a pot you can place and rightclick with an empty hand to take the white potash from, whole stack at once.