Potash is a mineral that can be created in Vintage Story. Its ore is called sylvite, which can be found inside of halite deposits.
Potash is made from sylvite ore, which is found in halite layers. Sylvite can be found as surface bits, or as ore blocks. Once broken, these blocks drop pieces of sylvite. These sylvite pieces must then be ground to create potash. One piece of sylvite creates two pieces of potash.
Potash's only use as of version 1.16 is as fertilizer. Uniquely, it permanently raises the maximum K capacity of a farmland block by 15%, in addition to adding 60% K to the block.