Salt is the body of any form of being, it is the vessel that holds the Mercury (Spirit) and Sulfur (Soul). Of the three principles, it is the most recently developed, added to the lexicon by the great physician and father of iatro-chemistry (pharmacology) Paracelsus.
The Salt of any form is the most terrestrial, or fixed, of the three principles as it is the vehicle for the Soul and the Spirit. Composed of natural elements, it is here before we arrive and remains after we die. It is the literal salts of the earth.
When the Sulfur and Mercury have evacuated the deceased body (corpse) they leave behind the minerals and elements that housed them for their terrestrial stay. When the body of anything is burned it produces an ash. That ash is composed of Carbon and salts in varying forms.
The ashes of the body can be refined further through leeching the salts: by dissolving the ash in water, pouring it through a filter, and allowing the water to evaporate. Followed through multiple times, this leaves you with pure water soluble mineral salts. In the plant kingdom this will yield various different types of salts, but primarily Potassium Carbonate (K2CO3).
Again this leads to an area of necessary research as it comes to the fungal realm. There are undoubtedly salts of varying forms, but as to which specific salts are present there is a need for further investigation and testing.
Philosophically this is the most physical application of the fungal kingdom as it relates to the earth. Primarily I think of Myco-remediation: the application of mycelium to remediate polluted environments and landscapes. Also, closely related here, is the understudied Myco-biome the fungus internal and innate in our biological processing and functioning.
Secondly I think of medicinal preparations of the mushrooms and cultivation methods in their most basic form. It would be good to note here that the more individual properties of each of these applications relate more to the Sulfur dynamic, the soul of the individual mushroom, or its unique expressions and needs. But as a whole, I find these processes fundamentally as correlated with the Salt principle in Alchemy.
I perceive the fungal realm as the Salt of the earth in enabling us to live and walk in a good way on this planet as we learn how to ally with this beneficial force.