Kedushas Levi on Parshat Korach

(19) All the sacred gifts that the Israelites set aside for ha-Shem I give to you, to your sons and to your daughters that are with you, as a due for all time. It shall be an everlasting covenant of salt (בְּרִית֩ מֶ֨לַח) before ha-Shem for you and for your offspring as well.

(13) You shall season your every offering of meal with salt; you shall not omit from your meal offering the salt of your covenant with God; with all your offerings you must offer salt.

(5) Surely you know that ha-Shem God of Israel gave David kingship over Israel forever—to him and his sons—by a covenant of salt.

You shall not cease to salt the offering, it is the covenant of your God. "Because a covenant was made during creation, assuring the subterranean waters that on the altar would be offered salt [from the sea] and a libation of water during the festival [of Sukkoth]." This is the language of Rashi, and it is a midrash of the sages. Rabbi Avraham [ibn Ezra] says that the plain meaning is, "God has entered you into a covenant and sworn you not to offer a bland sacrifice, nor should such a sacrifice be eaten because it is humiliating." And because there is a sacrificial covenant, the Torah also uses this covenant as a model for other covenants, as both the priestly covenant (Bamidbar 18:19) and the Davidic covenant (Divrei ha-Yamim II 13:5) are called `covenant of salt' because they are upheld just as the sacrificial covenant of salt. But Rabbi Avraham [ibn Ezra] explains (Ibn Ezra on Bamidbar 18:19), "A covenant was made that land (Yirmiyahu 17:6) should be barren [of produce, just as a field of salt does not bring forth grasses at all, or, similarly, a place of sulfurous salt.]" His reasoning is incorrect, because the verse here says, 'covenant of your God,' and not 'covenant of Hashem,' as is the language of the passage and the general tendency of the sacrificial instructions, or even `covenant of Hashem your God.' I am of the opinion that the significance is that the salt is water, which by the power of the sun that comes is made into salt, while the water saturates the Earth, impregnates it and makes plants flourish. Afterwards, salt cuts through every place that is not seeded and does not flourish. This covenant is a part of all of these processes, and the water and fire come together in it. {Unclear phrase}. The first government of the kingdom of Hashem came like salt, which gives taste to all kinds of foods, and upheld it and agreed to its salting. The salt is like a covenant, so the Torah says, (Divrei ha-Yamim II 13:5) "Surely you know that Hashem, the God of Israel, gave a kingdom to David over Israel eternally, for his descendants, a covenant of salt." Eternally, because this is also an aspect of [the] David[ic covenant]. Similarly it says about the offerings (Bamidbar 18:19), "it is an eternal covenant of salt," because the covenant of salt is in the world upheld. And I have already taught (Ramban on Shmot 31:13) to understand from our words in other places this exact phrase: "it is an eternal covenant."

“It is an eternal, salt-like, covenant ‎before Hashem.” (Bamidbar 18:19)
This statement had become necessary ‎after the revolt of Korach, who claimed that all Israelites are ‎entitled to be priests by dint of all of them having stood at Mount ‎Sinai. Korach had considered his status as a Levi, as a demotion. ‎He had not understood that the priests symbolize the attribute of ‎חסד‎ loving kindness, whereas the Levites symbolize the attribute ‎of ‎דין‎, justice. If both the Levites and the priests had symbolized ‎loving kindness, there would have been no room in the world for ‎the attribute of justice. ha-Shem’s universe can function only when ‎both these attributes perform their respective ‎functions.
Ramban, commenting on the significance of ‎מלח‎, salt, writes that salt is a combination of the raw materials ‎fire and water, i.e. justice and loving kindness. Between them ‎these two “raw materials” are the foundation of our civilization. ‎This concept had to be brought to the attention not only of the ‎Levites, but to the attention of all the Israelites, so that they ‎would understand that both the role of the Levite and the role of ‎the priest are indispensable to the Jewish nation, and are not to ‎be regarded as part of a pyramid, the priests representing the ‎pinnacle.‎

"You shall surely salt all of your sacrifices." In order to coronate God as master of all opposites which are found in the world and which have caused many to utter the heresy that one divinity cannot be the source of two opposites. Behold, salt holds in its very nature a thing and its opposite, for it has the power of fire and warmth, and it also has the derivative of water. The Kabbalists say that these are akin to God's attribute of Justice and Mercy. Thus, it is called the Covenant of your Lord because when you bring salt on the alter, you are coming into a covenantal relationship with God and giving him reign over all opposites. All Mincha offerings (except that of the Kohen) were given to the Kohen to eat. Thus, the Mincha is a kind of Tzedaka which itself is compared to salt on account of its ability to preserve and sustain meat. If you lack salt, you lack money. The Tzedaka of the sacrifice is greater than the sacrifice itself as it says (Mishlei 21:3), "To do what is right and just is more desired by God than sacrifice." When the verse says, "With all of your sacrifices bring salt," it means that the Tzedaka (salt) which is brought with every sacrifice is more dear to God than the sacrifice itself. The simple understanding of "All your sacrifices" includes even meat sacrifices (and not just Mincha offereings). (DBS translation)

The sages of Athens then asked another question: When salt is spoiling, what do we use to preserve it? Rabbi Yehoshua said to them: With the placenta of a mule. They said to him: But is there a placenta of a mule? Rabbi Yehoshua said to them: And does salt spoil?