How Salt Lose its Saltiness: Is it Possible?
If the reference was about an alternate usage such as heating the oven, then it would be of no problem if it loses its saltiness, since it was not meant for condiment, but for another purpose. You wouldn't blame the salt if you are utilizing it for some other use, rendering it inedible. The common explanation is as follows: NET Bible footnote on Matt 5:13.
13sn Salt was used as seasoning or fertilizer (BDAG 41 s.v. ἅλας a), or as a preservative. If salt ceased to be useful, it was thrown away. With this illustration Jesus warned about a disciple who ceased to follow him.
14sn The difficulty of this saying is understanding how salt could lose its flavor since its chemical properties cannot change. It is thus often assumed that Jesus was referring to chemically impure salt, perhaps a natural salt which, when exposed to the elements, had all the genuine salt leached out, leaving only the sediment or impurities behind. Others have suggested that the background of the saying is the use of salt blocks by Arab bakers to line the floor of their ovens; under the intense heat these blocks would eventually crystallize and undergo a change in chemical composition, finally being thrown out as unserviceable. A saying in the Talmud (b. Bekhorot 8b) attributed to R. Joshua ben Chananja (ca. a.d. 90), when asked the question “When salt loses its flavor, how can it be made salty again?” is said to have replied, “By salting it with the afterbirth of a mule.” He was then asked, “Then does the mule (being sterile) bear young?” to which he replied: “Can salt lose its flavor?” The point appears to be that both are impossible. The saying, while admittedly late, suggests that culturally the loss of flavor by salt was regarded as an impossibility. Genuine salt can never lose its flavor. In this case the saying by Jesus here may be similar to Matt 19:24, where it is likewise impossible for the camel to go through the eye of a sewing needle.
Craig Keener, in IVP commentary writes,
"Although the salt recovered from impure salt substances taken from the Dead Sea could dissolve, leaving only the impurities behind.. [...] Real salt does not lose its saltiness; but if it did, what would you do to restore its salty flavor—salt it? Unsalty salt was worthless."
(Wikipedia) Salt itself, sodium chloride (NaCl), is extremely stable and cannot lose its flavour.
- R. T. France (1985 commentary on Matthew) notes that Jesus was giving a lesson in moral philosophy and "not teaching chemistry"; to him, whether or not the proverbial image is factually accurate is of little relevance to the actual message of this verse.
- John Nolland, (commentary on Matthew: B. Eerdmans Publishing, 2005) considers the impossibility of what is described as deliberate, it is counter to nature that salt lose its flavour, just as it is counter to God's will that the disciples lose faith.
However, these explanations that suggests that it's impossible for salt to become useless, don't explain the passage well, since the passages of NT and Talmud presuppose the real danger of the corruption of salt. These protestant scholars also seem to explain the passage from their faith alone dogmas, as they suggest that it's impossible for the (real) salt to lose its taste, indicating a very low chance or even impossibility of apostasy, turning the teachings into sarcasm. Regardless of the scientific objection, it seems safe to assume that salt can get spoiled for some reasons, turning into a useless powder, worthy of being thrown into waste, and to be trampled under feet.
You Are the Manure of the Earth
I found the accurate explanation from this article by Ray Hermann, that the passage Ὑμεῖς ἐστε τὸ ἅλας τῆς γῆς Ye are the salt of the land (YLT) refers to the salt of the soil, for agricultural manure; the taste (Greek word is actually μωραίνω: verb, to become dull, useless, foolish) and seasoning pertains to its inherent functional value, rather than its condimental purpose. The manure function of the salt must have been the primary usage for the ancient Israelites living near the Dead Sea, and the teaching of salt for the ground fits with the theme of the necessity of bearing fruits to achieve life. Ray writes,
The best explanation “is that what would have been called salt of that era was quite impure, containing a wide array of other compounds” besides sodium chloride. And since the more pure the salt, the higher the price, most ancient people used salt of a lesser quality. “The salts in Jesus’ day were mixtures of chlorides of sodium, magnesium, and potassium [all called salts by chemists], with small amounts of calcium sulfate (gypsum). Some of these would dissolve more quickly than others, while some were better able to withstand the [environmental] elements.
“These hardier . . . salts were generally more valuable in an agricultural context because that meant their benefits would last longer.” So, during any times of high moisture, rain, or flooding, the sodium chloride salt, being the most soluble in water, would dissolve and wash away, leaving a white powder looking just like salt, but not having any flavor or preservative properties.
While too much modern refined salt will produce sterility and barrenness in the ground,14 most ancient salt samples didn’t have enough sodium chloride to be dangerous and, because of the other salt compounds, could even be useful as fertilizer to improve the soil.
Author Anthony Bradley wrote about an article he read in Biblical Archaeology Review where the writer15 “argued that in Matthew 5:13, Mark 9:50, and Luke 14:34–35, Jesus was speaking not primarily of salt’s household use but of its agricultural use.” According to the writer, “several kinds of salt are found in Palestine that are different from the kind we’re familiar with. There is rock salt, salt evaporated from Dead Sea water, salt pits (Zephaniah 2:9), and more.” Additionally there are many references in agricultural literature to the use of various salt compounds as a fertilizer. He noted that “the value of salt in small quantities appears to have been known in ancient times; many sources record its power to improving herbage of pastures.16 Although various kinds of chemical compounds are necessary for the soil to be fruitful, soil that is nothing but sulfur and sodium chloride salt is a desert wasteland (Psalm 107:34).
So, changing our perspective about salt can change our perspective about what Jesus said. He may have been implying that his disciples were like the salt that made soil more fertile in an agricultural sense. He was warning them that they should not lose their ability to bring about life and growth and change. The “salt of the earth” could very well be “salt for the earth” or “salt for the soil.” If we lose our saltiness, we are of no further value in spreading the good news. It is evident that Christians are not here just to merely season or preserve the world from decay, but to stimulate growth in parts of the world that are barren—to encourage this Christian life and make sure it grows and spreads.
15. Reference was to Eugene Deatrick, former head of the soils department at West Virginia University, in his article “Salt, Soil, Savior” in Biblical Archaeology Review.
16. Bradley, Anthony B., “You Are the Manure of the Earth,” (Christianity Today Magazine, vol. 60, no. 8, 23 September 2016).
The article requires paid subscription to be read, so the link is useless