The white crystalline compound of sodium chloride (NaCl), known as common salt. There are in the earth vast underground deposits of rock salt, some hundreds of meters thick. The oceans of the world contain about 3.5 percent salts, mostly sodium chloride. This may seem to be very little, yet a cubic kilometer of seawater holds nearly 27 million metric tons of salt. The Dead Sea (Salt Sea) in Palestine is about nine times as salty. (Ge 14:3) Salt was readily available to the Israelites. Evaporation of the Dead Sea waters furnished an ample supply, although of poor quality. There were salt-bearing hills near the southern end of the Dead Sea, not far from where Lot’s wife became a pillar of salt. (Ge 19:26; Zep 2:9) Supplies of salt in northern Palestine may have come, at least partly, from the Phoenicians, who, it is said, obtained it by evaporation from the Mediterranean.

Notwithstanding such virtually inexhaustible supplies, salt has not always been readily available to man. Wars and revolutions have been fought for it. In ancient China salt was second to gold in value. Wives and children have been sold into slavery just for common salt. Caesar’s soldiers received money to buy salt, the sum being called a salarium, from which comes the English word “salary.”​—Compare Ezr 4:14.

The Bible takes note of salt as an essential part of man’s diet, as a seasoning for food. (Job 6:6) Under the Mosaic Law anything offered on the altar to Jehovah had to be salted, not because of flavor, but doubtless because salt represented freedom from corruption or decay. (Le 2:11, 13; Eze 43:24) Large quantities of salt evidently were stored on the temple grounds for this purpose. Ezra saw to it that plenty was on hand for the sacrifices. (Ezr 6:9; 7:21, 22) It is reported that Antiochus III (c. 198 B.C.E.) gave 375 medimni (c. 20 kl; 562 bu) of salt to the temple service.

Certain healing, medicinal, and antiseptic values are attributed to salt. Newborn babies were sometimes rubbed with salt at birth. (Eze 16:4) In limited quantities salt is beneficial on certain acid soils or when mixed with manure, but if allowed to accumulate in the soil, it kills vegetation and the land becomes barren and unfruitful, as was the case with the once-fertile Euphrates Valley. A city condemned to total destruction was sometimes deliberately sown with salt, this act expressing the desire that the place be perpetually barren and sterile.​—De 29:22, 23; Jg 9:45; Job 39:5, 6; Jer 17:6.

Figurative Use. Salt is often used in the Bible figuratively. Jesus told his disciples: “You are the salt of the earth,” a preserving influence on others, preventing spiritual putrefaction and moral decay. The good news they carried would preserve life. However, he went on to say to them: “But if the salt loses its strength, how will its saltness be restored? It is no longer usable for anything but to be thrown outside to be trampled on by men.” (Mt 5:13; Mr 9:50; Lu 14:34, 35) One Bible commentator remarks on Matthew 5:13: “The salt used in this country [United States] is a chemical compound​—muriate of soda—​and if the saltness were lost, or it were to lose its savour, there would be nothing remaining. It enters into the very nature of the substance. In eastern countries, however, the salt used was impure, mingled with vegetable and earthy substances; so that it might lose the whole of its saltness, and a considerable quantity of earthy matter remain. This was good for nothing, except that it was used, as it is said, to place in paths, or walks, as we use gravel. This kind of salt is common still in that country. It is found in the earth in veins or layers, and when exposed to the sun and rain, loses its saltness entirely.”​—Barnes’ Notes on the New Testament, 1974.

Because salt prevented decay, it became a symbol of stability and permanence. Often when covenants were made, the parties ate together​—eating salt together—​denoting perpetual loyalty and fidelity to one another in the covenant relationship. “A covenant of salt” therefore was considered very binding. (Nu 18:19) Accordingly, Judean King Abijah’s statement that Jehovah had made “a covenant of salt” with David and his sons meant that the covenant with David’s line for the kingship would stand forever. Jesus Christ the “son of David” and “the root of David” proves to be the one holding the Kingdom and administering its affairs forever.​—2Ch 13:4, 5; Ps 18:50; Mt 1:1; Re 5:5; Isa 9:6, 7.

Jesus said: “For everyone must be salted with fire.” The context here points to a salting with the fire of Gehenna in the case of all who stumble into  a life of sin or who are responsible for so stumbling others.​—Mr 9:42-49.

Using the term to convey a different sense, Jesus thereafter said: “Have salt in yourselves, and keep peace between one another.” (Mr 9:50) The apostle Paul used it in a similar way, saying: “Let your utterance be always with graciousness, seasoned with salt, so as to know how you ought to give an answer to each one.” (Col 4:6) One’s conduct and speech should always be in good taste, considerate, wholesome, and tend toward preserving the lives of others.