While salt obviously was an important commodity of some worth, I doubt it could be called "rare" - hardly more so than fish or olives for example. After all, Mediterranean is quite salty and Rome itself was built amidst friggin' salt marshes.
AFAIK the Latin salt marshes developed after Rome's ascension vs the Latin League (after which Latium went into decline). Salt was a highly valued spice. (Salt continued to be used to preserve foods until the spread of refrigerators.) For comparison, copper was a valuable metal until it was replaced by iron; iron weapons could be afforded by many more than bronze weapons.
(Wikipedia salt quote: )
Aside from being a contributing factor in the development of civilization, salt was also used in the military practice of
salting the earth by various peoples, beginning with the
Assyrians.
It is commonly believed that Roman soldiers were at certain times paid with salt.
[1] [2] This, however, is a misconception:
'salary' derives from the Latin word salārium, meaning money given to soldiers so they could buy salt.
[3] The
Roman Republic and
Empire controlled the price of salt, increasing it to raise money for wars, or lowering it to be sure that the poorest citizens could easily afford this important part of the diet.
It was also of high value to the
Hebrews,
Greeks, the
Chinese and other peoples of antiquity.
Already in the early years of the
Roman Republic, with the growth of the city of
Rome, roads were built to make transportation of salt to the capital city easier. An example was the
Via Salaria (originally a
Sabine trail), leading from Rome to the
Adriatic Sea. The Adriatic Sea, having a high salinity due to its shallow depth, had more productive
solar ponds compared with those of the
Tyrrhenian Sea, much closer to Rome.
During the late
Roman Empire and throughout the
Middle Ages salt was a precious commodity carried along the
salt roads into the heartland of the
Germanic tribes.
Caravans consisting of as many as forty thousand
camels traversed four hundred miles of the
Sahara bearing salt to inland markets in the
Sahel, sometimes trading salt for
slaves:
Timbuktu was a huge salt and slave market.
In the Old Testament,
Mosaic law called for salt to be added to all burnt
animal sacrifices (Lev. 2:13).
The
Book of Ezra (550 BC to 450 BC) associated accepting salt from a person with being in that person's service. In
Ezra 4:14
, the servants of
Artaxerxes I of
Persia explain their loyalty to the King. When translated, it is either stated literally as "because we have eaten the salt of the palace" or more figuratively as "because we have maintenance from the king." In the New Testament, Matthew 5:13 Jesus said, "You are the
salt of the earth." He added that if the salt loses its flavor, it is good for nothing but to be trampled. Jesus said this in order to show his disciples how valuable they were and this saying is commonly used today to describe someone who is of particular value to society[
original research?]. In addition, the preservative quality of salt is in view here to show how the disciples were called to preserve the society and the world around them from moral decay. On another occasion according to the
Gospels, Jesus commanded his followers to "have salt within them".