It is also well-documented that at the end of the third Punic War, Rome salted the fields of Carthage, as part of their effort to systematically, totally, and irrevocably destroy the nation. I think the contemporary layman really only realizes half the significance of this -- the social significance of "thirsting so strongly for vengeance that they laid waste to the land" -- and misses the economic side of the issue, because we're just not used to thinking of salt as a valuable, costly resource.
How costly? Well, let's crunch some numbers.
I. The Economy of Salt
According to the Stoke-on-Trent Pottery Museum, Diocletian set a price cap in 301 CE of 100 denarii per modius of salt. The denarius was a Roman unit of currency; the modius was a unit of dry measure equaling approximately 8 liters, or .2825 cubic feet. The density of dry salt, according to the Salt Institute, is 72 pounds per cubic foot, so a modius of salt weighs about 20 lbs. This means that salt sold, a few centuries after the birth of Christ, at 5 denarii per pound.
This is all well and good, but what's a denarius worth? Well, the pottery museum says that a private in the Roman army, in the same era as the price cap quoted above, earned 750 denarii per year. This is a job that seems roughly equivalent to a minimum-wage job today, so let's set 750 early-CE denarii as equal to $11,000 in today's currency.
To put some perspective on this, let's look at today's prices. Table salt can be found at any supermarket, assuming that one buys in bulk at a decent price, for 50 cents per pound. Of course, one doesn't need refined salt to destroy fields; a more appropriate comparison might be rock salt -- which one can buy for water purifiers or for de-icing -- that can be bought in bulk, according to the Akron Beacon-Journal, for 8 cents per pound or less. The lowest price I was able to find reliably quoted was industrial salt, at $30 per ton (less than 2 cents per pound), but I have no idea how pure or usable it is for field-sowing purposes.
In Roman times, that minimum-wage salary would buy a soldier 150 pounds of salt per year. Today, minimum wage -- if paid in rock salt -- would work out to over 137,500 pounds of salt per year. This is approximately a thousand-fold difference. (And illustrates quite well why we're not paid in salt any more.)
To put this another way, if salt cost today what it did in Roman times, we'd be paying eighty dollars per pound for it. Now, it's a foregone conclusion that the Roman government wasn't paying full retail price; but even so, it's not unreasonable to assume that salt's true cost of production was on the same order of magnitude as its sale price -- perhaps $40 per pound.
And Rome dumped enough salt on all of Carthage's fields to turn it into uninhabitable wasteland.
II. The Dead Fields
It's difficult to figure out how much salt they actually used, but salt is still occasionally used in the modern day as a herbicide, and modern examples can give us a suggestion of the staggering quantity of material involved. Kansas State University (PDF file) says that an application of 20 tons per acre is typical to kill vegetation; as an acre is 43,560 square feet, we can safely assume the Romans used about 1 pound per square foot of farmland. And just how much farmland was there?
Again, such figures are almost impossible to come by; however, we can again take a cue from the modern world. Science News cites geographer Vaclav Smil as saying that 0.07 hectares per person is the bare minimum required to sustainably feed a population today without resorting to techniques at the extreme limits of technology or human safety. Most of the world, in practice, uses far more cropland per capita (due to antiquated farming practices, crop diversity, the planting of non-food crops, and especially the consumption of meat); even highly poor and heavily populated China uses 0.1 ha/person (while still importing about 3 percent of their food), and industrialized countries tend to stray closer to 0.5 ha/person, but I will be using the 0.07 figure in order to err on the side of caution.
As GORP claims that the population of Carthage was from 400,000 to 700,000 (I will be using a figure of 500,000 for convenience), we can use these estimates to calculate the rough square footage that Rome salted:
500,000 people * 0.07 ha/person * 107,600 sq ft/ha =
3,766,000,000 square feet of farmland.
And the cost? At 1 pound of salt per square foot, and $40 per pound of salt, the Romans spent the modern equivalent of 150 billion dollars.
To salt the fields of a single city.
Meanwhile, the United States of America -- with the world's most modern, mechanical military, and a population of 300 million people to bear the costs -- spent merely $53 billion on procurement in 2000. Rome's purchase was the equivalent of three years of equipment spending from today's world's biggest superpower.
To salt the fields of a single city.
Now that's revenge.
First of all, I should emphasize that this essay is the result of a few hours of Internet research and amateur number-crunching; this is not a scientific assessment of the Punic War period. This paper is not peer-reviewed (except to the extent that my peers are browsing through my site). The figure above should not be taken as authoritative, or even necessarily correct, but my final number is an estimate that I think has its basis in common sense.
I have necessarily made a number of assumptions to arrive at my figures. I have cited all of my sources, but my primary assumption is that the figures they gave me are reliable; this is not necessarily true in the Internet age. (Caveat reador.) My second assumption is that all of my assumptions, while necessarily creating deviations from the correct salt expenditure figure, will create errors that tend to cancel each other out. (For instance, my figure for the Roman price per pound of salt is almost certainly high, as it's based on the maximum price set by Diocletian; on the other hand, my figure for the equivalent in today's dollars of a denarius is almost certainly high, because even a minimum-wage worker in 2001 America has actual wealth far surpassing that of a middle-class Roman. These two inaccuracies will, to some degree, cancel.)
In certain circumstances, I have deliberately erred conservatively, such as with the hectare/person value for cropland; hopefully, this will produce a net result for the $150 billion figure that is lower than the actual value. On the other hand, certain assumptions (such as: "The majority of Carthage's food was grown there, instead of imported") could, if wrong, slash my figure by a factor of 3 or worse. As such, my conclusion should only be taken as an order-of-magnitude approximation, in the absence of any actual figures taken from ancient Roman records. (There may actually be such documents in a library somewhere. If this is a subject you're interested in, I recommend further research, and would be very happy to be informed of your results.)
So what use is the approximation I present, besides a way to entertain me for several hours of web-surfing time? Well, for one, it's a great tidbit to toss around at parties. It also may stimulate someone to actually produce an authoritative paper on the subject, which would be an even better tidbit to toss around at parties. One way or another, I hope this has been an enjoyable read -- or at least hasn't made anyone's brain explode.
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