Salting the earth

Humans have fought bugs since the dawn of civilization, and while we’ve dramatically improved our ability to wage chemical warfare, we still do close-quarters battle with melee weapons. The Bug-A-Salt changes all that.

Millenia of human innovation culminated in this, the pneumatic salt-firing shotgun. It alters the world’s balance of power, and with great power comes a great question: “Is it safe to shoot near potted plants?”

Be aware of what’s beyond your target, said Beth. If it’s a tiny herb garden, recall the phrase, “salting the earth.” Legend has it that ancient conquerors practiced scorched-earth tactics by sprinkling salt on enemy fields.

“Surely,” said I, “that’s just symbolic.” Salt is a rock. To become an herbicide, wouldn’t it take a huge quantity? In the age when fly swatters were still new technology, salt was relatively precious. Wouldn’t that be wasteful?

This begged for an experiment. First, we’d need a plant to test, something that grows reasonably fast and indoors so we could control the weather. Wheatgrass fit the bill, so we ordered a growing kit advertised as “ready to eat in 7 to 10 days.” Apparently wheatgrass is sold as food (for humans).

The kit came with four separate tubs, each with a pre-measured seed and soil quantity. Intending to control for variations in sunlight and other conditions, we distributed the soil and seeds from one of the four into an egg carton. Of the carton’s twelve cups, four egg cups received no salt, four received a sprinkling, and four received a heavy salt coat.

Likewise, the remaining three manufacturer-supplied supplied containers received received no salt, a sprinkling and heavy salt. We call “lightly salted” about the heaviest amount you might reasonably call “sprinkling.” The heavy amount was outrageous, enough that we will later see salt deposits climb the walls of its container, presumably thanks to evaporation and capillary action.

Three containers showing amount of salt
Before planting

We watered every three days and recorded growth. I expected we would need to do something tedious like count the blades of grass in each container, but the difference couldn’t have been more stark.

Control grew about an inch. Of the salted, only postsalted sprouted at all.
Day 4

In the egg carton, we salted the second column lightly and the third column heavily, repeating the pattern for the remaining six cups not shown. We salted the front row (presalted) before planting, and the back row (postsalted) when the seeds had sprouted, three days after planting.

The presalted egg cups never sprouted, and the postsalted cups withered dramatically. Even grass in unsalted cups adjacent to salted cups struggled, particularly one that I accidentally splashed while watering its salted neighbor.

Nearest cups, particularly on the left are unsalted, but grass is much shorter than control in background
Cups nearest and furthest from the camera are unsalted

None of the salted egg cups ever grew, but the lightly salted tub did manage to sprout on day 9. At first only a handful of sprouts appeared, but more followed for the next several days. By day 15, some of the hardier blades were catching up to their unsalted brethren.

Lots of grass in the unsalted tub, sparse grass in the lightly salted tub
Day 15

Despite its valiant effort, however, the salted grass remained sparse. We stopped the experiment on day 26.

While not that fabled “nothing will ever grow again” effect, it looks like a sprinkling of salt could cause devastating famine. In this experiment, we used about a teaspoon of salt on the lightly salted 3 3/4 inch square container. A 3 pound box of Morton’s kosher salt comes in a 90 cubic-inch box, so this works out to a little more than an ounce and a half of salt per square foot. The egg cup spillover suggests that a smaller amount could still be effective, so we’ll round down to 1 ounce per square foot.

Knowing this, how much salt will we need to starve our enemies if we aspired to warlording? We might decide to make a name for ourselves by sacking Carthage, with its population of 700 thousand, according to Strabo XVII Ch.3ยง15. According to Wikipedia, The Economics of Agriculture on Roman Imperial Estates in North Africa estimates that a family of six needed 7 to 12 acres of farmland to feed itself. Thus, at 2 acres of Carthagian farmland per person, we need to cover 1.4 million acres. If planted nearby, this would surround the city for some 30 miles in all directions and we would need about two million tons of salt to cover it.

We can use Emperor Diocletian’s Edict on Maximum Prices to put this in perspective. The edict listed salt at 100 Denarii per modius, a volume of around 530 cubic inches. Using our previous density estimate of 30 cubic inches per pound, this works out to 6 Denarii per pound or 12 thousand per ton. Since the edict was an attempt at price control, it’s safe to say that commodities it listed actually cost more, so a ton of salt might have cost about the same as a soldier’s annual wage. Cost-conscious generals would suggest using our two million ton salt fund to hire a gargantuan army instead, a million strong. With various sites estimating the Roman force that actually destroyed Carthage at 80 thousand infantry, our salt budget could scale it up by an order of magnitude.

Even using enslaved Carthagians to harvest and sprinkle, salting doesn’t look practical, but today’s budding emperor can take heart that Alibaba has road salt priced at a mere $100 per ton. On the other hand, today we have much more effective herbicides, so we don’t need to salt enemy fields. The important question, however, remains: will our potted plants become casualties of Bug-A-Salt conflict?

Suppose we have a planter that covers one square foot. The Bug-A-Salt’s magazine holds about a tablespoon of salt, good for 50 or so shots. Our ratio of 1 teaspoon per 14 square inches works out to 10 teaspoons per square foot, or three full Bug-A-Salt magazines per plant. Thus, if we kill 150 flies on our plant we would do collateral damage equivalent to this high, if not total, wheatgrass kill rate:

Bushy control beside sparse lightly-salted tub
Day 26