Gardeners Saved Eggshells For Years. Researchers Watched Slugs Walk Right Over Them.

Researchers surrounded a lettuce leaf with crushed eggshells and waited. The slugs crossed anyway — no hesitation, no detour.

That experiment, run by the All About Slugs group and reported by McGill University’s Office for Science and Society, dismantled one of the most-shared gardening tips on the internet. The Royal Horticultural Society later ran its own six-week trial. Plants ringed with eggshells fared no better than plants with nothing at all.

Every spring, the same advice circulates on Facebook: don’t throw eggshells away. Crush them for slugs. Bury them for tomatoes. Sprinkle them to fix your soil. The posts get millions of shares.

Most of the advice doesn’t survive contact with the research.

The Tomato Claim Universities Keep Correcting

The most persistent version involves blossom end rot — the dark, leathery patch that ruins tomatoes, peppers, and squash from the bottom up.

The logic sounds airtight. Blossom end rot signals calcium deficiency. Eggshells are roughly 95% calcium carbonate. Problem solved.

It isn’t.

“Using eggshells really doesn’t fix anything,” Mississippi State University Extension states in its guidance. The service explains that the deficiency almost never comes from the soil. It comes from water. When moisture swings, roots can’t move calcium into developing fruit — even when the soil is saturated with it.

University of Minnesota Extension reaches the same conclusion. Most soils already carry plenty of calcium for garden plants, according to the service, and the shortfall reflects a transport problem inside the plant.

The fix is consistent watering and mulch. Not shells.

The Timeline Nobody Mentions

There’s a second problem, and it’s about the clock.

Eggshells break down slowly. Anyone who has turned a compost pile knows it — the shells are still recognizable months later. University of Minnesota Extension notes the same holds true in garden soil, and that eggshells generally decompose too slowly to be effective even where calcium is genuinely lacking.

Yet the viral posts routinely claim the opposite. The Newsner listicle that helped spread this advice states flatly that eggshells “are broken down quickly.” That’s backwards.

Finer grinding speeds decomposition. It doesn’t make it fast.

Where The Advice Turns From Useless To Risky

The gardening claims waste eggshells. The eating claims raise a different question.

The same posts increasingly instruct readers to boil, dry, grind, and swallow eggshell powder as a homemade calcium supplement — usually with a specific dose attached and no medical sourcing behind it.

Raw eggshells can carry Salmonella and E. coli. Published sterilization research uses sustained heat treatment well beyond the quick boil these posts describe. The gap between a lab protocol and a Facebook tip is not a detail.

Dosage matters too. NIH sets adult calcium intake at 1,000 mg per day, rising to 1,200 for women over 50 and men over 70. The upper limit sits between 2,000 and 2,500 mg. NIH also notes Women’s Health Initiative findings linking higher supplemental calcium to increased kidney stone risk, though subsequent reviews found no such association.

None of that is knowable from a kitchen scale.

What We Know
Eggshell is roughly 95% calcium carbonate — the chemistry in the viral posts is correct
Blossom end rot stems from inconsistent watering, not low soil calcium, per Mississippi State and Minnesota extension services
Eggshells decompose too slowly in soil to work as a practical calcium amendment
Slugs crossed eggshell barriers in testing reported by McGill’s Office for Science and Society; the RHS found no protective effect over six weeks
Raw eggshells can harbor Salmonella and E. coli
NIH sets the adult calcium RDA at 1,000–1,200 mg daily, with an upper limit of 2,000–2,500 mg
Mississippi State recommends a soil test as the only reliable way to know whether calcium is actually short
Why This Matters

The eggshell hack is harmless in isolation. Someone loses a growing season and a few tomatoes.

The pattern isn’t harmless. Advice that sounds scientific — real chemistry, real minerals, real percentages — travels further than advice that admits uncertainty. A listicle citing 95% calcium carbonate reads like evidence. It isn’t. It’s a true fact welded to a claim it doesn’t support.

That same structure carries health advice into millions of American feeds. The Newsner piece first ran in 2018 and was quietly refreshed in October 2025, seven years of accumulated shares behind it, still telling readers what to eat.

Extension services exist to answer these questions. Most are free. Most states run soil testing labs for a nominal fee.

The information is there. It just doesn’t share as well.

Mississippi State’s advice on calcium runs three words long, and it works for far more than tomatoes: don’t guess — test.