There is a specific heartbreak of cultivating and tending to your tomatoes with love and care in Spring and Summer, only to discover that the bottom is a concave, almost leathery black crater rather than plump redness.
The first time I encountered blossom end rot (the culprit responsible) was a few years ago when I tried to grow San Marzano (the best tomato for making sauces). From the top angle, it was plump and delicious-looking; however, the bottom looked like it had been scorched by fire!
So I had a look online and did what I was told by viral articles, which was to use crushed eggshells and calcium additives in the soil, but this didn’t work!
So I stopped adding calcium and studied what was actually happening to the tomatoes.
From my own studies, I found that blossom end rot is not often actually a calcium deficiency in the soil. I found out that it was actually a calcium transportation issue! Essentially, the calcium that tomatoes require was already present in the soil at the right levels, but the tomato plant couldn’t transport it to where it was needed.
This, I found, was a crucial distinction!
Debunking the Eggshell Myth With a Real Test
So I was sceptical of the old egg shell advice that gets recycled around the internet on social media and on blogs, so I decided to put it to the test myself.
I ran a very simple side-by-side comparison experiment with two similar tomato plants in the same 30-litre containers…
Plant A had a really decent hand of finely ground eggshells (it was omelettes for breakfast before this experiment!), and I even threw in calcium tablets too.
Plant B didn’t have any added calcium at all in any form. What I did instead is I set up a consistent drip irrigation schedule to keep the soil evenly moist (avoiding any fluctuations due to drought or heat waves) and add a layer of homemade compost mulch to lock in moisture and shield the topsoil from direct sunlight.
Plant A actually had blossom end rot with black, charred bases despite the added calcium. The reason (as my hypothesis was testing) is that the eggshells take months (and sometimes years) to decompose into a form that roots can actually draw up, so by the time its bioavailble, you have run out of tomato season.
However, plant B stayed healthy, and each flush of tomatoes was without blossom end rot.
The reason for this? It’s all down to watering…if you’re a haphazard, erratic, forget-for-a-week, water (like me!), then the tomato plants’ roots cannot transport the calciu that its in the soil to the fruiting tomatoes. It’s a problem of watering, not necessarily a lack of calcium, as the calcium relies on consistent moisture to reach the tomatoes.
My advice is to get yourself an automated watering system as I did! It kept the soil moist, mitigated any effects of the heat waves (in heat waves, pots can dry out in a day), and my tomatoes looked great!
The Overfeeding Mistake Nobody Talks About
At the start of my tomato growing, I naively had a more is more attitude, so I bought tomato fertiliser and added it early in the season.
What I wasn’t aware of is that cation antagonism is a big factor in tomato plant nutrition, and excess amounts of potassium, nitrogen and magnesium in the soil all directly compete with calcium for uptake at the roots.
The ions are chemically similar, so that the tomato plant’s root cells uptake whichever is most abundant, which can lead to a calcium deficiency.
With my gusto of adding too much fertiliser, I accidentally prevented the tomato plant from uptaking the very nutrient that the fruit required to build cell walls, which meant the leaves looked great but the fruit rotted!
Here’s what to do… You need to learn from my mistakes and avoid fertilising the tomatoes when the fruit starts forming. Instead, what you need is a more moderate and balanced approach, as this allows the tomato plant to uptake calcium normally!
So if you are getting blossom end rot, ease off the high potassium fertilisers first and foremost and give the soil a good soak, which can flush excess water soluble nutrients like nitrogen and potassium from the soil, and it can help with the uptake of calcium. The next round of tomatoes is likely to be healthier without black blossom end rot.
The Humidity Stall Nobody Diagnoses
Perhaps the most surprising cause of blossom end rot I have encountered and read about is to do with humidity.
Calcium actually moves through a tomato plant to a large extent by transpiration, which is when water evaporates from the leaves, pulling the calcium-rich water up through its roots like a straw!
This affected my tomatoes when there was a big heatwave 2 summers ago, and blossom end rot ruined some tomatoes despite a practically perfect water routine on a timer.
When there is very high humidity, the transpiration from leaves slows down dramatically, and if the air around the leaves is saturated with damp air, then the water from the leaves has nowhere to evaporate to, and the straw stops working.
If the leaves aren’t losing water, the tomato plant cannot draw up the calcium it needs from the soil, and the rapidly developing fruit ends up with root rot despite calcium and moisture being present in the soil.
So what do we do? Obviously, adding calcium or water isn’t the answer, so I moved my potted tomatoes to a more shaded area (I placed a wind break that I usually use at the beach in front of the tomatoes I planted in the ground).
I then bought a handy little solar fan from Amazon, and it got some airflow moving around the tomato plant, which restarted the transpiration and meant the roots could uptake the calcium that the fruits needed. The other developing tomatoes were able to grow without tomato blossom end rot.
The Diagnostic Summary
| Cause | What’s Actually Happening | Fix |
|---|---|---|
| Inconsistent watering | Eggshells/calcium supplements alone | Consistent drip irrigation, mulch to retain moisture |
| Overfeeding (potassium/nitrogen) | Cation antagonism blocks calcium uptake at the root | Reduce concentrated feed, balance nutrients |
| High humidity / stagnant air | Transpiration stalls, calcium can’t be pulled upward | Improve airflow, shade during extreme heat |
| Eggshells / calcium supplements alone | Too slow to break down to be useful this season | Address the transport issue, not the supply |
What Actually Works
Blossom end rot is best dealt with by consistency rather than a dramatic intervention as such.
For example, consistent drip irrigation is better than infrequent soaking, and it is better than any soil amendment.
I love using mulch, and I think it’s really underrated as a way to keep the soil moist and protect it from direct sunlight. It’s the days of intense sun and high temperatures that can do damage, so mulch and consistent watering are a great way to mitigate against this.
Moderate balanced fertiliser use is a great way to support your tomato growth without risking blossom end rot.
On hot and humid days, a fan works best!
Dealing with blossom end rot and not sure which cause fits your situation? Tell me about your watering routine, recent weather, and feeding schedule in the comments, I’ll help you work it out.