📌 Quick Summary: The Great Calcium vs. Potassium Mix-Up
- 🍅 The BER Mistake: Blossom End Rot (BER) is caused by a lack of Calcium, not Potassium. Many growers confuse the two! Feeding a plant suffering from BER with Potassium will actually make the rot much worse.
- 🛑 Nutrient Lockout: Potassium and Calcium are antagonistic elements in the soil. If you dump too much Potassium into your beds, the plant roots literally cannot absorb Calcium, even if it is present in the soil.
- 🍂 Does Potassium Cause Leaf Mold? A weakened plant is a magnet for diseases. While Potassium doesn’t directly create Tomato Leaf Mold (Cladosporium), triggering a severe Calcium deficiency by overfeeding Potassium severely weakens the plant’s immune system.
- 🧽 The Power of the Pressure Washer: Eradicating Leaf Mold from a greenhouse takes extreme willpower. Growers share their success using pressure washers, laundry soap, and copper sulfate to scrub the greenhouse walls in the fall.
Photo author: MrsFlower
Marat wrote:
Last year I fed them with Potassium to treat Blossom End Rot. This year I excluded that feeding. Somewhere on this forum, I read that Potassium plays a role in the development of Cladosporium (Leaf Mold)…
Actually, no plant can grow without Potassium, especially a tomato. Potassium makes up more than 50% of the total nutrients inside a tomato fruit! You probably have soil that is already very rich in Potassium if your tomatoes grow well without additional feedings. Do you have loamy soil?
Aleksan9ra wrote:
How could Potassium possibly provoke Leaf Mold…
Good morning! Last year I also had a terrible case of Leaf Mold. I decided to remove the entire top layer of soil in the beds and replace it with fresh soil and humus.
I also thoroughly washed the greenhouse. First, I just hosed the dirt off with plain water, then I scrubbed everything with laundry soap, rinsed it again, and finally sprayed the entire inside surface with copper sulfate. I don’t know if this is what helped, but my tomatoes are still standing healthy in the greenhouse right now! I’ll probably pick the red ones this weekend and leave the green ones on the vine.
Aleksan9ra wrote:
Marat, I still think it is a pure coincidence.
I am not ruling out a coincidence, but I am not denying it either. We will live and see. But the phrase “feed with potassium and you get leaf mold” is stuck in my head from reading it here.
gala7496 wrote:
I also thoroughly washed the greenhouse. First, I just hosed the dirt off with plain water…
By the way, I also washed my greenhouse in the fall using a pressure washer…
Marat wrote:
…a very severe case of Blossom End Rot appeared. Someone advised me to feed them with Potassium…
Forgive me for being a nerd, but…
First: Putting wood ash in the planting hole provides macro and microelements, primarily Calcium and Potassium. Therefore, you already had enough Potassium in your loamy soil, and you added even more with the ash.
Second: Blossom End Rot is treated by feeding with Calcium, not Potassium! You got them confused, and instead of Calcium, you fed them Potassium again.
Third: There must be significantly more Calcium in the soil than Potassium. If this balance is broken, Potassium blocks the uptake of Calcium, which your plants were already lacking!
Fourth: BER is most often caused by underwatering. When you water, the moisture doesn’t penetrate deep enough into the soil. I advise checking soil moisture not just on the surface, but deep down, especially in hot weather.
Fifth (Conclusion): I am not stating this as an absolute fact, but it is highly likely your plants were lacking Calcium, and you made the situation much worse by feeding them Potassium. The plants became incredibly weak and caught various diseases, including Cladosporium. This year, you limited the Potassium, so the plants were able to absorb Calcium properly. They became healthier and resisted the disease. Congratulations!
Marat wrote:
By the way, I also washed my greenhouse in the fall using a pressure washer…
So two people weren’t too lazy to scrub their greenhouses, and now neither of them has Leaf Mold… Maybe I should wash mine too? The amount of willpower that must take!!!
