That's seems overwatered. Don't push the plant to absorb water more than it's limit. If your plant look dry, mist it or put it among other plants to maintain it's environment humidity and control leaves evaporation rate. It's look severe, but you can try checking the root. Remove the soft rotted part. If there's still some fresh part left, give fungicide then replant in good drainage soil. Don't use manure as part of the soil mixture since manure have bad drainage. If you wanna use manure, put it on top of the soil.

Hi Hajer:

Congratulations on killing a Calathea. Welcome to the club, we outnumber the club that CAN grow Calathea, and our numbers are growing every day.

As to what goes wrong and why so many do not grow this plant, my opinion is Calathea are hardcore humidity junkies. You have to provide a ridiculous amount humidity or the leaves curl/dry up and die, BUT if it's a little too much humidity (because it begged you for it) the leaves get disease and die. So it O.D.ed on what you tried so hard to provide.

Calathea are difficult, yet people CAN grow them. I hope they come and tell you how. I hope you try again. One attempt is not enough to write off a plant as pretty as this one.

I am talking as if your current plant is dead. It's close. The fun thing to do is excuse yourself, go buy a new plant and toss that one.

Good luck.

Good for you.

Your plant will put energy into trying to repair those damaged leaves, so the plant would benefit from those being removed, just leave some leaf material for the plant to still be able to produce food. Here is a quote regarding watering Calathea

WATER

Calathea Plants are very sensitive to the water you use. Hard water, soft water, fluoridated water, or water of poor quality causes the leaves of a Calathea Plant to turn brown from leaf burn. The best way to water a Calathea Plant is to use distilled water, rain water, or allow your tap water to sit out over night before using it. Keep the soil of a Calathea Plant moist but never soggy. Allow the top 2-3” to dry out before watering. Never let a Calathea Plant sit in water.

And here is the link to that site:https://www.houseplant411.com/houseplant/calathea-how-to-grow-care-tips

Moist soil should still allow for the top two or 3 inches to become dry between waterings,a bit like a ferns prefers.

Check your soil - all plants need air in their root zone, as well as moisture, and for plants that you want to keep consistently moist it is extra important that the soil structure allows for this by being chunky and free draining. I use pine bark and pumice in mine so that even immediately after thoroughly watering it is still aerated throughout.

If you’re using a finer denser soil then it will be staying too wet across the bottom of the pot, watered that often. Use a wooden skewer or chopstick to check the moisture deep in the bottom under the roots. You’re obviously growing several other plants nicely, if they’re all in the same mix it may be that they are more tolerant whereas calatheas are quite unforgiving.

Watch for pests too - spider mites can leave whitish dust across the underside of leaves, or fine webbing, mealy bugs hide in cottony blobs. These may not be the initial cause of the problem, but are more likely in a sickly plant.

Hajer, your plants are crazy healthy and beautiful. You know what your'e doing. Hope the Cal improves.

Research your soil options first - it’s worth reading more posts on this forum, about calathea in particular but also about 5:1:1 soil mix in general, and threads by tapla(Al) such as Good Growing Practices - An Overview.

I use a 511 type mix of pine bark, peat and pumice, using small grade orchid bark for the pine bark and bagged potting mix for the peat component. And pumice instead of perlite because it’s easy to get here and I prefer it.

But I do vary the proportions a bit - one calathea is in 411, for the other it’s more like 111. Some people use potting mix and perlite in a 50:50 ratio. Not many use a bought mix without amending it in some way to improve drainage. You’ll probably have to read the soil threads for any of that to make sense!

Do you know the humidity where your plants are?

Thanks robin for your help. I just have to tell you I am living on the other side from the world and in the place I live no one helps that much like you guys doing in this forum, that is why I registered here, so basically I know very little about soils, like what do you mean with 511? If you please put me some links that would help me understand all these things, I would be grateful

I will read Al article too, thanks

How can I know the humidity of the place where I put my plants in?

5:1:1 refers to the ratios of three ingredients : 5 parts pine bark, 1 part peat, 1 part perlite/pumice. I don’t know how to link sorry but you can type in the search box at the top of the screen. Read Als article first though. Your soil looks exactly like the sort of fine stuff it’s hard to get the watering right in. The roots I can see look ok. You want them firm, whitish and not smelly.

Was just wondering if you knew the humidity, you don’t have to know it exactly. Some people have gadgets measuring this, sometimes it’s part of the houses climate control system, or they run humidifiers which tell them. Here in damp New Zealand we have DEhumidifiers, so I plugged that in to check the reading, think it was 50-60% in winter, but higher in summer when we’re not heating the house which dries the air. Yours can’t be too bad, looking at your other plants.

If only simple testing to get best spot, you can use wet handkerchief or shock. Longer it's take to dry, more humid that spot.

It's a method to compare multiple spots in your house. If you try one spot and got 6 hours, you can try other spots using same handkerchief, maybe on other spot you get 4 or 7 so you can decide best spot for certain plant. You can't compare with mine since we don't have same handkerchief ^_^.. Btw my problem here is different. What i seek is most dry spot and i use towel. If i put my towel at wrong spot, my morning shower will end up with wet towel...