Usually chili plants need full sun, but in hotter countries, they do a bit better with some shade in the hottest part of the day. Seedlings do not appreciate a lot of sun, but once they get larger and become good size plants, they cope perfectly well. This is probably why your seedlings in the sunny area are not doing so well as those with more shade.

@Bamboo Wow, makes perfect sense. For the next batch of plants that germinate I'll keep this in mind.
Thanks. I appreciate :)

Also how do I know when is the perfect time to move them to full sun?

When they've become healthy, bushy plants at least 6 inches high with side branches and plenty of leaves. The ones in your sunny picture above would still be classed as seedlings.

The square pot looks as if the surface is cracking.

Yes, the soil in the square pot has a high clay content for a potting compost. That is going to be a problem for growing most plants. Do you have any way of checking whether the compost is acid or alkaline? A stop-gap measure would be to add extra calcium to the compost to "flocculate" the clay--limestone if the compost is acid, or gypsum if the compost is alkaline. Next crop, use a compost that contains no natural soil.

Thanks, bamboo, I notice that the ones in the sunny picture are just beginning to let out tips of side branches. I'll wait till they be bushy.

@scotsgran, I watered the pot sufficiently, yet it starts to crack by the end of the noon everyday.

Thanks for explaining so nicely Tugbrethil, I don't have a way to measure if the soil is alkaline or acid, How about litmus paper? I might manage to get some. And then I might know what to add.

Also what do you mean by saying "use a compost that contains no natural soil"? Didn't get that part.

(I am not a hardcore gardener, just do it for pleasure so my questions might sound silly)

Where you live, Chillies, I suspect its common practice to just scoop up some soil from outdoors and put in a pot and grow plants in it, rather than going off to the store and buying a bag of potting soil, perhaps because potting soils are not available where you are. If that's what you've done, then it will have a high clay content because of where you live, that's what the soil's like; over here, we buy special potting soils to use in pots and don't use garden soil, and that's what Tugbrethil is talking about, something you buy from a garden supply store.

If you and your neighbours usually grow all your potted plants in the local soil from the ground in pots successfully, and you have no potting soil on sale there, then carry on growing in the soil you've used. If potting soil can be purchased somewhere, its always best to use that instead.

I wasn't sure what your local resources were, Chillies, or whether, in India, the UK term "potting compost" or the U.S. term "potting soil" were used. I say to use whatever works, though I would mix up a combination of sifted compost, coarse sand, and shredded coir, to avoid drainage problems with the clay.

Break up the top layer of cracked clay then check and see if the soil is damp below that cracking. If not then breaking it up will allow the water to seep down and be retained in the soil. If that top layer is not broken up the water will run off and down the sides of the pot and be lost to the plant. When you are happy that the soil is wet I would put a layer of straw or some other mulch over the soil to stop it evaporating so quickly.

Yes, some kind of mulch will help stop the cracking, and the formation of a waterproof clay crust. As for the litmus paper, that should work for checking whether the soil is alkaline or acid. I would make a slurry of equal parts soil and whatever water you use for watering--since you have to check the whole system, not just the soil. Then I would filter the soil out with a fine-textured cloth, a scrap of unwanted silk would be ideal, letting it drip onto a glass, glazed ceramic or hard plastic surface. Touching the edge of the drips with the end of the litmus strip should let enough liquid to soak onto it to get a good measurement.

I so much apologies for being away and not responding to all your precious comments, I was on a trip. Apologies again.
All my pots have garden soil from a nearby garden.
And yeah Bamboo you are right, special potting soils are not available.
I sometimes used urine as a manure, but I think I might have overdone it and it might have caused some acidity. Thanks for explaining in details how to measure it Tugbrethil.
And yeah Scotsgran, I am gonna crack up the top layer right away.
Btw, the chili plants which are growing nicely and those which are not are both have the same soil.
I think the excess exposure to sunlight and probably (acidity) excess use of urine might have caused the problem.
I tied piece of thin cloth which reduces the intensity of sunlight.
I stopped using urine and started using pigeon poop instead as it's available in abundance due to lots of pigeons sitting on my terraces.
Thanks again for all the precious comments, all you lovely folks!
I will update soon with the pH measurement.

I've seen this type of soil before from someone else in your country - if I'm right, its not very free draining, looks like slightly sandy or reddish coloured mud when its wet, always pans and cracks on top as it dries, but, nonetheless, appears to grow plants pretty well, even in pots. With few other resources available, so long as the plants grow, that's all that counts.

In regard to pigeon poop, if the pigeons are wild and not kept by somebody, be careful - the guano can carry some pretty noxious diseases. If other local people use it all the time without problems, then it may be okay.

Acidity is a minor problem when using urine as a fertilizer. The main problem is with the sodium in the urine: it is toxic to plants, and keeps them from absorbing enough water.

You are right bamboo, it's not very free draining and a bit muddy kind of soil.
Well, the pigeons are wild and probably I might be the only guy in this locality using them(no exaggeration). People here are crazy just for the plants which they consider sacred, so mainly they grow tulsi/holy basil & I never saw anybody using manures in pots. The purpose being the tulsi plant should stay alive so that they can pray. So, I guess I got to evaluate the pigeon poop stuff myself.

The chili plants in the sunny picture have grown to 12 inches with finger length big leaves and it's my assumption that the poop is helping them, yet I will keep observing. They are not bushy yet with no side branching so as you told previously I am keeping them in semi shaded place.
The ones in square pot still show no growth.

Tugbrethil, I think I might have already done the damage, how do I check the damage now? Anything that can help neutralize the excess sodium?

I also have a rangoon creeper which is not growing well, the leaves stay small and grow twisted and get yellowed soon. I guess I should post a new thread with pictures for some expert opinion.

The trouble is, many of the pathogens wild or feral pigeons can carry are lethal to humans rather than your plants - that's what I meant about taking great care (mask, gloves, preferably don't use it at all)...

Well, calcium helps keep the sodium ions from binding to the soil particles--part of the process that helps limestone or gypsum break up the clay. Watering deeply when you water, so water comes out of the drainage holes, also helps reduce sodium problems. Depending on the local geology, river water and well water can be relatively high in sodium, as well, adding to the problem, without deep watering. Because of this, occasional watering with very pure water, such as captured rainwater, is recommended, too.

Bamboo, I got a replacement for the poop. Before planting anything in an empty pot I remove half the soil, stuff it with all vegetable waste from the kitchen cover it up again and then keep watering it, so I guess after a reasonable period of time it would rot and leech nutrients into the soil.
I am not very sure about the water content, but the water in my place is typically "hard water". So, I guess (not sure) it might be calcium salts which make it hard?
For pure water, I can use reverse osmosis treated water, or wait till the next rain.
There is a positive update about the chili plants. The ones in the small pots are growing vigorously with finger sized leaves and have grown to around 50 cm height.
Plus protecting the saplings from direct sunlight in my square pot is having positive results. I don't let the direct sun touch them and the leaves have slowly uncurled and it seems like the plant has started to recover.
Thanks for the excellent tips.

Hmm, well when direct composting like that, there should be 6 inches of soil over the vegetable scraps, so not sure how well its gonna work out over time.

I actually half empty the pots and then do it, so yeah it should be well above 6 inches :)

It doesn't work well in my climate, because when the heat starts rising, the decay becomes anaerobic, and the compost starts emitting methane and hydrogen sulfide, which kills the roots of whatever is planted above.

Oh actually I never thought of that. Till now I don't notice any bad smell, so does that mean it's doing alright?

Well, so far, so good! I never argue with success! :)

Sorry, tug. Since I was active on the other threads I missed this one.
You seem like a genius chemist-gardener sort of person lol. (no offense) :)

I'm an Arizona Master Gardener, and the college major I took before included a bit of chemistry. Here in southern Arizona, we are also always having to deal with alkaline and salty soil, so we learn a few tricks.

Your comments speak for you. This forum is a perfect place to get perfect tips from awesome and expert people :)