Hello guys, is anything wrong about this yellowing leaves on seedlings?

No that’s normal if you started out flowering it that young, not getting enough light for greener foliage, but it would get greener as it get older

Drop your ph to 5.8, and as they grow you increase it. Let them develop there roots, I would started them off in a smaller pot to built that root better. Don’t feed them anything, drop that ph and leave them alone.

Well 6.5 PH for grow period is too high my friend, thats the correct PH on flowering where the plant dont need to absorb that much nitrogen for example (there is a whole table on the internet explaining what nutrients are blocked by a too high or too low PH) and the light green yellow part inside the leaves is iron, i had these issues before, there is also lot of information around that on the internet, leaves should turn that color by excess of water, excess of water make the leaves hanging like without energy like it doesnt want to live anymore🤣, in my opinion i keep saying its iron caused by an incorrect PH in your case too high as its on 6.5, but maybe somebody else thinks different, just my opinion from experiences i had previously

hello friend, about fertilization, it's not guano (guano is bat feces, right?). My substrate is basically inert (peat and perlite at a concentration of 85% + 15% humus). And I don't think It would be a Ph problem, because I am using bottled water (Ph 6,5 from the label). Anyways, I am ordering a phmeter to test that I was quite in doubt about whether or not I was iron deficient, but I no longer believe that this is the problem because the seedling got a lot of sun in the morning and the excess water evaporated so the leaves got better (not able tô upload anymore)

Ah, sorry! And about starting fertilizers, you're using guano if I understand correctly? Do you happen to know the micronutrients available in that guano? But it seems strange to me that you would need fertilizers already. Look, I use a light mix for most of my grows, and I don't start giving them anything until around week 3 or so. With guano, they should have enough food for their first few weeks. Maybe when adjusting the pH, check out the micro from Advanced Nutrition; it has all kinds of micronutrients. Since you're using guano, you could use micro as a supplement in low doses. I've been using this for quite some time, and I've never had any problems—no yellow leaves or deficiencies of any kind, always nice and green. Obviously, this is just advice; it all depends on the initial guano mix you mentioned because maybe that guano already has everything you need.

Hey again, organic or not, it's always advisable to measure the pH. There are meters on Amazon for affordable prices (in case you don't have one) and just adjusting the pH won't solve it, yes and no. Let me explain, if you're currently using Canna iron, for example, you'll continue to suffer from deficiency since your plant will never be able to absorb the iron (and other substances) properly, and it's going to be an endless cycle. I would start by having a good pH around 5.5 (and in flowering 6 - 6.5) and measure it once you have the fertilizers mixed. Surely just adjusting the pH will solve your problem since there's probably enough iron available in the soil, but the incorrect pH is blocking absorption. If after adjusting the pH we see that in a week the new leaves are still yellow on the inside, it would be time to look for another solution (like Canna that sells iron alone) or something similar from your preferred grow. And about watering, well, it depends on the climate. Where I live right now, there's quite a bit of humidity, and I water them in the morning, and my indoor every 2 days, but of course, that depends. You can never say x amount for x time because every climate changes, so no problem, just make sure not to overwater, but if the soil is dry and cracked, then it's better to water. 🤣

Ohh, and the soil I m using: sphagnum and perlite (85%) + worm cast (15%)

No, I am not checking soil pH and I thought It was not going to bem necessary since I will use organic nutrition. Regardless to watering, If I don't do It by night the soil Will be cracking really dry. At least on early growth, I don't think would be viable to water 600-700ml early morning because at 4-5h the soil would be like a desert, but I can try it. about the deficiency, only way I could solve it would be correcting the ph? by any chance could this be the plant asking to iniciate the fertilization?

Are you checking the pH? It looks like a lack of iron, my friend, but this could be due to an incorrect pH since it blocks nutrient absorption. In my opinion, there's no need to water twice a day; just because the soil is dry doesn't mean they are thirsty. Give them a good watering early in the morning when the sun is coming up and leave them like that. The next day, check if they need water (normally, such a small plant doesn't need that much water; indoors, you water every 3-4 days).

Is It due to underwatering? High temps? Lack of nutes (i ve not started nutrition yet)? Sunburn maybe?

even so, around 2pm the soil is already quite dry. I've been trying to fix it by increasing the watering over the days. In dead, I've started watering about 100ml day and 100ml night like 4-5 days ago and I ve been incriasing 50-100ml day. Despite that all 9 seedlings has been acquiring yellowish tones mainly in the new leaves that are born.