Black/brown spots appearing on my leaves

Need some help with an issue I never before encountered.

I'll start with some relevant background information. I'm using soil, nothing else.This particular soil is a good brand that many in Sweden use for growing and is highly recommended, also I have previous experience with it and it worked well before. I'm using water from the tap, clean drinking water. The light is 150watt hps + 23watt CFL, 2 x 140mm computer fans blowing. Temperature of about 80-86F.

I have 5 seeds, 1 PinkBubble and 4 Papaya. Pinkbubble was planted around 12th February, and the Papayas around 24th February. We can exclude issues due to seed genetics, as its 2 different strains from 2 different sellers, the odds of getting same issue would be very small.

So that leaves us with either the soil, or the water.

The water has worked fine before and should be no issues with it, so lets say it's a small chance the water cleaning stations have changed the composition, perhaps more chlorine or something else, and this is the issue.

So the soil, what could it be, since its supposed to be a quality soil, I think deficiency can be discounted, unless its deficiency in some micro nutrients?

Could it be something in the soil in terms of bacteria?

The issue appeared on PinkBubble about 12-14 daysafter planting it, and I noticed first spots on the Papayas just 2 days ago, which is when they also are about 12 days old.

The spots do not appear randomly however, they seem to appear first on the bottom leaves and starting from the tip usually, but not exclusively, and tend to move from the tip and inwards untill the whole leaf gets kind of brown/black and dries out, and then it moves up the latter to next set of leaves.

Greatly appreciate any help and tips to handle this.

Looks like the soil is too rich in nutrients.

If it is nutrient burn, would it not fix itself after a week or 2, when some amount of the nutrients have been used up and down to normal level. Because the plant doesnt seem to be stunted or stop growing, the newest leaves are nice and no signs or any spots on them. It has only affected the older leaves and it progresses upwards as time goes. Also what strikes me odd is that it doesnt show all over the plant, somehow It seems logical that the newest leaves would also be affected. Another curious thing is that it started to show after about 2 weeks, if it was that the soil is too rich in nutrients, shouldnt a 4-5 day seedling be much more vulnerable then a 2 week?

Its over watering damage

Lol waiting for someone to say calmag.

I.would nutes it. Npk are mobile nutes. So if your
Lacking food for your plant. Older leafs show signs first. And not treaded will go t o new growth too.

Give it some nutes

It's guaranteed not over watering damage, as my leaves have not even once drooped down as they do with over watering, I tend to them daily several times and am really careful with the water.
If they lack some of the NPK nutes, it must be either P or K, since obviously its very green and nicely colored.

I have experienced low nutrient soils before and usually you would notice the N shortage first, as it mostly needs N in the beggining, so the leaves would start to get yellow at the bottom quickly. My have no signs of yellow almost 1 month old now.

I assume it could be PH imbalance. My soil says its between 5.6 - 6.4 PH, perhaps a bit on the low side so that it cant absorb some nutrient (?)

the brown leaf spots are sign of P def. Plants use P at the beginning of their lives for root development. Keep your pH between 5.8-6.2. Try doing a slurry test with your soil. Take some of the soil, dig in there a little bit. Then take very little water and make it so its muddy. Then measure the pH and EC of it. But yeah looks like you could give it some more. And then blast it will some cal/mag! jk. dont.