Help! Lower fan leave pestioles weak, wilting, AF Royal Critical LED setup

Hello, this is my first post so I apologize if there is any info missing.
I am a begginer and this is my second grow ever, and the first one using LED lights.
Heres my setup.

• Type of medium- soil
• Indoor
• Royal Critical from Royal Queen Seeds
• 36 days old
• 4 plants under 2 100W led panels, full spectrum. Note that I have only 1 Royal critical.
• How far from the lights? 10-12 inches approx
• What is your watering frequency and source of water? tap water, every 3-5 days approx, i just let it dry up till i feel no moisture in the first 2-3 inches of soil. I add half a cap of fertilizer every 10days.
• NPK? 6-2-4 Fertifox fertilizer
• What is the medium/runoff pH and PPM if in hydro? Not measuring PH right now, is this crucial?
• What are the temps and humidity in the room? 60-75 F, not measuring humidity
• What size pots? 15L / 4gallon pots
• Any bugs? 1 or 2 small flies in the whole indoor, they dont seem to be causing any harm at all.

I just noticed that the lower fan leaves and pestioles of the Royal Critical have started to wilt the last two days. I havent watered in 3 days so I dont think its an overwatering issue? The pestioles/stems holding the lower fan leaves have a purple colour on the top portion, while remaining green on the bottom. They feel extremely weak when I touch them, as if theyre not able to hold the weight of the leaves. The color of the actual leaves look good for the most part.

If there is any other info I could provide to help let me know.

The Royal Critical plant Im talking about is the first one that appears in the picture below,

The plant looks good overall, those lower leaves are going to die and fall off eventually. All I can say is make sure the pot is dry before watering (should be nice and light) don't over feed ether. Sometimes the lower parts that don't get light drop and die. Keep us posted

Thank you for the feedback, I was worried since I thought that would only start happening once the plant is already fully into the flowering stage.
One thing I did just notice is that the soil is very compact, like I have to make an effort to actually dig in those 2-3inches to feel the moisture in the soil. Is there anything I can do to "aerate" the soil at this point?

Ill keep you updated on how it progresses.

What kind of soil is this again? It does look really compact, which could end up be a problem. If it gets worse I would transplant them to slightly larger pots or add some very airy soil with like 50% perlite under the plant in its currents pot, also scratch up the surface a lil bit and add some airy soil. I don't think there's much else you could do?

Hey, it seems that it was a watering issue, not overwatering but underwatering I guess. Watered em last night, about 1/4 gallon of tap water per pot, without saturating the medium. This morning the fan leaves and pestioles I was worried about two days ago are standing up again, looking just fine. I will post pictures later.
Havent done anything about the compact soil yet, fixing to add airy soil with the perlites under the plants when I get back from work later.

Thanks once again, Ill keep you posted.