I currently have a living soil set up four week ol...

@Organoman is pretty spot on. Plus not having a diary doesn't help. If you got the cal/mag in your medium, it should be ok. If the yellowing gets worse, check your PH. If it's an auto, you're transitioning. Or pretty close. Also, should be close to feeding time, week 4. Google over feed or nute burn, then check the pics. You don't have it.

Hey mate You need to add a foliar spray, some Epson salt sprays will help. No matter what's going on below you can attach the top without causing much issue or worrying about ph. So, mix 150ppm or so (optimally Epsom salt 150 ppm added into tap) lower your light to 25% just enough to keep photosynthesis going. Make sure you have good airflow as if it dries In slowly it'll block the stomata vents. But, they need mag so they should take it in pretty quickly with good airflow. Also, if you've a calmag, check the label and see if you can use it as a spray. It would take a few days or to fix it in the soil base. So you can add some in but a spray will stop it getting worse until the soil is fixed. If you need more help. Feel free to reach out.

No, a plant "absorbs" nutrients from the entire pot, not just the top or bottom...........roots will be filling the whole volume of the pot. I can't see any real issues with nutrient "burn", but some of the plants do look like they are developing a magnesium deficiency.

Also living soil doesnt like to dry out that much, your plants are looking thirsty/droopy this could be to other reasons too though. A diary would help to diagnose this. Try to keep the soil at least a little moist at all times if the plant is in a proportional size for its pot if possible, or just let the surface dry a little bit before doing a full watering again.

Hey, having nutrient burn with living soil isnt that common. Your plants look a bit light green in my opinion you can feed them more. The issue youre having is clearly a magnesium deficiency. Add a CalMag supplement! For your next grow you could prepare the soil with amendments like raw gypsum (calcium sulphate), ultra fine basalt or granite rock dust (besides calcium and magnesium it will provide many micro nutes like iron, copper, zinc, silica,... aswell), , epsom salts (magnesium sulfate) and garden lime (calcium carbonate). Its good to have those 4 + a CalMag supplement on hand anyways for every grower in my opinion, especially when growing with LED´s which for some reason force a higher demand in plants for calcium and magnesium. Good luck and happy growing!