Burned leaf tips and rust spots

Hello Thank you for any help!

I've been having problems with yellow leaf tips since vegetative day 35. In the past few days, they have turned into burned tips, but only on some leaves. Others, on the other hand, are not affected at all or only minimally. Additionally, there are a few rust spots.
The lowest leaves turn completely yellow, and those pressing against the tent and receiving very little light turn brown and essentially die off (I think that's normal?).

Strain: Gelato (Zamnesia, Feminized)
Plant size: 100cm
Pot size (liters): 11 liters
Cultivation area (m²): 0.16m2
Medium: Soil, Plagron Light Mix
Duration of vegetative phase: 40 days
Duration of flowering phase: 36 days
Current veg or flower day: 36th day of flowering (after switching to 12/12)
Fertilizer (type, frequency, and quantity, N-P-K): Plagron Bloom (NPK 2-2-4). Since day 22 of flowering, 5ml with every second watering.
Lighting (wattage and distance): Sanlight Q1W, 50 watts, at 100% intensity with a 20cm distance.
pH/EC (if measuring device available): pH: 6.5
Air humidity: Currently 45-55%
Air temperature: 25-29°C
Soil temperature: 25-29°C
Watering: Quantity: 3.7l for an 11l pot, minimal runoff
Watering: Frequency: every 3 days
Water source: Original EC and pH values - use of additives (e.g., pH minus, etc.) - how it is prepared, dosed, and measured.
pH: 7.5, EC: 0.4. Let the water sit for 48 hours before watering. Add fertilizer, then measure and adjust the pH level.
Pest treatments (frequency and what type): -

SliceOfBread said:

The lowest leaves turn completely yellow,

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Classic sign of N deficiency. From your photos, the rust spots look to me like maybe Ca deficiency. I'm not familiar with your nutrients, you say you use 5mL every other watering, but don't say with what quantity of water. Are you following label instructions?

At some point a ways back they got too much N, the older dark shiny leaves show that, but now it looks like it’s burned through the N and may need some more. Be careful, as too much N can affect Ca uptake.

simpleleaf said:

you say you use 5mL every other watering, but don't say with what quantity of water. Are you following label instructions?

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No, im not following label instructions. Im watering 3,7l (about 1 gallon) every 3 days. Should i add some Ca/Mg next time?

i'm not a dirt guy but the yellow leaves in the shade are no big deal. they aren't getting enough light to be of use to the plant

SliceOfBread said:

No, im not following label instructions. Im watering 3,7l (about 1 gallon) every 3 days. Should i add some Ca/Mg next time?

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Hello This is my first post here so you can take it with a grain of salt, but yes, I would add cal mag. It also has a bit of nitrogen and will likely give you enough N to balance you nutrient needs.

SliceOfBread said:

No, im not following label instructions. Im watering 3,7l (about 1 gallon) every 3 days. Should i add some Ca/Mg next time?

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I found a usage label here. They call for 5 mL per 1L with every watering (unless I'm misinterpreting their chart). You're getting great growth at a very small fraction of their recommended strength. Extra calcium would be a good idea, but you might also consider bumping up your usage strength -- just a little. Be careful, Ca is antagonistic to the absorption of other nutrients.

Thank you, @simpleleaf I am aware of the chart, but I deliberately choose not to fertilize according to it. I want to learn how to recognize plant deficiencies myself and then adjust the fertilizer accordingly. Moreover, manufacturers often recommend higher fertilizer quantities to sell more of their product. I will add Cal + Mag during the next watering. Thanks for all the answers and help!