3 week old seedling showing rust spots/yellowing HELP!

Hey guys so I have a 3 week old seedling in a solo cup growing under some cfls and it seems to have some sort of deficiency or is burned from nutes to early in its life. I put it in some regular potting soil ( i know....dum of me) mixed with some starting soil and some earth worm castings. I am watering every few days with reverse osmosis water and letting the soil completely dry out and the PH is around 6.5. I am not sure weather this is a cal/mag issue because of the RO water or nute burn from the potting soil I used. The leaf tips are not burned at all. The yellowing and rust is starting at the bottom of the plant and working up and also in the middle of the leaves. Any opinions are greatly appreciated.

Pictures.

time to feed it 1/4 strength of your nutrients . if it has rust spots with put pic I have to say its prob hungry. your using potting soil just black stuff. that don't have much nutrients in the soil at all . so the plant used up the available nutrients in the potting soil .

with out pic

Urinate in the pot and if u have a cat, scoop out some feces from the litter box, apply ontop of soil then thats when u urinate, the nitrogen in your urine will help and the cat poop is loaded with beneficial microbes

could be the earthworm castings burning it.. i think its like 10-15% of total soil used should be castings, and if early feeding was a problem, recovery needs to happen before anything else yaknow..how do rust spots on the leaves mean the plants hungry? rust spots are usually like calmag issue or or other nute burn/def, if the plant was hungry and its this young, the first sign of hunger would be yellowing leaves due to lack of N

EDIT: or could be bugs, check underside of the leaves for tiny little eggs, bugs jumpin around the soil?

double EDIT: and the "black stuff" youre thinking of is topsoil, potting soil is usually amended with nutrients...little (depending on the brand) but some..

triple EDIT: are you using ph up or ph down? is it a good amount?

Jussblaz3420 said:

Urinate in the pot and if u have a cat, scoop out some feces from the litter box, apply ontop of soil then thats when u urinate, the nitrogen in your urine will help and the cat poop is loaded with beneficial microbes

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Why do the parts of my outdoor garden where the local cats shit grow like shit then?

FlashBabylon said:

Why do the parts of my outdoor garden where the local cats shit grow like shit then?

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Those cats have a bad diet

Hate to be the one to say it sounds like spider mites they have tiny pin kike rust spot and the more they infest higher they get on plant look under the rust spoted leaves for mites