Brown spots on leaves, young plant

[Ataraxia]

I'm growing indoor under 250W HPS and the plant in question is potted in MG Organic Choice soil with perlite. I have added no nutrients yet, and the plant's current age is 20 days old. I noticed some slight yellowing on the lower leaves about a week ago and added some lime to the soil as suggested by bombbudpuffa. They yellowing appeared to subside a little bit, but now there's brown spots on one leaf, and another leaf is almost totally brown and curling up. Can somebody please tell me what's causing this, or what I can do to help make it better?

pics below

 

[Mr.Wakenbake]

Looks like you got Phosporous defficiency. However i can't really tell .. What is the pH ataraxia?

 

[Ataraxia]

Thanks for lookin in Mr. Wakenbake. I'm not absolutely sure of the pH, my meter is in the mail, (shipping from Australia, and taking it's sweet time). I water with Reverse Osmosis water with a pH of 7 which I know by pH test strips. That's the best I can do for now. If it is indeed a phosphorous deffeciency, what can I do to add just phosphorous? Is it common to have a deffeciency this early? Well, I guess it is 20 days old, just kinda small. Should I add some type of nutrient solution? I have Botanicare 3-1.5-4, Tetracycle .03-.002-.03, and Fish Emulsion 5-1-1 at my disposal to use for organics right now. Is there any simple organic way to boost phosphorous?

Thanks

 

[Runbyhemp]

It could also be the Mg soil. Although some people have good results with it, I think for most, it causes nothing but problems.

 

[Ataraxia]

I repotted the sick plant as gently as I could, and hopefully it responds well. After seeing her problems, I repotted her week younger sister into Fox Farm Ocean forest with great results. It's now my biggest plant. No improvement yet with the sick one, but it's only been a day in the new soil. Hoping for the best. Thanks for the input everyone

 

[papabeach1]

like i told other growers in these forums it came to my sense, same thing to my plants, i realized what i done, i feed them with my water from sink in past, i gets these yellow spots, brown, too, i did tested it with fresh bottled water, i see no brown no yellow, so it came from chlorine to prevent it, use bottle water please, u want grow best buds, use bottled water and u can sleep good tonight and again tomorrow sleep good too