N or P problem???

So I've been doing alot of research on plant problems. My largest & oldest plant is looking sick (well, not that large & old-only 3 nodes). The bottom leaves are turning yellow and drying up (actually the 2 most bottom nodes). The top leaves are nice and green, and look healthy. The stem is also purple. I checked the pH and its between 6-6.5. I have close to 1,000 lumens 2-3" away from plant. I was over watering (with distilled water), but I stopped that. Now I only give water when the soil 1" deep is dry. This is the only plant that is doing this, and it really started to do it once I stopped watering it so much.

Could this be a N or P deficiency? If so, what exactly should I use to correct it? The plant is too small to start fert. I'm worried if I start to give it fert, it will burn the young plant.

I will have photos soon. Thanks everyone.

What is your medium...if your in soil, the soil pH needs to be 6.5 - 6.9, if your in hydro your pH needs to be 5.5 - 5.9, those are the ideal ranges for the medium. Have you been giving them nutes? Your also using a distilled water, and i think, that water is missing some needed micro nutes like Ca, Mg, etc...

I forgot to add, I have not given it any fert. I am using "Garden Magic Potting Soil." The only kind of ingredient list says "Peat Moss, Sand, Perlite 80%." This leads me to wonder what the other 20% is. It says nothing about any fert added. I mixed the soil 2:1 with added perlite because I noticed the soil can be quite compact on its own.

What type of water would you recommend? Tap water left out for a few days? Rain water???

the peat will keep your soil running hot (low pH), there could be some nutes in there that they are not saying, hard to tell, should be somewhere on the package, but that could be the 'magic' part of the soil. I use stuff called "Black Earth" and add perilite to it, i cant say enough about it, and there is another medium that is made by FoxFarms and its an organic soilless mix that does not have coco in it.

You can use the distilled water, you just have to add the micro's into it if you are or you will get nute def's. The tap water is fine as long as your not well water, and if you are, you just have to check and see if its got alot of stuff in it, other wise known as ppm's (parts per million). A normal city water will have somewhere in the 100-300ppm range, and that has your micro's in it that your plant needs.

To get your soil pH up, try doing a flush with pH adjusted water....and capture that out the bottom of the pot and see what it is, then you can go from there, just keep in mind you want your medium (since your soil) to be a pH of 6.5 - 6.9 and that is of the water coming out of the bottom of the pot.

Hope that helps