Sooooo... I'm new here but definitely not to this world. You have a couple of things going on here.
1) Your soil uhhh sucks. But you can fix that.
2) You have a nutrient deficiency going on. The plant needs more than it is getting. But... you can fix that.
First, GENTLY switch your light cycle back to 18/6. That means gradually. Like... today you go to 14/10... hold for a few days. Then 16/8... hold for a few days. Then 18/6 and stay there. Your plant is WAY too small to flower if you are going for excellent yield. If you are space limited, then say so and we can fix that quick too. But you need to be in veg at this point.
So for your soil... Go to your local whatever store and get a bag of perlite, good potting soil, and some cow shit. Walmart sells it. You may have a small amount of perlite in there, but it is not enough... no where close. In a 5 gallon bucket mix up a few cups of perlite, a few cups of cow shit, and the rest will be that potting soil. Mix it up. Mix it up REALLY good. You don't want those roots in pure cow shit. The nitrogen will eat them up.
Take your plant out of the pot. If you see a bunch of roots jammed on the sides, then you are root bound. Do the bonsai thing. Take a very sharp knife and cut a 1/4 inch of soil and roots from the sides and an inch from the bottom. Sounds scary... but it's not.
Take some of your new good soil, and pack a good 3/4 of an inch to an inch in the bottom of your existing pot. Set your plant on top of that new dirt. Then pack in more good soil around the sides. And then fill up the top until it is even with the top rim of the cup. Dirt is going to settle when you water anyway.
This all solves your dirt issue.
Now nutrient deficiency? That's also very easy to cure. Whatever nutrients you use for veg, mix them up but mix them weak, like half strength. Water vigorously. Soak it slowly. Keep the nutrient solution off the leaves. Those two little yellow leaves at the bottom? Trim them off. They are taking more from the plant than they are giving. Leave any growth shoots alone at this point. That plant needs all of the energy it can take in at this point.
Next, leave the plant alone. Observe the soil. When it appears to be dry and the leave JUST start to droop under light, then flush with fresh water. This will ensure you don't get nutrient locked. Salts can build up in any growth medium. You don't want that to happen. So in between weekly ferts, clear water but ONLY when the plant needs it. Over water can cause mold, fungus, root root and a whole host of other nefarious shit.
You'll see the plant seemingly pause for a couple of days, green up, and then blast off.
I know this sounds like a lot, but after you have a couple of years under your belt it will all be second nature. Being tuned into your plants' needs, strain quirks, and familiarity with nutrients helps a lot.
A note on nutrients? Don't believe all of the bullshit hype coming out of fertilizer and hydro companies. Faster grows and more nutrients are not always better. I use the following and will simply stack plants and yields against anyone:
1) The best of the best in soil mix. This ensure the plant is getting exactly what it needs to thrive.
2) Fall over, but miracle grow? Best supplemental fertilizer I have used in soil gardening.
3) 5 Gallon Cloth Grow Bags for smaller strains (Low Ryder, anything predominantly Indica).
4) 10 Gallon Cloth Grow Bags for anything predominantly Sativa.
5) Use indoor lighting for supplemental only at night when there is no sun.
6) Outside sun (if you can without risk) always before lights.
Here are two pics so that you can see I'm not full of shit lol! The first pic is Pineapple Express. It's in veg and being trained. I'll be ready to flower it in about 2 weeks when those side shoots are straight out and about 8 - 10 inches in length. PE grows really tall. Bushing it? Don't like to supercrop these. I like tight tall nugs. IMO this is the best way to get them. It will get re-tied over the weekend which just involves sliding those nooses out more towards the outside of each shoot instead of on the inside.
The second pic is White Widow. It's about 2 weeks into its flowering cycle. It's starting to cola up. Side branches were trained like the PE but not topped like the PE. The side branches have shoots growing straight up reaching for the sun that average 6 - 8 inches each. When it's done, they will all look like mini colas. Again, dense tight large nugs.
