What's going on with my marijuana?

A question from a fellow grower:

Looking for some advise here. My setup is as follows. Grow tent is 3X3x7 and I am using a 400W Metal Halide lamp. Single plant per pot, pots are probably 3 or 4 gal in size. I’m using Miracle Grow potting soil with moisture control. Don’t know what the PH of my water is as I have no tester. Humidity has been running 50 to 58 percent and I have not used any nutrients as of yet. I don’t know what strain this is, as I got it from a bag of Mexican dirt weed. The plant has grown nicely and has many blooms on it. About 3 weeks ago, I noticed these brown spots starting to grow on a few leaves. over time it is getting worse and spreading and I can’t find anything online in any of the blogs or forums to tell me what might be happening. I was hoping someone here could tell by looking at my pictures. Also, I am growing a couple of White Widow Autoflowers that I purchased from you. One WW has grown to a height of about 11 inches and is getting covered in buds, just not growing much. The second WW has been on a growth spurt for the last 2 weeks and just amazes me everyday. It has grown an average of 1.7 inches every day. In eleven days it has gone from 13 inches tall to 32 3/4 inches tall. WOW! It is also forming buds up and down the stem as it grows.

looks like potassium Deficiency

check out the Chart - Cannabis

Flowering plants need lots of Potassium (k)
Vegging or growing plants need Nitrogen (n)

Thanks for your reply walter white. Going to research a fix now. The plant is still in the veg stage and was looking pretty dang good, until this spotting problem. It now has several leaves that have turn completely brown and curled up. Not having this problem on any of my other plants, so didn’t think it was a bug or insect problem. Although I do have some tiny, tiny somethings that fly around. Not big enough that I can tell what they are. Thanks again and I’ll try to update this thread to let you know how it goes.

Yup, it could be a nutrient imbalance, this is a great chart. You need to know your PH, this is the main reason for nutrient imbalances, sorry no way getting around this.

And fungus gnats/soil gnats are often a sign of over watering which can also cause nutrient imbalances.