Leaves turning yellow in late flower

The leaves are turning yellow on some of my plants. It started in the middle part of most of the plants. It's yellowing on the older leaves, and some of the tips are turning up. One plant has brown patches on the yellow leaf. I know it's probably a nut def., but I don't know which one. Any suggestions?

Soil Growers:
1. Are you growing from seed or clones? Clones
2. How old are your plants? 6 weeks into flower
3. How tall are your plants? 25"
4. What size containers are they planted in? 8" diameter by 5" deep
5. What is your soil mix? Fox farms ocean forest
6. How often do you water and what type of water do you use? Every other day, and tap water.
7. What is the pH of your water? 6.0
8. What kind of fertilizer do you use and what is its NPK ratio? Don't use
9. Do you foliar feed or spray your plants with anything? No
10. What kind of lights do you use and how many watts combined? (HPS, MH, fluorescent, halogen, incandescent "plant lights") 400 HPS
11. How close are your lights to the plants? 12"
12. What size is your grow space in square feet? 16 square feet
13. What is the temperature and humidity in your grow space? 60-75
14. What is the pH of the soil? Don't know
15. Have you noticed any insect activity in your grow space? No
16. How much experience do you have growing? This is my first grow

if ur plants are mid to late flower its totaly normal for leaves to yellow and even drop!! if u think neut burn is an issue just take ur plants to the bath and flush them with teppid (luke warm) water 3 times the amount ov soil. ul be fine

yeah their good, when the plants nearing the end it will start using stored up energy in its leaves.

Thanks! I thought that I had a bad problem, but now I can sleep.

yes u can sleep fella,ur plants are using upthe very last stored energy in the leaves now for the bud,from now on ur buds should realy start to swell, if any leafes are realy yellow and almost droppin just pick them off at the stem, they should almost fall into ur hand, the plant no longer needs them and may be takin up precious energy that could be directed to more usefull places AS IN THOSE JUICE NUGZ LOL