As far as I know, the bottom leaves are supposed to turn yellow during the flowering stage. Otherwise, check this thread for numerous deficiencies with pics: http://forum.grasscity.com/general-indoor-growing/374679-essential-plant-nutrients.html
best guess could be nute burn. need more information really. What is your soil PH, water PH (after nutes), runoff PH. What nutes are you using and how much? What is your grow temp? if the tips are turning brown then you could have an issue with to low humidity.
Its not humid enough and its too hot.. Thats a given, most of my nutrients are like 1-1-1 and I have big daddy with is 0-0-0. It could be the nutrients, but I think I need to add humidity to the plants, euh how do I do that? I'll add more fans to circulate the air and cool it down.
Everything you need to know about pH and your plants can be found here. Its worth while reading.. keep in mind that this is the short version. The full document is at http://growersunderground.com/pH_Manifesto.pdf Hope this helps EDIT: you want to add humidity? - get a humidifier!