What's wrong with my plants (pics attached!)

This is my fourth CFL closet grow and as ever i'm using autos. The larger plant is an Automatic Haze (around 5 - 6 weeks old) and the smaller a Blue Mystic (about 4 - 5). The plants are kept in a large cupboard under a 300watt CFL and reflector with two medium fans for ventilation. I am pretty sure that the problem is not due to the setup as my other grows have been under the exact same conditions and have always yielded around half an ounce per plant (dry).

As you can see, the bottom leaves are yellowing and eventually dying. This started about a week ago on the very lowest leaves and is now working upwards towards the top of the plants. I am feeding them with Canna Flores, around 5-10ml diluted in 2 litres of water every week or so. I also water every 3 or 4 days inbetween when the soil feels dry below the surface.

Is this a nutrient problem? If so, too little or too much? I've heard of plants taking nitrogen from expendable fan leaves to make up for difficiencies - could this be the case here? I'm sure I fed my last plants a little more often but didn't want to burn them so have kinda held back a bit this time!

Help!!

Looks like nitrogen deficiency. Do you know the ph of your runoff? You may just need to adjust your ph so it can utilize available nitrogen.

I don't have any way of measuring the PH unfortunately. I know it can be increased by adding a but of bicarbonate soda to water. Is this worth a shot?

they look like they need nitogen, as xara says, if you want to have a succesful grow get yourself a ph meter. even a cheap one is better than nothing. if your not phing the water then its locking out certain nutrients. also dont water just because the top of the pot is dry, they could still be ringing wet in the bottom and damp soil can also lock out nutes. lift the pot and feel how heavy it is when dry and wet. these plants love to go dry wet dry wet. never damp damp soggy!
lu

no dont raise PH. odds are ur tap water is 7 or higher. nutes are made of acid usually and will lower ph to 6.5 if ur water is ph 7

u want 6-7