Possible PH problem or nutrient issue??

[WombatJr]

I'm a new grower at about 5 weeks into veg from seed. I'm growing in soil-less mix "Sunshine Mix" growing in 3.5 gallon containers. I have 400W HPS lights (3) over a crop of 26 plants (1/3 Blue Apollo, 1/3 Trainwreck, and 1/3 Northern Lights).
I'm watering with tap water mixed with Botanicare PureBlend Pro Grow organic plant food (3-1.5-4) at 1tbsp per gal strength.
Plants have been growing great up until about a week ago when I started seeing some lower and middle leaves on some of the plans turning yellow, brown and drying up.
I've noticed that my PH might be a bit high.. it's between 7.0 and 7.5. I've got some PH down to try to correct that with the next watering. Could that be all that's wrong? Or are my nutrients messed up?

I've attached a couple of pics from some of the plants. Any ideas?

Thanks in advance for your insight.

WombatJr.

[The Green Man]

Have you flushed your plants yet?

It could be a salt build up locking out nutes. When you use nutes you need to flush at least every 2 weeks.

[WombatJr]

Well, I flushed one of them... the one that I noticed the worst problem on first. Watered it in the tub with a LOT of plain tap water and let it drain out a couple of times. I can do that to the rest, if that is the solution, and/or if the first one starts to pick up.
I'm also trying to lower the PH of all the plants as I see that they are a bit above 7 right now. so I've added some PHDown to the water to try to get it down to 6.2 - 6.8 which is what I've read is correct for soilless mediums.

Thanks for your input.

Any other input??

[The Green Man]

If your using tap water that could be a problem. Tap water has chlorine in it and it needs a few days of sitting out to evaporate. Just a quick thought.

[homegrown4life]

Originally Posted by The Green Man

If your using tap water that could be a problem. Tap water has chlorine in it and it needs a few days of sitting out to evaporate. Just a quick thought.

What about tap water that is filtered from the fridge??

[The Green Man]

Well Im guessing since its going through a filter it would be fine. Do you know how good that filter is? I would test the ph just to be sure. You can also taste a difference in tap water straight out of the tap, and tap water that goes through a filter first. At least where I live you can, if it tastes freaking gross like my tap water I wouldn't give it to em lol.

[schmedrickk]

Wombat, it looks like your plants have a Calcium/Magnesium deficiency. Did you PH test your fert solution before adding it to the soil? I have found that when using water with a PH 7.0 and then adding ferts to it, the ferts can drop the PH a full 3 points in a heartbeat. Also the breakdown in soil plus the ferts can also hammer your overall PH.

Try adding 2 tsp per gallon of water of Cal-Mag or, if you don't have Cal-Mag, try 2 tsp per gallon of water of Epsom Salts. I have found that Epsom Salts are not very effective in reversing the problem, but will at least halt the problem. Cal-Mag seems to be the best from my experiences.

In any event, you'll need to get a handle on this fairly quickly, because a Calcium/Magnesium deficiency can significantly stunt the growth of your plants.