Yellowing Leaves

I have a few plants developing some yellowing conditions in the leaves.  The appearance differs somewhat from plant to plant.
 
Some taller baccatums and annuums have lower leaves that are yellowing and dropping off (no pic).  By "taller" I mean 8" so these are not old leaves.  I wonder if this could be from a magnesium deficiency.  I have begun a foliar Epsom salts spray.
 
The pic below shows three plants (top - Bhut Jolokia, left - 7 Pot Yellow, right - Reaper) displaying yellowing in newer growth.  The yellowing is more solid in the 7 Pot  versus an irregular pattern of inter vein yellowing in the other two.  
 
I've read some suggestions that these conditions may arise from iron deficiency brought about by high pH.  I've also read arguments that pH in containers is of much less an influence than in the ground (which makes sense to me given we pretty much control the delivery of nutrients to plants in containers with our watering regimen).
 
Growing medium is MG Potting Mix
I try not to over water.
I did soil testing today on the 7 Pot and the Bhut soil.  I used the Mosser Lee test kit.  
pH 7 Pot 7.5
pH Bhut 6.5
nitrogen - both high
phosphorus - both medium
potassium - both high
 
As the MG is supposed  to have fertilizer I have not fertilized much - only watered with Fish Emulsion (5-1-1) a couple times.  (I use Fish Emulsion because I love the smell)
 
Thoughts on causes?   And remedies?
If an iron deficiency what's the simplest way to correct it?  And do I need to test for iron levels?
 

Firstly I'd quarantine the 3 plants just in case, wash your hand before handling any unaffected plants.  It seems very strange to me that three different varieties would be exhibiting the same symptoms.
 
MG "Potting Mix" sometimes needs to be cut with something like perlite or sphagnum.  I'd almost think the roots are getting choked.  Any idea what they look like?  All three are C. chinense plants...this species is an aggressive rooter.  They might already need to get potted up

can you get a better picture of the one in the lower left? the other two look fine.
 
the light is all yellowish and cant really see anything.
 
iron should start in the back of the midrib, moving forward. it will get pretty much pure white.  cant really tell from this picture, but you could be correct.

According to one reference, high levels of potassium can block iron absorption -- "nutrient lock" is the term many people use. Iron uptake can also be blocked by higher pH -- above 6.5 is of concern for many plants, but i think 7.0-7.5 is about right for peppers.

My experience with peppers is minimal, but most plants that manifest yellowing of young foliage are suffering from iron deficiency. The other, less likely, prospects are zinc or manganese.
All three have been addressed in a product referred to as 'trace micronutrients', or "trace mikes" -- a foliar-feed spray. I still have a package i purchased 15 years ago. It's basically refined chemical salts of the trace metals involved.

More natural sources for the trace mikes are: iron-blood meal; zinc-wheat germ; manganese-kelp.
Or a seaweed fertilizer, of any sort, can provide all three. Note that most of the liquid seaweed fertilizers are almost as fast-acting as the trace-mikes, when applied as foliar feed.

Typically, when apical (growing-tip) buds and young leaves start yellowing, I spray trace-mikes or liquid seaweed fertilizer on foliage and supplement with kelp granules in the soil. If i have the space, i pre-ferment it in a large pot or bucket with an approximately equal volume of peat. Keep moist, but never dripping (if you wet it that much, you're flushing away the nutrients you wanted). Aerate by transferring to another bucket... daily in warm weather, 2-3 times/week in cool (below 65°F) weather.

One of the best sources for trace mikes i ever accquired was several pounds of stale Orange Pekoe teabags, from a restaraunt owner... although it was tasteless from a culinary view, it was rich in all three minerals. Teabushes are a very dark green (they're a species of Camellia), and the Orange Pekoe grade is apical foliage -- buds and very young leaves. Black tea, being pre-fermented, breaks down quickly -- particularly when it's ground up to a fine powder.
[Note to other bachelor slobs: it would be wise to wipe out the coffee grinder with a paper towel, then purge all working surfaces of tea scent by grinding a handful of bran or rolled oats, the wipe out the grinder again... my then-girlfriend made the next pot of coffee, and i still haven't lived that one down.]

If you are using a similar grow regimen for tomatoes or basil, you'll likely need seaweed (or other trace mike sources) for their health too.

Two of mine was like this, i gave them potassium and they now looks fine :-)
 
Before Potassiun

 
3,5 weeks after using potassium

 
I dont have any good picuture of the plant today, but i can take one later.

Plants that young might be getting over fertilized.
With the MG, I'd just stick to water until they are a bit larger.

Dave

My plant today that had yellow leaves. I have used potassium phosphate on the plants that had yellow leaves. All my plants are under LED light.