three things come to mind:

1) Excessive Salinity in the soil (did you use "washed" sand?)
2) Too porous growing media. 50%+ is a bit high.
3) How much (and which type) Osmocote did you use per volume of soil?

Thanks Igo,

1. I'll check the sand bag later tonight to see if it says anything about washed. It was labeled as "play sand," if that means anything.

2. Does too porous mean that plants to be watered more frequently? Or does this somehow affect root growth? Other problems associated with this?

3. Re Osmocote, I'm not sure at this time, but I think it is the "green" label vegetable slow release formula...14-14-14. As to how much, I don't recall. I do know that I used less than was recommended. Maybe about 1/2 teaspoon per gallon of potting medium.

You have an immobile nutrient deficiency. And possible root problems from adding so much fine sand.
Salt would have caused problems months ago.
Sand and perlite do not hold nutrients so what you added in spring is likely gone by now.
Go buy some seaweed extract and foliar feed as well as add to the pots. More lime too, my guess is calcium or iron deficiency.

Here is a link that might be useful: http://edis.ifas.ufl.edu/LyraEDISServlet?command=getImageDetail&image_soid=FIGURE%206&document_soid=SS530&document_version=42330

Actually, under normal circumstances of salt from just watering, Perlite has a tendency to hold onto salts to the extent that it needs to be replaced every few years. So it is conceiveable that it has reached a level to become toxic to the plant if the sand had not been washed out before mixing the medium. The additional problem could be that since the medium is so porous, you are leaching out nutrients at an accelerated rate and subecting the plant to more frequent droughts. Iron deficiency is not the cause as it results in intervenial yellowing and your leaves show signs of total yellowing and necrosis, not just between the veins. My quick solution at this point is to change your mix ratio to a lower % of sand and ensure you use washed sand. May want to use a different brand. Some recommend waiting until your trees are dormant to do this. That may be ok but your growing tips look like they are dying. I routinely do bare roots repotting on figs throughout the season with no harm to the plants. I simply submerse the root ball in a bucket of water and work all the dirt loose then fill with the new mix from the bottom up with lots of shaking of the planter to get the mix between the roots and then water by submersing the potted plant once again and pressing down to remove any remaining air pockets. Good luck!

Actually you are making things up. Like I said, perlite and sand do not hold nutrients, or salt, or carbonate ions, prove otherwise. Salt damage is interveinal necrosis by the way, see any of that in the pics igo? I will also add that F. carica is salt tolerant.

Geoff-There is nothing normal about salt accumulation from watering, unless you are using softened water, but you did not mention that. Barerooting is a bad idea, it would help but only because the tree will get the nutrients it needs from the new mix. Just add lime and micronutrients and ride it out for the season. No need to stress the plant (young cutting with tender roots) more.

I appreciate the differing perspectives. H-banana, do I need to be concerned about fertilizing so late in the season? These plants will be going dormant in October or so (zone 6).

Thanks,
Geoff

Nitrogen Fertilizer would cause you problems, especially time release. Seaweed extract does not contain N and will actually help the wood harden because of the K. It will not hurt one bit. I can see your plants have plenty of nitrogen because the lower leaves are not yellowing. If you used MG perlite it had fert as well, so adding osmocote was not necessary.
It could solely be a ph problem. If you can test the mix with some litmus paper and ph neutral water you will know for sure. Nitrogen fertilizer tends to acidify potting mix when it gets washed out, so I really think that is what happened. Too much fert and now they are too acidic. Hope you figure it out.

Hoosier, apparently you know something that universities and the professional growers do not. They must like wasting money to increase profits. As you requested, read the following links for education. FYI, sodium chloride is not the only "salt" out there. Moving on...

http://text.lsuagcenter.com/en/communications/publications/agmag/Archive/2002/Summer/Recycling+Perlite+for+More+Profit+in+Greenhouse+Tomatoes.htm

http://aggie-horticulture.tamu.edu/greenhouse/nursery/guides/ghhdbk/media.html

The first article says that greenhouse tomato growers need to replace their perlite because of salt and pathogen buildup, OR they can take care of both with a hot water process. It is obscure to this discussion because: 1. Geoff's plants are not in a greenhouse so they are leached by the rain 2.The timeframe is still wrong (1-2) years before replacement, not 4 months 3. Tomatoes are less salt tolerant than figs, and this article outlines successful tomato growing only.4. there are dead tomato roots in this equation, they prolly contain some salt.

The article clearly states that salts can be leached from perlite, it tells you to use hot water but remember they are killing insects and diseases too. So your article proved what I said earlier, perlite does not hold salt, because it can be leached easily.

The second web address is to a basic potting media guide that makes no mention of the topic.? If you think I missed something important then why not post the quote so everyone does not have to copy and paste those links into their searchbar and read through to find what you are talking about. Because they should not just take your word for it, the info you provided is obscure and irrelevant like I said.

There was one gem that applies to Geoff's situation though, the play sand specifically. His mix has poor drainage and aeration as a result of fine sands.

"Fine sands (0.05mm - 0.25mm) do little to improve the physical properties of a growing media and may result in reduced drainage and aeration."

FYI I know there are more types of salts. Your response to Geoff's question was gobbldygook related to this very topic. You say that "salts" are accumulating in his media, but fertilizer salts, you called them "nutrients", are leaching because the mix is too "porous". Care to explain how that is possible?