Blood Meal, 12-0-0 should work and it is easy to find at most farm stores. I use it and also worm castings for general all around health. You could also use liquid fertilzer like MG. I would go light because you don't want a lot of new growth with the cold comes around.
Just my .02 worth
1eyedJack and the Dawg

Nitrogen leaches out of the soil very quickly so many soil tests don't even test for Nitrogen. Unless you test right after fertilizing, almost any test will show you need nitrogen.

True, nitrogen comes and goes very quickly sometimes, but if your plants are showing poor performance, then the test is probably right.

I am a little perplexed, though, because my experience has been that clay soils hold nitrogen fairly well. Did you ever mix wood chips or another high-carbon material into the soil?

Jack is right about blood meal. It's an excellent quick fix. Corn meal or other seed meals will work great too. They are cheaper, but may be harder to find and seem to take a little longer than blood meal to start producing results.

-Diggity

One thing to note about blood meal is that since it's as high in N as it is and is fast release, you can easily cause nitrogen burn if you apply too much (unlike with most organic fertilizers).

Really quick fix, (no need to go to the store to buy anything) diluted urine. it will add other nutrients as well though.
A little slower fix would be to brew some green manure teas using something like alfalfa or comfrey leaves that can be used to foliar feed while you wait for the seed meal pellets you might use on the soil to become available to the roots.

This was, apparently, one of those very unreliable home soil test kits that may or may not give you anything near what you really have. Soil Nitrogen is very dependant on soil temperature, and the activity of the Soil Food Web in your soil. That that test showed no Nitrogen showed tell you that the test is at fault, because there would be some Nitrogen if the plants there are growing at all.
Contact your local office of the Texas A & M University USDA Cooeprative Extension Service and have a real, good, reliable, soil test done before doing anything. It is highly unlikely they will test for N but if those trees and shrubs grew at all there is N in your soil.
What is the level of organic matter there?
What is the soil life activity, the number of earthworms?
How is the drainage"
What does that soil smell like?
How well does the soil stick together and then crumble?

We live on a big clay hill in the Big Thicket area of East Texas. Our land was all forest a few years ago. The fig trees and crepe myrtles were planted with a lot of compost. A big hole was dug and back-filled with the compost and manure.

Our soil is terrible. No earthworms. We could go into the adobe brick-making business! I can't understand how this can be the best place in the world to grow trees, and the worst place to try and grow anything else.

I have alfalfa tea, already brewed. I will pour that on them today until I can get some blood meal in town. Thanks for the suggestions.

If you need a quick nitrogen fix, just use some Miracle Grow.

MG will get your plants the nitrogen it needs now.. You might also want to start mulching grass clippings or something else around the plants so it get a steady nitrogen source as it breaks down.

It's the best place in the world to grow trees so long as it's the native trees you're trying to grow. they're already adapted to the local soil and water conditions. Your garden trees are from other conditions and won't necessarily do well in heavy clay. We have the opposite problem here. We have some of the most valuable timber growing land in the world here in western WA, but garden plants struggle to survive in our gravelly sandy soils - the same soils that produce some of the largest and oldest trees in the world.

Worms are not native everywhere. If your land was recently cleared of native forest, and you have no worms, they're probably not native to your area. Worms tend not to like coniferous forests and I think East Texas is pine forest country.

Try planting locally native stuff! I bet that will grow well. And lay off the compost and manure in the planting hole. You created clay pots in the ground when you dug out a hole in the clay and filled it with compost and manure. Clay pots with no drain holes, no less. You can mix in some compost with the clay backfill if you insist, but you should not totally replace the native soil in the planting hole. I bet that has something to do with your trees not growing well.

If the land was clearcut and bulldozed, the soil life is probably all dead now. Compost tea will help it grow back (not alfalfa tea). Micorhizal (sp?) products can help too. Trees need micorhizal fungi in the soil in order to grow and removing the native trees and disturbing the soil usually kills them all off. Many trees are not able to pick up soil nutrients and water very well without their fungal partners. Clay soil should have some nitrogen in it, after all.

It may not be a nitrogen deficiency at all. Your story suggests some more likely culprits.

Really quick fix, (no need to go to the store to buy anything) diluted urine. it will add other nutrients as well though.
* * *
I feel a sixpack of Corona coming on...

I would listen to the above posts but I have a few other thoughts:
In your specific case with heavy clay, etc., backfilling the planting hole with compost and manure might have been needed. However, that is no longer a generally reccommended practice for most tree planting. How big an area was backfilled with the amended mix? How deep? How wide? If it was a relatively small area that was amended, the trees' roots might not be spreading far enough from the trunk since that nice loose and juicy compost and manure is a path of least resistance compared to clay and provides easy food for a while. Are the trees mulched? I have to admit that figs and crepe myrtles are not my forte so take the free advice for whatever it is worth.

Here is a link that might be useful: Purdue on Figs

Actually the clay in Texas is really nice and full of good stuff....the problem is it's locked up. Over here in Dallas it's calcium that's the problem so you can't use gypsum to break it up like you can in Houston.

You need to find out what you can use to help break up the soil (call county extension--that's what they are there for) and then just start composting everything. Put 3-4 inches of good compost on top and trust me it will work.......it'll take awhile. Meanwhile use some type of slow release fertilizer to get the plants through it.

I know it seems impossible but it will get better if you're consistent.