Possible nutrient deficiency?

[DeXodus]

Dear gardeners,

I am growing couple of black locust seedlings and one of them started to show an interesting signs. On of the leaves started to make a yellow spots on it and after more carefull inspection it seems that the whole area between veins is getting a bit yellow. From what I have read it may be nitrogen deficiency (although I started fertilizing with nitrogen rich fertilizer). Could it be something else? The pH is something around 6.7-7 (could be lower I guess).
Here is the picture:

 

[DeXodus]

Quick update:
so a new leave is a bit yellow as well. I am attaching a picture:

I have the all true leaves numbered in an order they appeared.

 

[Beth_B]

It's in a fairly small container. Possible it's getting too much water? Do you let it dry out completely between watering?

Based on your photos, and the fact that overwatering is the #1 most common mistake gardeners make, that's my guess.

 

[DeXodus]

Yes, it dries completely every time. I already went through that road

 

[Beth_B]

Well shoot, there goes that theory.
Do you think they might benefit from larger containers with a good potting soil? I don't think that would hurt.

This spring I started heirloom tomato seeds...some in little peat pots and others in large planters. All tbe same soil. They all germinated about the same time but after a month the ones in the containers with room to grow were twice the size and healthier than the ones in the little peat pots. My theory is that some plants just appreciate more room to stretch out. And locust is a fast-growing tree.

 

[DeXodus]

The pots are made of a mesh as I am trying out air pruning technique, because I want to have no circular roots around the inside and I want to make the root system much more dense. Thus I want to keep them in the pot for another couple weeks before repoting. Soil is (I think) good as it is mixture of vermiculite and coconut choir and fertiliser.

 

[LIcenter]

My guess, and this is only a guess, is your soil is too fluffy. Black locust love to sprout up in dry hardpan soil. Very rarely will you see a locust in nice cultivated earth.