The technical term is "interveinal chlorosis" (chlorosis between the veins). It can indicate various mineral deficiencies, not only iron.

Here's a link to a chart about tomato symptoms re. minerals and other nutrients:

http://fivepointgardens.com/nutrientinfo.html

I'll leave it to others to make recommendations and better explanations.

If it's on the lower (older) leaves, it's more likely an Mg deficiency.

Yes, if the interveinal chlorosis is on the lower leaves that means the mineral the plant needs is mobile ... that the plant is able to move a supply of that mineral from one place in the plant to another place in the plant. Typically, the plant will rob the mineral from the lower (spent) leaves and move it to the newer and emerging growth. The result is chlorosis in the lower leaves.

If the mineral is immobile ... the plant isn't able to move the mineral from old growth to new grown ... then the new growth shows the chlorosis.

The deficiency can be caused by lack of the micro-nutrient in the soil or growing medium. More often the deficiency is caused by a soil condition that injures or shuts down the roots ... like over-saturated soil from too much watering, or dried out growing medium and extreme heat scorching the root ball ... or more frequently a too acid or too alkaline soil condition that locks up the mineral making it unavailable to the roots.

And interveinal chlorosis indicates micro-nutrient deficiency while general chlorosis of the lower leaves indicates nitrogen deficiency.

Since you have interveinal chlorosis of the lower leaves we need to look and see which micro-nutrients are mobile ...

Look Here for Deficiency Signs and Fixes (includes pictures, etc.)

The plants are grown in the ground in raised beds.I brought in dirt from a guy who supposedly makes organic dirts. He has a good reputation. I added some organic fertilizer (lady bug brand) and some stuff that had molasses, and cotton meal, I added compost also. I watered them with Epsom's salt and gave them some fertilizer about three weeks ago. It was a rushed affair and it is my first veg garden. I forget all I did, because I was in a big rush with my job, and had a sister visiting. Did I mention , the deer are helping themselves to the branches that wander outside the large cage.

I took the leaves down to another store here in Austin that sells organic products and has a service where thy look at your sick offerings and come up with a diagnosis. She says that it looks like I am not watering it enough. We are suffering 40 days of above 100 degrees. I have been watering it deeply twice a week and she says I need to water lightly in-between every day. She thinks that there are some signs of herbicide in the soil ( asymmetrical shaped leaves) probably from the compose that I bought, and a phosphorous deficiency possibly. I did throw her for a loop because I carried in the leaves in a plastic bag that had held a cinnamon bun. The sugar glaze particles confused her. OOPs.