Look at this.
You might be wondering why on earth the veins are turning yellow. Paling or yellowing of the leaf is a sign of mild chlorosis; but if you see that your normally green leaves have yellow veins, there might be a larger problem. Learn more in this article.
www.gardeningknowhow.com
Might be time for a decent soil sample. Iron could be amnded with milogranite. Common to get at a lawn n gardne store.
Not enoug of a nutrient could be another nutrient is there way too much blocking uptake.
Any knowns about your soil, used to be this or that at the location? Any other plant / tree/ shrub issues there? A certain prevalent weed or shrub or tree unique to the area? Sometime something flourishes in a habitat because other plants will not thrive.
Blck walnuts are nasty buggers for habitat. Supposedly deer do not eat the nuts too. Something makes them disappear at my home, hoping to remember to put a trail camera up this august / sept.
Black walnut trees are armed with a substance that kills their competition, including your tomatoes. I share my experience with this natural born killer.
milkweedrising.com