Outdoor Grower In SERIOUS Need Of Help!

I've had a few problems with my plants since it's the first time I've tried growing. Stunted growth, bad weather, animals, etc. etc. I've turned to this website on MANY occasions to help me with my babies. Well, I'm really not sure what to do this time & i'm hoping someone here has a clue.
So this is whats happened- my plants were slow to grow (which i assumed was because of the weather in New England) but eventually took off. I also began using plant food for tomatoes, as well as a pesticide for tomatoes. Everything was going fine and then I went on vacation for 2 weeks and left the plants in the care of a friend. Well, I got back yesterday and saw that the leaves on the BOTTOM, (not on the top like they were burned by the sun) had turned completely yellow. and not just the leaves, but the whole entire stem from the plant to the individual leaf had also turned yellow. They are still soft like the rest of the plant and haven't turned brown or turned in, they look exactly the same but are clearly yellow. So I'm hoping that someone out there with outdoor growing experience can send some advice my way, i'm in desperate need. Should I stop using the plant food? Do you think it is a deficiency and I need to add something to the soil? I have them outside, potted, on a wood board with a fence around them. They are pretty close together, should I space them apart more? I can send a pic later.. I'm at work ha.. buut any help would be greatly appreciated!

Hey HT. Need more info. Are you in vegg or flower? As a plant nears the end of its flowering cycle, some tend to start yellowing, beginning at the bottom and gradually moving up. As the plant nears the end, it starts using all of its stored nutes in bud production, in a desperate attempt to get fertized.

If you are in vegg still, yellowing of the bottom leaves could indicate a nitrogen deficiency. Also, it could be genetic - in some plants as it matures, the older, lower leaves will yellow and die. As long as it doesn't spread and your mid and higher leaves are nice and green, you should be ok.

Also, it could be that your plant is very bushy and those lower branches and leaves aren't getting enough light (even outdoors).

Hope this helps and good luck.