What's wrong with my plants?

[98HornGirl]

I've got two 4'x4' raised beds in the back yard, right next to each other. One of the beds is doing GREAT, but in the other one, all of my herb/veggie plants are turning yellow. I've got dill, catnip, bee balm, cilantro, and a zucchini plant in there now. The edges of the leaves are yellow, and esp. on the zucchini plant the leaves are almost entirely yellow. On the bee balm, the leaves are browning and up and completely dying back. I've been giving the same treatment to both beds, so I don't understand why one is doing so much better than the other. I've been doing some research and I think I have either an iron or nitrogen deficiency, but I don't know which one. Does anyone have any pointers?

 

[gecko]

It sounds like this bed is getting too much water, but that doesn't seem likely given its proximity to the other bed.