Whats Wrong With These Plants?

[Walt B.]

I'm trying to figure out why one of my tomato boxes is struggling a bit. I have two 7'x4' SIPs filled with Raybo's 3-2-1 mix. One of my boxes with 2 cherry tomatoes looks fine....no wilt or leaf discoloration......the other box with 6 plants looks stressed. Some leaf curling and purplish veins on the underside of the leaves. I mixed in Tomato-Tone initially and have been top water fertilizing with Texas Tomato Food weekly for the last two weeks (seeds started in late January....planted on 3/21). The weather has been pleasant all spring (high 70's) . I'm leaning towards phosphorous deficiency but would like to hear more seasoned opinions.