Under normal circumstances, time to water means that the top 1/4 - 1/2 inch or so of the soil is dry to the touch. They have likely been a little too moist recently. They were transplanted from plastic cups to their 4 1/4 gallon pots last Wednesday and were watered liberally before I left for the weekend and I noticed the yellowing when I got home. The stems also exhibit a pink/purplish tone, which I believe is also a symptom of nitrogen deficiency. It hasn't slowed them down too much and I've already taken measures to increase ventilation into and out of the grow area.

One last thing; after feeding them yesterday, I noticed that the run off water had a rather putrid odor. There was also a film that floated to the top of the run off and so I emptied the watering trays fearing this would invite either more pests or disease.