I am not a soil grower but I have heard others talking about organic grows and it seems like, if yu aren't doing a feed when watering, that the nutes in the soil are slow to get going to sustain the plants that are a few weeks old and starting to get real hungry. It's hard to tell much from yer pic about the yellowing but apparently it's not too bad yet. Also yu said it rained on them so that could have thrown off the ph. That is certainly a viable cause if yu haven't noticed any problem until after the rain. I personally would get more water and PH balance it and do a one time flush, let it run through good but not so much that it carries away all the nutrients in the soil, but enough to take care of the possible PH issue. PH seems to be the biggest stumbling block for all growers (even the most experienced) And PH can lockout the nutes like yu suggested and make the grower think its a deficiency issue. I would also look into getting some organic (liquid) nutes to suppliment the soil until the microbes catch up. good grow mojo