Yellow Leafs on my tomato plants
 

I have been starting my seeds the last few years with success for the most part. But this year has been a little different, the seedlings looked great for 3-4 weeks then i started noticing the bottom true leafs were turning yellow. I dont know if its a watering thing or nutrients, or something else. But i dont think its normal. And also they are the only leaves that are yellow the rest are pretty and green like they are suppose to be. I did add some orgainc fertilizer a couple days ago to see if that would help. I also went ahead and got all the yellow leafs off as most of them would just be barely hanging on. Any help would be appreciated.