plant yellowing suddenly

my ph for dirt and water seem ok between 6 and 7 is kinda hard to tell with my kit(hate it) The N tests ok but the p and k barely register, one out of three plants is yellowing badly, it looks more yellow now than it did 12 hours ago, and I was wondering if it might be a too much or too little watering prob?Im always worried that the tiny seedlings will drown so instead of soaking the whole pot of dirt down, I have just been spraying the dirt near the plant and doing it a few times a day, could this be my problem? Also my plants arent near as large as others at approx 2 weeks, dont have pics though...sorry.Thanks in advance for any help or advice.