Too many folk panic once they see a little yellow even a yellow leave the first leaves to come are the first to go leaves do just die of natural causes.but if you have yellowing in the top area of the plant then something is wrong unless it is in bud.but if you do get yellowing in the top section at any time just flush the plant with a ph of around 5.5 till the water runs clear leave her to dry then carry on feeding.only feed a couple of times a week if in soil let the top 3 ince dry out b4 re watering.Get a large bucket the biggest you can find or test the ph of the water in the tap if it is under 6 you are fine you can go ahead and keep pooring water over the grow medium till it runs clear.If the water is a little high then set it around 5.0/5.5 and keep pooring till it runs clear leave till dry then re~feed but if you have made a fresh batch up then add the feed at after your final flush,But don't just flush at the end of a grow a little work never hert anyone i flush at least 3 times during veg and 4 including the last one during flower good luck.Cannabis growing in the wild would get all the salt and other nastys out by flushing when it rains so you have to do what nature would my yeilds have increased alot since doing this plus it is not very often that i have yellowing or any other problems,That i think are all down to excess salt build ups.You willknow when you have flushed enougth because the water will be dark at first and will clear always flush when makeing a fresh batch of nutes then your final flush you can put new nutrients back in that way you are not loseing valuble growing time and the nutrients lost will be re gained.