I'm new to all this, and I have a question. But first, plants are 14 days old, about 4" tall and look good, they are in potting soil. My son keeps telling me to feed them episons salt. Now the potting soil has nutrients in it, and was told should last 3 weeks. So is my son right about adding episons salt? Thanks.
Just adding epsom by itself won't do much. It's best to think of the main elements (N-P-K-C-M) like specifically placed blocks in an arch. Then think of all the other elements (like iron) as the cement filling the gaps. If a block is knocked out for some reason, one being cement falling out, then the arch will slowly begin to collapse if nothing is done.
You can use single or 2 part bottles that will cover all the elements needed, but that will cost you more money for the convenience. At this point in time it's your best option. Later, if you care to, read up a lot on powder nutrients and you can begin to make your own arch so to speak. It requires more learning but the upside is it costs a lot less for the same yield, if done right. For some people it can increase yield since you have a better understanding of what or how much is and isn't needed.
Guess I might as well stick with the arch thing. If you understand how much P is needed, the block will fit in place fine. If you don't, and give way too much P like many people do, the block is too big, the other blocks don't slot in properly and things begin to slip until something falls out. This to me is why it is worth learning as best you can about how much of what you should give.