I am new at growing too, and im still finding it quite hard to distinguish between nute burn and deficiency BUT I do know a few things. If the bottom leaves are the ones yellowing, and the yellow is sorta spreading upwards, its probably a nitrogen deficiency. However, if you dont want to take my word for it you can always flush them out really good, and if the yellowing continues you know it is a deficiency and not a burn. Hope that helps. lol