Magnesium deficiency?

Also if your leaves are yellow, it's most likely Nitrogen at this stage of growth, they don't look too bad to me tho mate.

I wouldn't go adding anything unless you are sure. If you think you have a deficiency research the symptoms. If you add something after a guess, chances are your guess may be wrong and you'll create the opposite of what you feared, an excess. Too much is just as bad as not enough. https://www.loudclouds.co/plant-nutrient-deficiency-chart/ Something like this chart should be your first go to with any deficiency fears. Look to the leaves. Look to the chart. Hope this helps buddy!