bottom leafs yellow?

This is normal in you plants stage of life.

It COULD be lack of nitrogen, it COULD be just that it's one of the first leaves and they will tend to yellow-up and there's nothing amiss with that... the problem here is that you don't have a diary so we can't tell what it is you've been doing with this grow. Don't know the age, don't know the nutes, don't know the heat, don't know the humidity, don't know the light distance... If it's your first grow and you don't want to "mess it up" and you want advice that's worth anything, do start a diary so that when you ask a question, the community will have as MUCH information as possible in order to help you out! Good luck, mate!

As the plant grow and when it reach a leaping period, it may requires a little more nutrients. So it may draw mobile nutrient like Nitrogen from the bottom leaves first. This set of bottom leaves will eventually have to be defoliated anyway, nothing to worry. Just monitor the upper leaves closely. If the green color turns a bit pale, you may just want to add a tiny bit more nitrogen (Grow formula). Add gently per week as adding too much too quickly at a time may cause nutrient burn/lockout. Also, you are growing in soil, so nutrient release from the soil's eco-system may take a bit more time. Go gently...