@Fishcake Hi there mate The plant looks to me to be very hungry!! Mainly calcium as fruitgrower said but also it's more than likely low on Magnesium iron nitrogen
etc So If you can I would feed it some Calimagic or something along those lines that have all of the trace elements in it. Also potassium is needed in slightly higher doses each week as the buds grow, before tapering off again @ around week 8 or so depending on the strain your growing. Cheers I hope to have answered ya question mate