is this n deficiency ?

i am 3 weeks into flowering with 3 bb blue cheese and 3 northern lights its my first grow and the leaves around the middle and the bottom of them are turning yellow and starting to go crispy is this n deficiency or just normal i didnt want to give it to much nutes i am giving it humbolt bloom natural but only giving it half the amount stated on bottle(5 ml per gallon im giving 2-3ml per gallon) until 2 days ago and i gave a full dose and still no change its a bit hard to see in the pics but some of the leaves are yellow and green nearly like a tiger stripe can anyone tell me what its is i thuoght it ight just be old growth falling off the new growth doesnt look to bad just a little pale any advice would be much appreciated

Yeah, looks like you have a magnesium deficiency starting, as well, and possibly others on their way. Either it's simply from underfeeding, or possibly due to the PH dropping, over time. Maybe both? Any idea what your soil PH is? What PH water/fert have you been giving them?