Nitrogen is the sole chemical responsible for the production of chlorophyll, amino acids and photosynthesis. Basically, it is  responsible for the growth of the plant, producing the food it lives off of and how healthy the plant is overall. It is the largest element that can travel to any part of the plant. You will first notice deficiency in the lower to middle part of the plant, taking out older leaves first. Yield can be greatly reduced and your leaves will turn violet, Violet and more yellow.

To keep from being confused with a magnesium deficiency, remember that magnesium will turn the entire leaf yellow excluding veins, white nitrogen deficiency starts at the tips and works on the back of leaves. You can also noticeably tell by the reduced growth rate that it is nitrogen. Towards the middle to end of flowering, your plant will experience nutrient deficiency with the yellowing and following of leaves, however, this is normal and should NOT be treated. You need the chemicals to wash out anyways. In rare cases, Nitrogen deficiency can also effect petioles.
Too much nitrogen in plants are dark green with delayed maturity because plants will focus on growing upwards with tall, weak stems. New growth should be very active and plant transpiration is usually high. Nitrogen toxicity is seen in very dry conditions, such as in a drought and leaves could appear burned. Ammonium based nutrients may show NH4+ toxicity, causing smaller plant growth and lesions on stems and roots as well as downward rolled leaf margins, giving a claw look. Tips will point down, though leaves are up and can even be twisted. Roots however are under developing and flowering is delayed, so decreased yields. Water uptake is decreased from vascular breakdown. Both potassium and nitrogen will cause calcium lockout when over supplied. Flush your plant with plain water. Using organic grow mediums makes it easier to deal with deficiencies. When flushing your soil, use an amount of water at least double the size of your grow medium. You may need to carefully shimmy and remove your plant from its pot to get thorough results.
Your plant being to cold will make it harder for your plant to absorb nitrogen. Also, keep your pH around 6-8 in soil grows, preferable under 7 to avoid lockout. Hydro and soilless mediums should be around 5.5 to 8, though under 6.5 is better. Ammonium nitrogen can affect other nutrients and delay flowering, but can be fixed by giving your plant more rational ratios of each main chemical. Chemical or organic fertilizers with nitrogen in them will fix a deficiency.

Peters 20-20-20, Miracle Grow, Miracle Grow Tomato are all good for this, though when using chemical solutions opposed to organic, use it at at least 1/2 dilution. To use blood meal 12 0 0 to quickly solve a deficiency,making it into a tea first makes it work much quicker, other wise absorption is pretty gradual and a great source for phosphorous. Other sources include bone meal(0-10-0), cotton seed meal (slow) , insect eating bat guano 10 2 0(fast),etc.

Fish meal and emulsion 5 1 1 is medium speed, worm castings are gradual, seabird guano and crab shell are slow and Fox Farm Grow Big is fast, though can bring down pH. More chemical nutrients include:Humboldt Nutrients (2-1-6), Floragro (2-1-6), MaxiGro (10-5-14), Age old Grow (12-6-6), Floranova (7 4 10), Dyna Gro (7 9 5 ).

Organic nutrients include

Marine Cuisine 10 7 7,Seaweed Plus Iron, Iguana Juice Grow (3-1-3), Pure Blend Pro (3-1.5-4), 30-0.25-0.25 Age Old Kelp, Neptune’s Harvest 2-3-1, Bio Thrive Grow 4-3-3, Mother Earth Grow (1.5-.75-1.5), MaxiCrop Seaweed (1-0-3), Super Tea (5-5-1), Sea Island Jamaican Bat Guano (1-10-.2), Kelp Meal (0.6-0.5-2.5).

Remember, blood meal, dried blood, guano, kelp meal, cottonseed meal, peat moss, sulfur ,fish mealbone meal, rock phosphate, wood ashes and shellfish compost will change your pH