After reading this post, be sure to see how different forms of nitrogen affect pH (LINK)
Nitrogen (N) is a vital nutrient for all plants and it should be made available to plants in all stages of growth, although it should be given in lesser amounts during flowering. It is always listed as a percentage of every fertilizer as the first number. For example, a fertilizer listed as 5-1-2 would contain 5% nitrogen. Nitrogen is needed in all organisms as part of their DNA, it is also needed in many enzymes which regulate all metabolic activity in plant cells. Nitrogen will promote increases in stem and leaf growth and causes leaves to have dark green growth.
Sign of a Nitrogen Deficiency -- Plants will exhibit slow growth and will be stunted. If the problem is not corrected yield will be significantly reduced. The first thing you will notice is that older leaves become yellow (chlorotic). Nitrogen deficient plants will exhibit uniform light green to yellow color on older leaves, these leaves may die and drop. This color change will begin at the tips of leaves at the bottom of plant especially older leaves and the color change / yellowing gradually spreads up the plant to the top. Please note that many plants will exhibit these symptoms during flowering which is normal.
Nitrogen Deficiency Causes -- You can get a deficiency in fast growing crops, so if you are growing a plant that grows fast feed it a lot of nitrogen (never more than recommended by the manufacture!). You will also get a deficiency if you grow in very sandy soils (sand promotes good drainage but holds no nutrients for plants) or if you grow plants in soils with low in organic material (See compost link). Also, excessively wet soils and high or low pH can cause nitrogen deficiency. After you check the pH and adjust it to your plants optimal pH you can add Nitrogen fertilizer. The actual number is not so important as making sure the first number is the highest.
Nitrogen Toxicity is possible, so never over do it! With nitrogen toxicity leaves are often dark green and in the early stages plants are abundant with foliage. Eventually leaves will dry and begin to fall off. If you think you killed your plants by over doing nitrogen check and see if the root system is under developed or deteriorated, this is a sure sign you added too much nitrogen.
Organic sources of nitrogen are best in my opinion, you can get nitrogen in blood meal, most guanos manures and for free in urine. One cup of urine per gallon is a free source of nitrogen.
Remember though that if you are going to flower or fruit your plants you want to reduce the nitrogen percentage and have phosphorus (the second number in fertilizers) be the highest… Too much nitrogen delays flowering. Plants should be allowed to become ALMOST nitrogen-deficient late in flowering so that it does not inhibit flowering and I have read low nitrogen late in flowering can increase fruit flavor.
Dr. E.R. Myers