The short answer is, very important! Nitrogen is one of the primary ingredients for plant growth. Nitrogen is rapidly depleted by vigorously growing plants so it must be replenished regularly. Nitrogen is available in two readily available forms. The first is organic nitrogen. Organic nitrogen can be found in the form of manure, soybean meal, blood meal, and sources from the yard such as grass clippings. Chemical nitrogen can be purchased in the form of ammonium sulfate, ammonium nitrate, calcium nitrate and sodium nitrate.
The suggested form to follow in looking for nitrogen content in chemical fertilizers is to look at the label. Nitrogen (N) is the first element listed on a fertilizer label. The first number listed on the label is the percentage of nitrogen in each pound of material. An example label that states, 20 - 10 - 10, would meant that 20% of the of the fertilizer is nitrogen.
Nitrogen application for your new garden can be done in the fall. There is no need to dig in nitrogen fertilizer into the soil. Water the soil or let the rainfall do it for you. Another method for nitrogen applications would be to use a green manure. Sow seeds like clover or rye and when they reach the height of 6 inches, cut the plants to the soil line and then incorporate the plant residue into the soil. The decaying plant material will release nitrogen and the soil should be ready to plant within a week after the incorporation of the plant residue.