Nitrogen is considered the most important nutrient in fertilizers.  It's difficult to measure nitrogen in soils day to day due to many elements.  Long periods of dry or cold weather slows microbes in the soil, which means less nitrogen is released.  Warm, moist weather increases microbial activity, in turn, uses more nitrogen.  Soils that are abundant in organic matter ensure a steady supply of nitrogen, therefore it cuts down on the amount of fertilizer you need. 

Types of Natural and Organic Fertilizers containing rich amounts of nitrogen for deficient soils:

Blood Meal 12-0-0 is moderately fast acting.  It should be used in the rate 3-5 lbs per 100 sq. ft.  Blood Meal is slightly acidifying.  Can burn plants, so keep away from stems and don't overuse.  Blood Meal will generally last in soil for 3-4 months.

Fish Meal 8-6-0 not only supplies nitrogen, but also micro-nutrients.  Relatively fast acting.  Apply 5-8 lbs per 100 sq. ft.  Odor is fishy, so work into soil.  Fish Meal last in the soil for approximately 6-8 months.

Feather Meal 12--0-0 is a slow release fertilizer.  Apply 5-6 lbs per 100 sq. ft.  incorporate into garden soil before spring plantings. Feather Meal will last in garden soils for 6-8 months. 

Cottonseed Meal 6-2-1 is slow acting and helps acidify the soil.  Apply 7-8 lbs per 100 sq. ft.  Cottonseed meal has a pleasant smell and will last in soils for up to 2 years. 

Bat Guano 9-3-1 provides one of the best macro and micro nutrients available.  While it is more expensive per pound than other fertilizers the rate used is much less, using 1-2 lbs per 100 sq. ft.  Bat Guano fast acting and is highly effective mixed into potting soils.   It is excellent for making liquid preparations. It will generally last in the soil for a year. 

Soybean Meal 7-2-1 is a by-product from processing soybean oil.  It is slow acting and should be applied at the rate of 7 pounds per 100 sq. ft.  Soybean meal lasts in the soil for 1-2 years. 

Plants use up nitrogen rather quickly, but avoid overfeeding nitrogen.  To much increases susceptibility to drought, pests, disease and winter hardiness.  It will also cause poor vegetable, flower and fruit quality with the exception of leafy crops such as lettuce, chard and kale.  If your unsure, it's best to apply less (such as half) the recommend rates more often, then more less often.