Fertilizing is a very misunderstood science. The first thing to know that fertilizing an organic lawn is different than fertilizing a chemically dependent lawn.  The fertilizer in a chemically dependent lawn is the primary food source for the grass.  Without sufficient fertilizer the grass becomes starved for nutrients, resulting in pale, thin growth.  With too much fertilizer the grass has rapid growth spurts (making mowing more difficult) and becomes susceptible to disease.  Chemical fertilizers need to be applied consistent with the needs of the grass, which change during the season.  In an organic lawn, the primary objective is to feed the soil so that the soil can continuously produce nutrients to feed the grass at a consistent rate.  Most organic fertilizers also provide some level of nutrients to supplement and balance the nutrient diet needed by the soil.  However, not all organic fertilizers are equal when it comes to feeding the soil.  The needs of your lawn can be determined through a soil test.