Proper fertilization of grass seed is essential to the success of your new lawn. While the exact type of fertilizer you will need will vary, depending on the soil conditions of your area and the type of grass seed you have chosen, what remains constant is the importance of neither over-fertilizing or under-fertilizing new grass seed.
Before sowing grass seed, perform a soil test. New grass seed needs the proper level of nutrients to germinate and grow, but you cannot provide the proper amounts unless you first know how much of those nutrients are already in the soil. While soil tests for nitrogen are deemed unnecessary because it often leaches outside of the root zone, it is essential to calculate the current levels of phosphorous and potassium in the soil to determine how much, if any, of these two components you will need to add. You can do a soil test yourself with a store-bought testing kit, or, for more accurate readings, you can gather the soil samples from the area and send them to a local soil-testing facility.
Fertilizer is comprised of three main, and essential, nutrients: nitrogen, phosphorous and potassium. When establishing a new lawn from seed, apply 1/2 lb. of nitrogen per 1,000 square feet of newly sown area. Rake the nitrogen into the soil to a depth of 1/2 to 1 inch. Two weeks after the initial nitrogen application, reapply a nitrogen fertilizer at the same rate. Phosphorous and potassium should be applied to the soil before sowing the grass seed to give these nutrients time to break down in the soil. The amount of each needed will depend on the results of your soil test. If sowing grass seed in sandy or sandy-loam soil, it is recommended not to exceed 2 lbs. of potassium per 1,000 square feet of area due to potassium's tendency to leach out of porous soil types.
Soil moisture levels play an important role in the fertilization and establishment of new grass seed. Always irrigate the area immediately after applying fertilizer, to successfully incorporate the fertilizer into the soil. During the initial two- to three-week period after sowing and fertilizing new grass seed, water the soil regularly to keep the soil continually moist. Watering to a depth of 4 to 6 inches is recommended.