Fertilizing New Grass Seeds and Lawn
Most of the nutrients your grass seed needs are already supplied to the roots of your grass in small, trace amounts through natural occurring soil processes. However, grass lawns require three especially important nutrients—nitrogen (N), phosphorus (P) and potassium (K)—in larger quantities than are readily available in most soils. Lawn fertilizers are formulated to provide these nutrients at higher levels than are typically available naturally within most soils.
All commercial fertilizers display three numbers somewhere on their bag, representing its particular combination of nitrogen, phosphorus and potassium. These numbers are the percentages of these nutrients in the fertilizer and are separated by dashes, such as 4-6-4 (or N-P-K). In the example of a 4-6-4 (totaling 14%), the remaining 86% of the fertilizer is made up of filler material (and sometimes other trace ingredients such as calcium, magnesium, iron, etc.) meant to give you the bulk needed to spread the fertilizer evenly over your yard. In the case of an organic based fertilizer, the additional weight is not filler, but organic matter, a much needed soil component in sandy and clayey soils.
Newly planted grass seeds need nitrogen, phosphorus and potassium in a much different combination than an already established lawn, and thus most fertilizers are not formulated to meet the needs of a newly seeded lawn. When planting your new lawn make sure to select a "starter" fertilizer, or an organic, "slow-release" fertilizer. Do not use "weed & feed" fertilizers intended for use on established lawns, as these include weed preventing chemicals in combination with the fertilizer that can severely damage or kill new seedlings.
The phosphorus and potassium in your fertilizer needs to be easily accessible to the seedlings and so it is best to work the fertilizer into the soil as one of the final steps before spreading your grass seed. To do this, use a seed spreader to apply the fertilizer evenly. Then incorporate it into the topsoil, either manually with a rake and shovel, or mechanically with the use of a tiller. Do not work the fertilizer deeper than 4" into the topsoil.
About 4-8 weeks after germination you should fertilize your grass again. Do not continue to use starter fertilizer for this second application. The higher phosphorus content of starter fertilizers is an important nutrient right after your grass seed has germinated, but once established, your lawn no longer needs phosphorus at such a high rate. Moreover, phosphorus runoff is a contributor to surface and groundwater pollution. Instead, use either a more traditional type fertilizer, or an organic, slow-release fertilizer such as an 18-1-8 formulation which is better suited for an existing lawn. Use your seed spreader to apply the fertilizer in two directions. Make sure not to over-apply by following the application rate provided on the fertilizer’s packing. Sweep any fertilizer granules that end up on the sidewalk or driveway back onto the lawn. Water the lawn immediately after fertilizing to allow the nutrients to filter down through the soil to the roots.