Most soil is not fertile enough to supply plants and grasses with all the nutrients they need. Fertilizer is food for these plants, providing nitrogen, phosphorus and potassium. The components are listed on the bag in this order: nitrogen-phosphorus-potassium. Phosphorus is the most essential nutrient so it should be the highest in starter fertilizer. Later on, once the grass' or plant's roots establish, nitrogen is important for growth.
Consider having the soil tested prior to applying starter fertilizer. The county Cooperative Extension Service office in your area can explain how to take soil samples and where to send them for testing. This will tell you which nutrients you need to add and which ones are already in abundance.
Choose a time to fertilize when the soil is moist, because it will best absorb the food. However, don't fertilize before it rains because the nutrients will wash away.
Rake soil to create a smooth surface. Break up clumps and make sure the soil has an even texture. This will make it easier for fertilizer to absorb.
Apply starter fertilizer to the soil when it's time to lay seed. Buy enough so you have 1 to 1 1/2 lb. of nitrogen and 1 1/2 to 3 lb. of phosphorus per 1,000 square feet of planting area.
Use a fertilizer with a ratio of 1-1-1 or 1-2-1. Ten to 20 lb. of a 10-10-10 or 10-20-10 starter fertilizer would work well.
Spread starter fertilizer over the surface of the planting bed by hand or with a spreader. Apply it evenly, avoiding clumps. Overlap the spread pattern so you don't miss any areas.
Rake starter fertilizer into the soil lightly. The goal is to mix it in with the top inch or so of soil.