New lawns, whether started by seed or sod, are voracious consumers of water and fertilizer. The former is needed to keep the seeds, soil and roots moist enough for germination and rapid growth. The latter is needed to replenish the nutrients that get washed away in the constant watering. Lawns should be fertilized when sown or laid, then again two weeks later and once to three times per year thereafter.
Use lawn starter fertilizer at the time grass seed is sown or the sod is laid down. Rake the starter fertilizer into the top inch of soil that has been amended with compost or well-aged manure before grass seed is cast over it. When using sod cast the starter fertilizer over top of the sod after it is laid down and water in lightly. Apply according to label dosing directions for your square footage and err on the side of less not more if in doubt.
Feed your new lawn for the second time with the lawn starter fertilizer two weeks after the seed has been sown or the sod laid down. Cast the fertilizer by hand or use a spreader tool to create an even layer over the entire lawn surface including the edges. Overlap each row a bit to prevent a striped effect on the lawn where areas were missed. Water in deeply but gently with a hose sprayer so as not to wash away the fertilizer but allow it to percolate down into the soil.
Establish an annual fertilizing regimen for your lawn in which you feed the grass between one and three times each year. Determine the fertilizing needs of your lawn grass according to the grass variety, your climate and by the fertilizer product used. Once a year slow-release granular formulas can be used for many grass types while others respond better to repeated feedings of diluted liquid fertilizer sprayed on.