Water Insoluble Nitrogen dissolves more slowly and will stay in the plant root zone longer. Lawns vary in their need for nitrogen. Maintaining an existing lawn requires less nitrogen than starting a new lawn. Maintenance applications should be split and applied over a few months, so there will be less danger of leaching. A healthy root system will absorb most of the leaching nutrients.
New lawns benefit from a high relative level of phosphorus.
This helps with seed germination and gets the roots off to a good start. Phosphorus is fairly stable and is held by the soil particles. There is a chemical balance of phosphorus in the soil, with some being readily available and some held in reserve. But if erosion occurs, phosphorus can move with soil particles and contribute to pollution. Phosphorus also contributes to algae growth, although not as seriously as nitrogen.