Slow-release N becomes available to the plant over a period of time depending on soil moisture, temperature and microbial activity. The balance of the N is water soluble nitrogen, which is readily available for plant uptake. The tradeoff is that slow-release N is usually more expensive. Natural organic fertilizers supply nitrogen in complex organic forms that are not immediately available to plants. Lawns grown on mostly sandy soils should rely more on slow-release nitrogen to reduce the possibility of N leaching out of the root zone.
Research shows that on most soils with some silt and clay, nitrogen leaching from lawns is a rare.
Consider different needs. Different species of grass have different needs, too. Kentucky bluegrass, for example, requires more nitrogen than fine leaf fescues. If bluegrass doesn't get enough N, it is less competitive against weeds and pests. Apply with care.