Urea is a common nitrogen fertiliser and an ingredient in many lawn fertilisers. Slow-release forms such as sulphur-coated urea are especially popular for lawns and golf courses. Adding urea to your lawn helps increase nitrogen levels in the soil.
Just like any other plant, turf grass needs nitrogen for growth. Nitrogen shortage can cause leaf yellowing and make for thin, unhealthy grass. Once added to the lawn, the urea begins to break down through a chemical reaction that forms ammonia and carbon dioxide; the ammonia is turned into nitrate by soil bacteria and can serve as a source of nitrogen.
When it's first applied, each particle of urea initially increases the pH in its immediate surroundings; ultimately, however, urea reduces the pH of the soil. It also increases the nitrogen content, which is beneficial for the grass -- up to a point. Excess nitrogen causes reduced temperature tolerance and moisture stress, so overfertilizing is not typically beneficial.
If applied in excess, urea can "burn" or injure the grass and cause serious damage. Slow-release formulations such as sulphur- or resin-coated urea help circumvent this problem -- one of the reasons these formulations are popular for lawn and golf course use. In general, urea has less potential to "burn" turf grass than ammonium nitrate or sulphate.