There are a number of reasons to apply a top dressing to your lawn including to give it a nutrient shot, fill holes or even it out. Dressing your lawn should be done immediately prior to its spring growing season. If you top dress your lawn when it is dormant, the lawn may be destroyed. That said, you are able to top dress for nutrients with an organic soil mix at any time throughout the year. This has numerous benefits such as improving drainage, encouraging micro-flora and fauna as well as feeding those nice little earthworms.