If you plan to topdress your lawn this year or you have been recommended to topdress, you NEED to read this first.
Why do we topdress?
One common reason to topdress is to level your lawn. If you're living in one of the older neighborhoods in Kingston Ontario for example you may have a bumpy lawn. This is from the constant freeze-thaw cycle which comes in the transition from winter to spring and again from fall to winter. This is actually a very beneficial process because it naturally reduces compaction in the soil. It may be difficult to level your lawn without starting a new lawn from scratch.
Another reason to topdress is to reverse erosion damage. If your property is on a slant, then water and wind can easily erode the soil down making it difficult to maintain a healthy lawn because of the decreasing level of topsoil. This is exacerbated by synthetic fertilization and mowing too short.
NEVER TOPDRESS TO "ADD" NUTRIENTS
I can't stress this enough, one all too common reason for topdressing is to "add nutrients". Your soil doesn't need more mineral nutrients, not from rock dust and not from topsoil. The only time when it is appropriate to add nutrients is after a soil test informs you of a deficiency. Even urban soils are so full of mineral nutrients that you will surely never need to correct a micronutrient deficiency.
Compost is a whole different story, when you topdress with compost you aren't doing so to "add nutrients" contrary to popular belief. For instance, the NPK value of most composts is in the range of 1-1-1. This is a low nutrient analysis relative to all fertilizers on the market. So why do we add compost? For the biology! When you topdress with compost you are adding organic matter to the soil which is critical for the development of a healthy soil structure.
Layers may be delicious for cake but they need not be involved in soil. By creating layered soil, say when you add a thick layer of topsoil, you actually disturb the flow of water. This is because of the suction forces of water between soil particles, water movement is not strictly determined by gravity. Layering soils creates a disturbance in water flow and can create future issues. Instead, topdress with compost as microbes will break the compost down and the microfauna will redistribute the organic matter throughout the soil.
If you're considering a topdressing, first consider the goal you are trying to achieve on your lawn. Topdressing can be a great way to add organic material to the soil and because it is much more affordable than topsoil, topdressing with quality compost is the best choice in most circumstances.