There are only two reasons to ever top dress lawns, the first is to even out uneven lawns or indentations in the lawn's surface, and the next is to add nutrients to the lawn as part of an overall lawn care regimen.
While top dressing lawns has been popular for many decades around Australia, it has often been done for the wrong reasons, which is not only unnecessary, but could even lead to the long term damaging of the lawn.
There are many richly organic soil mixes available now which can be applied to lawns to not only add nutrients, but organic matter too, which all those soil-borne friendly bacteria and micro-organisms need to feed on and need to thrive. These soil mixtures add yet another source of not only feeding the lawn itself, but feeding the soil which supports the entire lawn. Top dressing with these organic soil mixtures are a great benefit to all lawns, and could easily be applied a couple of times a year as an added boost to our lawn care practices.
Organic lawn mixtures only need to be applied sparingly, the topsoil mix is added to the lawn in small amounts and raked into the green leaf and thatch of the lawn. If applied correctly, the mixture should almost disappear into the green leaf altogether after watering.
Organic top-dressing mixtures should not be used to fill in any lawn indentations as they could have the potential to create water traps inside the original depression, as well as adding organic matter in quantities which may be too rich for the lawn to handle.
Levelling uneven lawns is relatively easy, but it is important to do it correctly, using only small amounts of soil in increments, using only soil from the property or sand, and never top dressing large depressions in the lawn, but instead to lift the lawn and add soil underneath.
We have a comprehensive article on top dressing uneven lawns which outlines how to best level uneven lawns.
There was once a belief by many people in Australia that top dressing lawns with yellow sand should be done every Spring. This was mistakenly done as part of caring for the lawn, every Spring the lawn owner would top dress this way, and a few weeks later the lawn would jump back to life with miraculous new green leaf growth.
The problem with doing this is that it adds nothing of benefit to the lawn, all it does is add to the thickness of the thatch layer by constantly adding more sterile matter. This practice can also run the risk of creating a water-resistant effect to the soil, and as the lawn thickness raises, the new roots sit only in yellow sand and dead lawn matter - preventing it from ever receiving adequate nutrients from the soil.
The only repair when this has occurred is to contact a professional Vertimowing Contractor to assess the damage and whether it can be Vertimowed, or bobcat and remove the affected lawn and start again.