Topdressing is the application of a thin layer of soil (usually no more than 1/4" — 1/2") to a turf. Topdressing is an effective way to modify thatch and smooth the turf surface.
Topdressing is quite expensive and requires a soil type that will not interfere with drainage into the existing soil. Uniform, regular applications of topdressing are rarely appropriate for home lawns, although small additions to selected sites can help correct constantly wet areas or particularly bumpy lawns.
The type of mix to be used varies according to your basic soil type. Normally, I use equal parts of sifted top-soil, sharp sand and peat. (If the lawn is in need of a feed, or for extra 'oomph' I use multi-purpose compost instead of peat! (It it often cheaper as well!) For Lawns on heavier soil, I would leave out the soil in the mix, and use 50/50 sharp sand and peat/compost.
Mix the ingredients well— on a dry day, ensuring that there are no lumps in the mix.
Lightly spread to mix over the lawn. It is better to do it lightly and in several applications throughout the season, rather than 'smother' the lawn grass in one go!
I normally use the back of my wide 'Landscaper's' rake to spread the mix and 'brush' it into the grasses of the lawn. A good stiff broom will also do the trick. Make sure that the mix goes down into the sward.
The grass should be showing through. For those little hollows, apply a little extra mix. Again not too much. Far better to do these in several applications throughout the season. 'Rough up' the area with your hand to get that grass showing through.
Lawns on poor dry sandy soils, silt, chalky soils, or shallow soils, can be improved by top dressing with an organic gardening compost. It can be used on other lawns too, but mixtures of loam, sharp sand and peat/coir/compost are more widely recommended.
On clay soils use a mix of compost and sharp sand. For perking up lawns generally, measured amounts of organic fertilizer can be used instead.
Top dressing is commonly used to even out hollows, but for this you really need loam not organic gardening compost. Restricting it to part of the lawn may lead to uneven lawn quality. For lawns over loam, organic matter and nutrients are well conserved by leaving the grass cuttings in place during the summer. This alone may perk up the lawn. Compost may help to break down thatch but always rake thatch out first.
Use fine and fully matured compost to top dress lawns. The job is done mainly in autumn but alternatively in spring. Brush in top dressing to no more than 1cm about 1/3 inch deep. Most grass blades will peek through. At this depth 1 cubic foot will cover 4 square yards.
In loamy soils mix the soil cores from hollow tine aeration into your organic gardening compost. (Compaction is no less reduced.) Top dressing lawns with garden compost need only be done every 2 to 3 years once the initial soil improvement is complete.