Topsoil is soil; compost is decomposed organic matter used to improve soil that already exists.

Topsoil builds up levels, fills, and provides a growing medium where there is none.

Compost changes the structure and biology of existing soil.

Buying topsoil to improve a tired lawn adds depth without addressing anything.

Buying compost to raise a level is expensive and settles considerably.

Confusing them is the commonest purchasing error in this category.

Organic matter does opposite things to opposite problems.

In sandy soil it holds water and nutrients that would otherwise drain through.

In heavy clay it opens the structure so water drains and roots penetrate.

Both effects come from the same physical and biological properties.

That is why compost is the default answer to almost any soil complaint.

Sand added to clay without organic matter can make matters worse rather than better.

By screening grade, stone content, weed burden, contamination history and pH - and it is worth specifying rather than assuming.

Quality varies enormously between suppliers.

Cheap topsoil frequently arrives full of perennial weed root that takes years to eliminate.

Contamination history matters on any soil from a construction source.

Screened grades are specified for turfing and fine work.

Asking for the specification, and inspecting a sample, is normal practice for any significant quantity.

Immature compost still decomposing draws nitrogen out of the soil and temporarily starves the plants.

The effect is the opposite of what was intended.

Well-made compost has a dark crumbly texture and an earthy smell.

Sour or ammonia smells indicate it is not ready.

Visible undecomposed material is another indicator.

Immature material can be stacked and left rather than discarded, since it will finish in time.

Slowly and cumulatively - a single application changes little.

Annual applications over several years transform a soil.

That makes this an investment across seasons rather than a treatment.

Expecting a visible result in one season leads to abandoning the approach early.

Existing plantings limit how much can be incorporated at once.

Top-dressing lawns annually is the standard way of improving soil without lifting anything.

Yes - top-dressing is the standard method, usually combined with aeration.

A thin layer of screened material is spread and worked into the sward.

Hollow-tine aeration first gives it somewhere to go.

Applying too thickly smothers the grass rather than improving it.

Repeating annually is what produces the improvement.

Material for top-dressing needs to be fine and screened, which is a different product from bulk soil improver.

Its use is restricted or being phased out in a growing number of markets on environmental grounds.

Peat extraction damages carbon-storing habitats.

Peat-free composts have improved considerably and are now the default in many places.

They behave differently in watering and feeding, which takes some adjustment.

Regulations and timetables differ substantially between countries.

What applies locally is the authority here, and it has been changing quickly.