Topsoil vs compost: Which one should you use, and when?

To the novice gardener, topsoil and compost appear to be similar in appearance and texture, but they have slightly different roles in your planting projects. 

“As the name suggests, topsoil is the top layer of natural soil and is where the plant roots gain most of their nutrients and water from,” Charles Carr, head of Hillier Garden Centres, told us. “Compost, on the other hand, is made from a blend of composted organic matter to create the right structure to grow plants in pots.” 

Carr explains that when it comes to knowing which one to use in your garden, it’s important to understand their properties. 

Topsoil has many uses in the garden. It essentially creates a base made from natural materials to plant grass seed into or lay turf on, form flower beds with, and fill plant containers. If your garden is paved, or doesn’t have areas of existing earth, topsoil can be used to transform these spaces into patches that can be planted into. 

But topsoil can also be added to existing earthy areas that need a little health boost, before adding compost and plants. 

“If your soil has a thin, or depleted topsoil, then this can be enriched with the addition of more topsoil, which will add nutrients and improve water storage,” Carr adds. “Soil improvements can be done with other soil conditioners, such as homemade compost, spent mushroom compost, well-rotted manure or a mulch of bark, which will be drawn into the soil.” 

So, topsoil can be used to cover ground, improve poor soil and create new areas or containers to plant into. 

If topsoil is the main ‘bulk’ of planting material, then you can think of compost as an additional nutritional boost for plants. It can also add structure to existing soil, giving plant roots more stability as they grow. 

“Compost can be mixed with an amount of topsoil for use in pots to increase water retention and give plants a valuable boost of nutrients,” Carr agrees. 

This is achieved thanks to the organic composition within compost. Travis Perkins explains how this works on its website: “When compost is added to soil, the organic material is eaten by microorganisms within the soil. As the microorganisms break the compost down, this produces a dark and spongy material called humus. This material contains carbon, and is rich in fibre, and additional nutrients such as potassium, phosphorus and nitrogen. These all help to promote plant growth and protect plants from disease and help with their reproduction.” 

There may be variations in the types of compost available. Usually, the product’s packaging will have instructions on when and how to apply it.  

Mixing topsoil and your chosen compost together will provide the best growing conditions for your plants. But the ratio of these two materials – along with sand, grit or stones added for drainage – will depend on what’s being planted and whether it’s in a hanging basket, raised bed or in the ground. There’s no one-size-fits–all solution, so once you know what you’ll be planting, it’s best to research optimal ratios for each type of plant.