Image credit: Shutterstock/Krasula
If you’ve decided to take on a new planting project, whether that’s building some raised beds, filling your hanging baskets, or improving your garden borders, then you’ll need some material. But when it comes to topsoil vs compost, which should you be using?
You may have had less than successful flowerbeds or vegetable patches in the past. The reason behind this could be what you’ve actually been planting into.
Though topsoil and compost look similar at first glance, they’re really quite different. We explain the purpose of each one, so you’ll know which to buy and how to use it in your garden.
Looks can be deceiving
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.
It provides the building blocks for planting in
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.
To avoid later problems, it is best to check the topsoil before buying it, warns the RHS. Look out for high stone content, thick fibrous roots, weeds and contaminants such as glass and brick, it says.
“Imported topsoil can sometimes introduce invasive plants, such as Japanese knotweed and couch grass, to the garden,” the RHS adds.
“Inspect the soil for signs of weed roots or shoots. Weed roots are often white and fleshy, sometimes with a brown covering, unlike the brown, dry, fibrous roots of trees and shrubs.”
It boosts nutrients and improves water retention
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.”
How well do you know your soil? If you’re hoping to start new planting projects in existing soil, it helps to understand the general soil type. This is usually done by looking at the way soil feels between your fingers, and how it responds to being handled.
“Pick up a handful and squeeze it together,” gardener Sarah Raven suggests on her blog.
“Sand feels gritty, and the grains do not stick together when squeezed. Loam feels velvety or flour-like when dry and forms a weak ball shape when wet, which crumbles apart when dry. Clay feels sticky but goes smooth when rubbed. Chalk will have large lumps in it and be hard to mould.”
Raven advises gardeners to remember the golden rule of 'right plant, right place' in order for your plants to thrive.
“All soil types have their good points and can be improved. Even the best loam soils will benefit from additional organic matter [such as compost].”
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