When you buy soil from a plant nursery or garden supplies store, you often buy what is called "topsoil". Topsoil is usually a premixed collection of decaying organic matter and microorganisms that are full of nutrients to support plants and retain moisture. It is very similar to mulch, which is an earlier stage in the process. Many organic gardeners and farmers will tell you there is no need to ever buy soil, but there are many circumstances where it might be preferable.

There is no other option

Sometimes when you move into a new house you are given a "builders garden". This is where you have only been given a very thin layer of topsoil in the garden, which hides layers of fill soil known as "subsoil" and clay underneath it. These lower layers are unsuitable for growing plants and purchasing topsoil may be required to restore the structure and nutrients that will allow your plants to flourish.

It is possible to create healthy topsoil from these lower layers, by adding compost or fertilizer to revive the nutrients in the soil to allow new plants to flourish. However, it requires you to wait an extended period of time for plant matter to decompose and break down into topsoil. When you purchase topsoil, you can start planting right away. If you are planning on doing some landscaping, find out how much topsoil is in your garden. If there is less than a foot of good soil, you might want to consider purchasing some additional topsoil.

Grading or regrading

It might be that you have a small garden and it currently slopes very gently down towards your house. While that doesn’t seem a problem at first, when it rains, the water may roll down into the foundations of the house, weakening the structure. In this case, you may want to buy soil in order to reshape the land to slope away from your house, perhaps towards water features. There are other landscaping supplies and landscaping materials that can do this, but if your garden is not big, buying soil might be the best way to go. Talk to your landscaping architect or the local nursery before moving forward.

Filling raised beds

An obvious reason to buy soil is to fill a raised garden bed. It’s important to consider what type of soil you are buying. If your soil only needs to fill an area to be compacted, you may want soil with less humus in it. There would be no need for premium topsoil that has an optimal pH balance and nutrient content for plant growth.

Constructing berms

If your garden is big enough, your landscaping designs may involve constructing a berm. This is an earthen or sod wall that is elevated in order to separate two areas. This sort of wall is usually placed next to a ditch and often used to control erosion by reducing runoff, or redirecting water away from erosion-prone areas.

If you decide to buy soil for your gardening or landscaping project, you will then need to estimate the volume or weight of soil needed. One cubic metre of soil – a ‘box’ of soil 1m high x 1m wide x 1m deep – weighs around 1.25-1.5 tonne. Handily, this usually equates to a trailer load – which is how soil is often sold.