Choosing soils and soil mixes for your garden

There are times when it can be useful to import soil mixes into your garden. Perhaps you have heavy clay or something akin to beach sand, in other words a soil that is just plain difficult whatever you do to it. Bringing in a ready-to-plant soil mix provides a convenient solution that will avoid a whole lot of back breaking work trying to improve a soil that may still be unsuitable in the end. The easiest way to introduce new soils to your garden is by creating raised beds with sleepers, treated pine logs or rocks that are then filled with whatever product you choose. A raised bed is a good idea as it automatically gives you very good drainage, important for growing a wide range of plants. The choice of what material to use will depend on price, availability and the types of plants you wish to grow in it.

Choosing top soil for the garden

Landscape suppliers obtain topsoils from various places and it is not surprising that there can be a lot of variation depending on where it originated. A simple test can help you determine whether you are purchasing a quality product. Take a handful of the topsoil and moisten it slightly and work it between your fingers. If it is sticky and can be formed into balls that do not break easily then it probably has too much clay in it and may form undesirable surface crusts or worse still, set hard. The ideal topsoil will allow water to move in and out readily (i.e. water does not form little beads on top of the soil) and can be easily crumbled in your fingers.

What is a garden mix?

The landscape supply industry has coined the term garden mix to describe products that can act as substitutes for topsoil. They normally consist of blends of organic matter and soil materials. The big difference is the much higher amount of organic matter, sometimes over 40% in garden mixes. Generally speaking this is a positive thing, resulting in better storage of water and nutrients in the soil. However, it is important to realise that the organic matter will break down over a period of months, causing it to shrink and the level of the soil to drop. This can create problems for long-term plantings of trees and shrubs. Therefore, if you are creating raised beds with garden mixes it is a good idea to put in short term plants such as annuals, herbaceous perennials or vegetables that can be easily moved. After the mix has been in place a few months it will have settled and your long-term trees and shrubs added at this point after you have topped up the level with more soil mix.

How much soil or soil mix do I need to order?

You can easily calculate the volume of soil mix required by measuring the area you wish to fill and then multiplying that by the depth of the garden bed. For instance, if the bed is 5 metres long by 2 metres wide then the area is 10 square metres. A reasonable depth would be 0.3 m meaning that the volume required would be 10 x 0.3, in other words 3 cubic metres. If you are planning to use the mix for long-term plantings such as trees and shrubs you should add an extra 10% to allow for shrinkage in the mix over time.

How do I choose the right soil or soil mix for the garden?

A vegetable garden or general garden shrubs and annuals such as roses, camellias and petunias will benefit greatly from plenty of organic matter in the mix, which will tend to be at the higher end of the price range. On the other hand if the mix is for Australian plants then something with a lot more sand will be best as well as being at the cheaper end of the spectrum. It is also a good idea to view the product you wish to purchase to make sure it is stored in a protected place free from wind blown weed seeds.

What is the best way to use the soil or mix in the garden?

There are a couple of ways to get the best use from whatever product you choose. The simplest way, as mentioned above, is to create a raised bed, particularly if you are growing shallow rooted plants such as vegetables, annuals or herbaceous perennials. If you are growing deeper rooted trees and shrubs and your existing garden soil is a heavy clay then you should try and loosen that soil before putting down the new soil mix so that the roots will have some chance of penetrating the existing soil underneath and develop a more stable root system. In addition to building raised beds another approach, particularly for large areas, is to spread a layer of imported soil mix and dig it into the top 10cm of your existing soil. This will have the effect of deepening your topsoil with the amount you add depending on the existing topsoil depth and the type of plants you are planning to use. Aim to create a 15cm topsoil for shallow-rooted annuals or 25cm for trees and shrubs.

A simple test for soils or soil mix quality

If you have any doubts over the quality of a product, particularly if it is a large job then test the mix by obtaining a sample and putting it in a pot. Sow some fast germinating seed such as radish and within a week you should have healthy green seedlings emerging if it is a good quality mix.