Soil and Your Raised Garden Bed
Good soil is the single most important ingredient for a good garden. Raised beds give you an immediate advantage over a regular garden, because when you fill it with a blend of soil that is superior to the native soil in your yard. Soil that is loose and rich with nutrients and organic matter will allow the roots of your plants to grow freel, and ensure that they have access to the water and nutrients they need to sustain healthy growth.
Choosing top soil for the garden
A simple test can help you determine whether you are purchasing a quality product. Take a handful of the top soil 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 undesireable 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 term garden mix is used 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 planting 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 annual, 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 multiplying the length, width and height of your garden bed together and then multiplying that number by 35.31.
For example, if your raised garden bed is 1200mm long x 600mm wide and 850mm high you would go 1.2 x 0.6 x 0.85 = 0.612 cubic metres x 35.614667 = 21.61 Cubic Feet. Multiplying the number by 35.31 converts the measurement from cubic metres to cubic feet.
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?
Expect to pay between $30 and $55 per cubic metre depending on the quality of ingredients used and the purpose you require it for. For instance, a vegetable garden or general garden shrubs and annuals such as roses, camellias and patunias will benefit greatly from plently 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 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.
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 greed seedlings emerging if it is a good quality mix.
Gardener’s Tip: If you are not planting within a week, cover the bed with a layer of straw to protect the soil,. This will, however, prevent the soil from warming up and drying oiut. So you will need to uncover the bed at least a week before planing warm-season crops.