It can be very tempting to look at the soil you have and think it is better to replace it with soil you can buy in. It looks all fresh and new, problem free and feels like a clean slate. Ignoring the environmental aspects of digging up soil, dumping it, creating new soil, relocating it etc, there is a good chance you are better off improving the soil you have rather than replacing it. Better off in that you will have both a superior soil and more money in your pocket.
Many soils you buy are constructed soils, so they are relatively uniform from one batch to the next. These soils can have no, or minimal, clay component, which is a problem in the medium to long term. Often deplored by gardeners, clay has many highly superior qualities and plays a vital role in good soil. You will end up with a beautiful rich dark fruitcake of a soil if you improve your existing clay based soil.
When discussing soil improvement, the first step is appreciating the benefits and essential nature of organic matter. No matter which literature source you access, whether peer reviewed journals, university fact sheets or garden blogs, it is impossible to escape the vital role organic matter plays in the soil.
For Australian soils adding organic matter is almost always extremely beneficial. Add in both compost and manures, spread out over the top and then dig in to around a spade depth. Do not over work the soil and try to work when soil is damp – not dry and not wet. The addition of organic matter allows the build up of worms, soil arthropods and soil micro-organisms; these decompose the organic material and in the process provide nutrients for plant uptake.
Very occasionally (almost never) an enthusiast will add in far too much organic matter resulting in salt build up, large nitrogen release, excessive phosphorus and an imbalance of other minerals. This is rarely a problem in Australia.
Keeping the soil moist – to a level of a wrung out sponge – is important in keeping the soil micro-organisms alive, and these are vital for good plant health. Mulch is very beneficial in improving soil moisture and adding organic material to the soil. As the soil micro-organisms break organic matter down and drag it into the soil, it needs to be replaced periodically. If the organic matter is not replaced, the soil micro-organisms will die and soil fertility will fall.
If you have resorted to importing soil it is important to avoid ‘textural interfaces’. This is where you have one type of soil laid directly over another type of soil; the resulting difference in pore spaces interferes with soil water movement and root development. Make a transition layer between the soils, mixing the fill (imported soil) with the soil underneath.