Soil is the basis of the garden, and soil health is key to growing healthy plants. This thin layer of mineral and organic material provides the medium for plant growth that goes a long way to sustaining life on earth, In this post we’ll look at the structure and layers of soil to understand how to manage and improve some of the most commonly encountered soils in Australia. The mineral components are sand silt and clay in decreasing order of particle size, whilst organic matter comes in all shapes and sizes and comprises anything that was once living which will eventually decompose to a black material called humus.
The term “soil profile” refers to the vertical layers that make up the soil of an area, from the organic matter that accumulates on the surface of the ground to the rocks underneath the topsoil. For the purposes of gardening and composting, we only need to look at the top few layers of the soil profile as this is where the vast majority of plant growth happens, especially the uppermost layer – the organic horizon. This top layer of soil is sometimes referred to as AO - “O” for organic, Mother Nature’s very own cold composting system – with the soil horizons A1 and A2 beneath it. These “A” layers make up the topsoil. Below that is the B horizon, also known as the subsoil. Further down are the C and D horizons, with D being the bedrock. We’ll be looking at the A and B horizons to see how they work and how to work with them in the garden.
Soil is a mixture of weathered rock and minerals with a wide range of organic materials. Most soils tend to form from the underlying bedrock but not always. Soil particles can be washed in by water flows, particularly floods, or can also be blown in by wind, such as sand dunes. While the underlying geology of the bedrock often impacts the nature of the soil, it is the top two layers that we can manage through adding soil conditioners and fertilizers such as comppst and mineral soil improvers such as lime gypsum. The organic horizon is where all the organic matter from above melds into a layer where composting naturally occurs at a slow rate. It’s like a miniature compost heap, with everything breaking down in a cold composting process. The rate of decomposition is very slow because of the lower temperature, but there are also many organisms that help the process along. The organic horizon supports a range of creatures, such as bacteria, fungi, nematodes, and worms, many of which are actively involved in the composting processes that go on there. The richer and more diverse this layer is, the more nutrients go back into the soil. Most ecosystems have a lot of their nutrients stored in the vegetation in the canopy above the soil, and this eventually and gradually cycles through the organic layer of topsoil. We can greatly enhance the organic layer, and hence plant growth by making and adding your own compost or worm castings back to the topsoil helps to foster a range of beneficial organisms in the organic horizon that not only help to release nutrients and improve soil structure, but also can help to prevent soil borne diseases such as root and collar rot.
Having looked at the topsoil layers that are so crucial to plant growth, let’s now look at the subsoil layers where lots of stuff accumulates over time. Fine particles such as clay or humus leach down through the topsoil to the subsoil (B horizon). This process can lead to a build-up of clay in the subsoil which can be detrimental to plant growth if that subsoil is dug up and becomes the medium you are trying tpo grow things in. Gypsum is a mineral soil conditioner that can be added to clay soils to help the particles bind together in a more crumb-like structure that greatly helps improve drainage and therefore the root growth of most plant species. By changing the structure of the soil like this, we can also improve the aeration of clayey soils, supplying oxygen needed by roots for respiration. Adding gypsum doesn’t generally affect the pH of soil, so if you also need to raise the pH (make the soil more alkaline), you can add lime or dolomite. Lime (calcium carbonate) will not only raise the pH, but also add calcium to the soil which improves the structure of clay particles in the same way as gypsum. Dolomite (calcium magnesium carbonate) is even better - it will raise the pH and provide valuable magnesium as well as calcium. If there are building works on a site, this can often lead to severe disturbance of the soil profile, with the (usually clayey) subsoil ending up as surface layers. This upheaval and exposure of subsoil is very bad for gardens, and the best way to counteract this is to strip off the topsoil and stockpile it. But if this is just not possible then using amendments such as gypsum and lime provide a practical alternative to get your subsoil ready for plant growth,.
The mineral component of most soils in Australia is some combination of sand, silt and clay. Sandy soil has large particle sizes and drains freely, but it can’t store a lot of nutrients or water, silt particles are smaller but behave in a very similar way to sand particles. Clayey soil has very fine particle sizes that stick together and don’t allow much aeration or drainage. Both of these soil types can be improved by digging in mature compost, as well as rock minerals and manure for hungrier plants like vegetables. How you condition your soil depends on what type of soil you start with, as well as what you want to grow. Soil profiles with sandy and clayey layers can be mixed together and improved by adding compost, binding the small and large particles into a better soil structure.
There are many types of soil across Australia, and it can vary quite a bit even in a small area. Doing a bit of research into your local soils is a great starting point for improving your garden soil. Any type of topsoil will benefit from the addition of good quality compost, as it helps improve structure, adds nutrients, and supports communities of beneficial organisms. Adding rock minerals depending on the type of soil and pH will also help to create a balanced and rich soil for gardening. Creating and supporting diversity in the topsoil is one of the best ways to improve soil health in the long term.