By Kate Wall
The type of soil you have will greatly impact on the types of plants which will grow happily in your garden and the degree of care they will need.
Australian soils can be highly variable, from very sandy soils to heavy sticky clay and everything in between. A good soil is called “loam” and lies somewhere in the middle of sandy and clay soils. A good quality loam is generally dark grey to black, and quite friable – meaning it is not too hard and not too crumbly but holds its structure well. A good loamy soil will be less susceptible to both drying out and water logging than will a poorer soil type, it will be nutrient rich and suitable for growing a wide range of plants. Most gardeners will dream of having a good loamy soil but not many will have such good soil naturally.
Sandy soils are very common in Australia. In some areas they are very deep and free draining, such as much of the Perth district. In other areas sandy soils are less deep and have a clay subsoil which, although it is free draining, can lead to water logging if the water table is high or the soil is particularly shallow. Sandy soils tend to be pale in colour and exceptionally free draining as they have very little to bind the soil particles together. They tend to dry out quickly and to hold very little nutrient for plants. Many Australian native plants are adapted to grow in our poor sandy soils, and have low water requirements and also low nutrient requirements.
Sandy soils hold very little nutrient and therefore will require significantly more fertilising than will clay soils. Slow release fertilisers should always be used in sandy soils. To find out more about which fertilisers will suit your garden click here.
All sandy soils will be improved through the addition of organic matter. You can determine if your soil is sandy by wetting (not saturating) a handful and trying to squeeze it into a shape – similar to making a ball or sausage from playdough. If the shape crumbles, the soil is sandy. The degree of sandiness can vary from pure sand found in many coastal gardens to light grey soils which do not bind together to form a shape when moulded. The lighter in colour and the more crumbly, the sandier the soil is and the more improvement may be required for successful gardening in such soil. Sandy loams may be quite a dark grey colour and form a reasonable shape on moulding but remain slightly crumbly and drain easily. These are ideal soils for many garden plants in addition to many natives.
Although sandy soils tend to be free draining, some sandy soils have a shallow clay subsoil which creates a difficulty in that the top soil is nutrient poor and free draining but below that water is trapped by the clay and they can water log easily. These soils can be improved by the addition of organic matter and may also need some drainage installed for successful gardening.
At the other end of the spectrum are the heavy clay soils. These soils are hard to dig and can compact easily. They tend to water log easily and become sticky and unworkable. When they dry out they can crack, leaving significant cracks in the drought affected ground. Plant roots can find clay soils hard to penetrate and the water logging can be very problematic in these soils, making them difficult to garden. Clay soils are common in Australia and are the dominant soil type in Brisbane. Clay soils are very common in new housing developments where the clay subsoil has been disturbed and left on the surface.
Clay soils can have highly variable colours from yellows, reds, browns, greys and black. When moistened and squeezed to form a shape, clay soils will hold that shape easily. A loamy clay will show some degree of crumbliness.
Most clay soils can be improved with gypsum and organic matter. Gypsum will only work on soils which when formed into a pellet and dropped into a glass of water, cause the water to become cloudy after time. If the water remains clear, gypsum will not benefit your clay soil.
With improvement clay soils can be highly fertile and hold water well, giving some degree of drought protection to the gardener. As they can be very poor draining, they suit plants with high water needs.
Many gardeners will find they have soil which lies somewhere between these two extremes of soil. All soil types will benefit from improvement with organic matter. As many Australian soils are notoriously depleted, the addition of rock minerals as well as organic matter will greatly enhance both the structure and the nutritional value of the soil, aiding in strong and healthy plant growth.
Understanding your soil will assist you to know how best to improve your soil and will help you choose suitable plants for growing within your soil type. To find out more about soil improvement products and which might be of most benefit to your soil click here. To assist with plant selection for sandy soils click here and for clay soils click here.