Why improve the soil?
Good soil is the foundation of a good garden. Soils have often been degraded or topsoil completely removed from a site during earthworks. Various grades of fill may have been imported. A variety of soil types from heavy clays or very porous sands are found in WA so depending upon the soil type and plants selected, soil amendment is usually required to improve soil condition for nutrient uptake and efficient water use.
Naturally occurring soils do not usually require improvement for local plant species, though this is questionable if natural vegetation has been removed for a period of time and the topsoil has been degraded.
Soil types in the Perth region
The majority of the Swan Coastal plain is sand. But there is a range of soil complexes in this broad category from shallow sands over limestone, black peaty sands around lakes and swamps, alluvial loams and clays along the rivers to sands and loams over clays. Then there are the lateritic (gravelly) and granite soils of the Darling Range in all their variety.
Identifying a soil type
To work out soil type the soil texture is important. This is the relative amount of sand, silt and clay in a soil. For specific agricultural and horticultural production purposes it may be helpful to undertake a more scientific analysis of a soil but for amenity landscaping the following broad categories and their characteristics are sufficient:
| SOILTYPE | WHEN DRY | WHEN WET |
|---|---|---|
| Clay | Sets hard, may crack | Sticky, drains very slowly |
| Clay-loam | Sets hard, may crack | Sticky, freer draining |
| Sandy-clay | Sets hard | Usually not sticky, drains slowly |
| Loam | Sets firm | A little sticky |
| Sandy-loam | Remains friable | Easy to work |
| Clayey-sand | Dries firm | A little sticky |
| Loamy-sand | Dries firm | Easy to work |
| Sand | Remains loose | Easy to work |
Improving the soil
All soils benefit from the incorporation of organic matter.
Organic matter is best added in the form of compost. Compost, which is the basis of quality soil improvers describes a range of naturally manufactured products. It is basically organic material which has undergone a controlled, biological process by which pasteurization (at temperatures above 55 degrees) and biological transformation occurs and a certain level of maturity is reached. It improves the structure of soils, the availability of nutrients and the water holding capacity of the soil.
A major advantage of professionally composted products is that due to the pasteurization process they are weed free. Manures, blood and bone etc are organic materials that can be classified as fertilizers but are often added to compost products to create a soil improver.
Clay soils
Soils in the clay range benefit from the addition of organic matter to improve soil structure, aeration and drainage. Gypsum can be added to reactive clay soils to improve structure and water penetration.
Sandy soils
Soils in the sandy range benefit from the addition of organic matter to improve water and nutrient holding capacity and enrich soil fauna.
Clay can be added to sandy soils to increase the nutrient and water holding capacity. The addition of a small proportion of clay can dramatically increase water-holding capacity and reduce water repellency. Another benefit is that the role of organic carbon in promoting water repellency due to wax build up decreases markedly with increasing clay content. There is also evidence that the addition of lime can decrease water repellency over time which can be a more cost effective solution than claying sandy soils but pH requirements will need to be considered.
Note: The terms soil improver, soil conditioner and soil concentrate are interchangeable. Products made to Australian standards AS 4454 are recommended.
Inorganic soil amendments
Spongelite and Zeolite are inorganic naturally occurring products that absorb water and nutrients and allow their subsequent slow release in the soil profile.
They can also extend the life of liquid wetting agents. These products are particularly useful for water repellent, sandy soils.
Wetting agents
A first requirement for efficient water use is to ensure that water rapidly infiltrates the soil. The application of wetting agents is highly recommended for water repellent sandy soils. They are available in liquid and granular forms.
The liquid wetting agents are faster acting and ensure initial wettability of soils but granules can last longer – so depending upon the application a combination of the products may be the best solution. Apply according to the manufacturers specifications. River-safe products are
Important Planting Tip
When planting in water repellent sandy soil, plant significantly lower than the surrounding soil level so water runs to the root ball rather than away. This simple technique will make a huge difference to effective watering. It is important to plant below existing ground level rather than mound up soil in a moat effect as this will rapidly wash or blow away in heavy weather. Also leave room for mulch when finalizing soil levels in retained beds and against paving to prevent run off and debris messing up paths, paved areas etc.
Incorporating organic matter
It is best to incorporate organic matter into root zone of the soil – to a depth of 250-400mm. The most economic and efficient method is to improve the individual planting hole. The roots benefit immediately, water storage is located at the base and sides of the plant where required. If the material is left at the surface it encourages weed growth and can hold water at the top of the soil profile encouraging shallow root systems that are less drought-tolerant.
Mounded gardens and sloping areas can be problematic for water penetration due to the rate of run off. Apply the planting tip below and if the slope is significant, ensure the edge of the planting hole slopes back towards the root ball to minimise run off.
Importing soil
It is not necessary to import large quantities of soil unless fill is required to change levels, back fill retaining walls, create mounded effects etc. It is preferable to use the existing soil and add soil improver and soil amendments as outlined above. If soil is required, there are a variety of sands and soils available for landscaping applications from brickies sand and fill sand to topsoil and Gin Gin loam. The loams are premium products with some clay content and good water holding properties.
Soil mixes which are designed for specific applications such as lawn mix, vegetable mix etc are useful products. A new development is a purpose designed soil mix available for use with drip irrigation which aids even dispersal.
Ensure that you are purchasing a quality compost product rather than sand, which has peat, added to it. There are Australian standards for garden soils – ‘Soils for landscaping and garden use AS 4419’.
