Sustainable Landscaping Blue Mountains City Council
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Soil Character & Properties

The soils of the Blue Mountains are ancient and fragile, and it is worth remembering that many millions of years have been spent in their formation. They are far too precious to lose or abuse! If you are planning on making the most of the sites ability to provide you with a trouble free garden, then it is vital that you preserve the existing topsoil.

The soil type found on any site is dependant on many factors. Foremost amongst these is the nature of the parent material (or rock type) from which the soils originate. How much of this soil has formed in situ, and how much has been transported through gravity, wind or water, as well as the degree and nature of ongoing geophysical and biological processes will also determine various soil characteristics. The position of the soil profile in the landscape has a major impact on these properties.

Assessing the Soil
The most important aspects that need to be considered in a basic soil assessment are depth, texture, structure and nutrient status (including pH).

Soil depthis the distance from the soil surface to solid bedrock or hardpan. This indicates the effective depth to which plant roots may penetrate. The soil may be differentiated into two or more "horizons", or layers, or it may be more or less uniform in character from top to bottom.

Soil texture reflects the proportions of sand, silt and clay in the soil. Soil texture affects the soils cohesion and potential for erosion, its ability to hold water and nutrients, the depth to restricting layers that may affect root growth or water movement and its drainage characteristics. It will also indicate how easily the soil can be compacted during construction. The texture of the "topsoil" (upper layers) and the "subsoil" (the lower layers) may or may not be the same. These differences are frequently used to classify soils because of its importance for plant growth and water movement.

Soil structure relates to the way soil particles are arranged and bound together. The size, shape and nature of these aggregates have a major impact on its permeability (the rate at which water moves through the soil and the proportion of surface runoff to infiltration). This is important because it will affect the sites ability to deal with peak stormwater flows, its coherence, and its ability to provide a suitable growing environment for plants.

Nutrient status is the level of nutrient present in the soil. The pH (or relative acidity or alkalinity) of the soil will determine which of these nutrients are available to plants. Generally, light sandy soils have low nutrient status while clay soils have a higher but variable nutrient status. Organic soils generally have a high to moderate nutrient status. The ability of the soil to hold and supply nutrients will in turn determine the type of plant ideally suited to the site.

Soil blends
There are many soil blends available. Some are promoted as top dressing, or premium mixes may not be appropriate. Where soil is imported to the site make sure that it meets these requirements:

  • Use a courser grain topsoil than the underlying subsoils. This will help water to penetrate rather than pooling on the surface.
  • Do not use clay or any mixture with clay in the topsoil as this will also create pooling of water.
  • Use a weed free soil blend.
  • Use "clean topsoil" make sure that it is not just river silt as depending on its origin it can contain detrimental nutrients and other harmful pollutants. Try an organic blend made up of 60% organic material and 40% sand.

Improving soil Water and nutrient holding capacity of sand and clay soils can be improved by the addition of organic matter such as manure, leaf mould and compost. Dig in to a depth of 15 to 25cm.
Gypsum and sand added to clay soils help break the clay into clumps, improving air space and drainage. Add gypsum at the rate of 0.5 to 1.0kg per square metre. A combination of gypsum, sand and composted organic matter will produce the best results in clay soils.
Chemical additives often produce a "quick fix" but may have adverse environmental impacts in the medium or long term. Natural methods are better. There is normally a desirable concentration range of plant macro and micronutrients for plants. Maintaining the correct nutrient status is most likely to produce vigorously growing healthy plants. These will in turn be better able to resist disease and pest attack, reducing the need for chemical controls.




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