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Understanding Soil & Their Layers


Looking beneath the soils surface where root systems grow helps us to understand the soils better. Soils are compromised of disintegrated rock and humus that provides the perfect growing medium for plants. The development of soils takes time, anywhere from hundreds to thousands of years, and consists of a diverse range of inorganic and organic materials.

Inorganic materials refer to the non-living aspect of the soil, such as the minerals and rocks whereas the organic material refers to the living parts of the soil such as the microorganisms.

Importance of Soil

Good quality soil is essential to the success of revegetation. Without fertile, nutritious soil, any efforts made to successfully germinate vegetation will be in vain, whether it is a smaller scale home garden or a larger scale civil project. In short, soil provides the necessary food, water and oxygen that is needed by plants to support healthy growth and development of seeds, so it is important to ensure that soil is of good quality.

Layers of Soil

Soil is separated into various different layers termed into horizons. This type of categorisation is known as the soils profile. Horizons range from fertile upper layers composed of top soil and humus, to the underlying rock layers composed of the subsoil, regolith and bedrock. Each layer is unique, boasting differences in makeup, texture, age and characteristics, each with their own benefits.

1.       The O-Horizon

The first layer, the O-Horizon, is the superficial top layer of soil that is primarily composed of decaying organic matter (humus), living organisms and fresh soil. This layer of the soil boasts a brown or black colour due to its organic composition and is usually very thin.

2.       The A-Horizon

The A-Horizon is known as the topsoil. It is the second layer and is made of highly decaying organic matter and minerals, ranging from a greyish to brownish colour. The A-Horizon is the layer that hosts many plants and supports many grassland areas. Because of this, seed germination and plant growth take place in this soil layer. There are multiple soil types in this layer, ranging from loam, clay, sand and silty soils. The topsoil is also the soil layer that is most prone to the erosive forces of wind and water.

3.       The E-Horizon

This is the third and the thinnest layer of soil. This layer is lighter colour wise when compared to the A-Horizon. Due to the leaching of minerals in this layer, it is termed as the eluviation layer. As a result of the leaching, the soil is depleted of aluminium, clay, organic compounds and other soluble elements. As the minerals drip down through the soil, all that is left is silt and sand textured soil.

4.       The B-Horizon

The subsoil is generally lighter in colour than the layers above it. It is mainly made up of clay, large rocks and bedrock, minerals and features little organic matter. The subsoil accumulates a lot of aluminium, clay, iron and organic compounds from the layers above leaching mineralised water through a process referred to as illuviation. This layer is usually chemically hostile to plant growth, with the roots of large trees ending in this subsoil layer.

5.       The C-Horizon

Known as the regolith, there is very little organic matter in the C-horizon, thus plant roots do not go through this layer. There is a lot of compacted sediment and cemented geological material in this layer which is made up of mostly fragmented pieces of bedrock.

6.       The R-Horizon

The R-Horizon is the final layer at the base of the soil profile, known as the bedrock. Materials here are mainly sloid unweathered rock and is compacted and cemented down. Rocks that can be found include limestone, granite and basalt some 50 meters from the top-soil.

SGA & Soil

For our rehabilitation and revegetation projects, our process begins with collecting and assessing soil samples to create optimum soil conditions. Soil testing provides us with essential information to determine what adjustments need to be made to assure a more favourable growing environment.

Biotic Soil Amendments (BSA’s) have been developed to improve the composition of poor soils that lack essential nutrients and bio activity after they have been disturbed by land activity. BSA’s act as a complete topsoil replacements and soil building erosion control product. They work by promoting the natural microbial activity and natural topsoil forming process to ensure a healthier vegetation support system by mimicking natures cycle. BSA’s have been formed to be hydraulically applied through hydroseeders, being spraying directly onto the surface in a consistent slurry.

Do you have an upcoming Rehabilitation or Revegetation project?

Contact our expert team at Spray Grass Australia for information about our Rehabilitation & Revegetation solutions and an obligation-free quote.

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