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What is Soil? - Guest Editorial by Katrina Walton

What is Soil? - Guest Editorial by Katrina Walton

Date Published: 04-Feb-15

Guest editorial in celebration of the International Year of Soil by ChemCentre soil expert, Katrina Walton.

2015 is the International Year of Soil. The International Year of Soil aims to raise awareness on the importance of sustainable soil management as the basis for food systems, fuel and fibre production, essential ecosystem functions and better adaptation to climate change for present and future generations.

ChemCentre is planning to do its bit for the International Year of Soil by digging up the facts on soil on a global and more importantly, a local level. Our first instalment is below.

What is soil?

Soil can be defined as the organic and inorganic materials on the surface of the Earth that provides the medium for plant growth and is defined as containing particles smaller than 2mm. Particles larger than 2mm are classified as stones or gravel; they are not soil. Soil develops very slowly over time and is composed of many different materials. Inorganic materials, or those materials that are not living, include weathered rocks and minerals. Weathering is the mechanical or chemical process by which rocks are broken down into smaller pieces. As rocks are broken down, they mix with organic materials, which originate from living organisms such as dead plants and animals. As the plants and animals decompose they release nutrients back into the soil.

There are three basic types of soil: sand, silt and clay. Most soils are composed of a combination of the different types. How they mix determines the texture of the soil. Soil texture is defined by the particle size distribution of the inorganic component. In Australia, soil is classified into the three groups based on particle diameters; sand (0.02 to 2 mm), silt (0.002 to 0.02 mm) and clay (less than 0.002 mm).

Sand within soil is actually small particles of weathered rock. Sand is fairly coarse and loose so water is able to drain through it easily. While this is good for drainage, it is not good for growing plants because sandy soil will not hold water or nutrients.

Silt can be thought of as fine sand, and it will hold water better than sand. If you were to hold a handful of dry silt in your hand, it would feel almost like flour. If you were to add water to the silt in your hand, it would do a fair job of holding the water and feels slick and smooth.

Clay is very fine-grained soil. Its particles are even smaller than silt, so there is very little space between the fine grains for air or water to circulate. Therefore, clay does not drain well or provide space for plant roots to flourish. In many of the eastern states gardening shows, it is common for products like gypsum to be promoted as a clay breaker. We don’t have much need for clay breakers in WA, apart from the WACA cricket pitch.

Soil scientists use tools like soil triangles (below) to classify a soils texture, after the sand, silt and clay particle size measurement is done in the laboratory.

Soil profiling triangle

Soil profiling triangle
Retrieved from:

“Typical” West Australian soil profile

Wondering why your garden maybe struggling this time of year? Well the diagram should be a big clue. As indicated in the diagram most Perth garden soils from the Swan Coastal Plain fall within the “sand” texture classification. Typical clay contents of Swan Coastal Plain sands range from 0.5 to 1.5%.

Typical Perth soil profile

Typical sandy profile of Perth soils
Retrieved from:

The sand plain soils (undisturbed or unamended) are characterised by extremely low fertility, poor nutrient retention and low water holding capacity. To create an attractive garden means healthy plants and soils. Adding clay to sandy soils does miracles to soil structure. Previous work carried out by the Department of Agriculture and ChemCentre has indicated the addition of 4-5 % clay in the top 10cm of the soil profile can improve water holding capacity dramatically and help you grow healthier plants!

-Katrina Walton

International Year of Soils

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