Basic Soil Properties

What gives a soil its basic properties?

Soils are created from the erosion, weathering and disintegration of rocks such as Granite, Shale, Quartz and Limestone over millions of years. The base rock is weathered by rain, sunlight, ice, organic acids (from plants decomposing that produce a weak acid called Humic acid), temperature (when the rock has the forces on it of expansion and contraction as the temperature changes), and other factors.

The soil type in a garden is determined by the rock that it was weathered from and how and where it was deposited. If the parent rock was a limestone then the soil will most likely be a sandy soil.

Three basic soil types exist in most gardens but can be further classified into Silty Soil, Peaty Soil and Chalky Soil though the three are less common in the back yard.

Of course, the soil in your garden may have been altered by people adding top soil, compost, mulch, green waste etc. to alter and improve the basic properties of the soil

The proportions of sand, silt & clay in your garden can be determined by placing some soil in a tall thin clear container, eg. an old milk bottle, shaking it well (not stirred) with some water, and letting it settle overnight into its respective layers. Measure each layer and determine the % of each of the sand, silt & clay and then use the below diagram to determine what category you soil falls into.

In NSW and north, the soil is slightly acidic so the plants that grow naturally in it prefer a slightly acidic soil. At 6-6.5 the soil is slightly acidic and all the nutrients are available in enough quantity without being in excess or deficient.

In areas where the soil is derived from limestone, (eg. in SA), the plants prefer the slightly alkaline soil such as Azaleas & Rhododendrons.

Water can also change the pH of the soil as well as added unwanted minerals.

Town water is quite variable depending where it is supplied from.In Brisbane, the water comes from a wide range of reservoirs.If you are watering your plants from a bore then it commonly has excessive iron in it. This Iron is not normally available to plants as it is locked up chemically but will discolour the leaves of plants.

To be sure of the water you will be using you will need to get it tested mainly for pH levels.

A local Nursery will often test the water for free or a nominal charge and then you will have a base level to work from.

The cheap pH kits based on a piece of soil that you add some powder to and then check the colour against a colour chart, are crude and will only give a value of + or – 1 unit, and about 30% of Australian males are partly colour blind so may not see any clear colour.

Today’s Did You Know…?

An amazing example of weathering and erosion and deposition of sediment is the Bodélé Depression. Located in Chad, in North Central Africa on the southern edge of the Sahara Desert, it was once a part of the Mega-Lake Chad. Drought and irrigation eventually reduced this great lake to 5% of its former size leaving silt and sediment to scorch and dry in the African sun and later to be eroded away by the wind and is deposited thousands of miles away in the Amazon Rainforest and the east coast of the United States.

Believe it or not, this sediment is “Diatomaceous Earth” and provides a valuable source of nutrients for the forest and the soil. (See the previous newsletter on Diatomaceous Earth)

“In winter, the depression produces an average of 700,000 tonnes of dust each day (Todd et al., 2007).”

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