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How to check the texture and structure of your soil

By Dave Forrest, lecturer at Wollongbar TAFE.

[Diagram of Soil Composition ]

SOIL texture and structure give important characteristics affecting crop selection and agricultural productivity.

They determine water infiltration rates, water retention rates, drainage capacity, root diseases, aeration, root penetration, surface crusting, seedling germination and suitability to cultivation.

To determine your soil textural class, you need to find the relative proportions of sand, silt and clay.

Sand particles are large enough to be seen by the naked eye. Silt particles are able to be seen with a standard microscope and clay particles are so small that an electron microscope is needed to identify individual particles.

A quick field method to determine the dominant particle involves first physically breaking any sod structure. Next pick up some moist soil and rub it between fingers and thumb. Is it gritty? If so, sand is present. Is it smooth? If so, clay is present.

To find the actual textural type, one third fill a jar with your soil. Add another one-third volume with water. Shake vigorously and leave it to settle overnight. The coarse sand particles will settle first followed by the silt and then the clay.

The relative proportions will give the soil's textural class and common name as shown on the textural triangle (see illustration).

Large amounts of alternative particle sized material are required to affect total textural type. For example, 3,000 tonnes/ha of coarse material is needed to vary the texture of a clay soil.

The textural types most suited to cultivation are dominated by the coarser particles. Soil structure describes how the textural particles group together.

The broad range is from unstructured, loose sand through granular and crumb structure in clay loams to blocky structure in heavy clays.

Soil structure is more easily manipulated for agricultural advantage. Organic levels are important in the development and maintenance of strong structure.

Under trees and pasture, organic matter levels rise and structure develops whereas cultivation lowers organic matter levels and breaks down soil structure.

The organic matter encases the clay and sand particles to form more ideal aggregate sizes. The clay particles, being so small, are also affected by mineral additions to the soil. Gypsum applied at 2.5 tonnes to the hectare will improve the soil structure by causing the clay particles to clump together.

Lime and dolomite also have this effect but to a lesser extent. They will also lift the pH. They are useful on acid sands and clay soils.

Readers' Comments

From: Warren Vidrine wv@vidrine.com
Date: 6/17/2003

I've seen soil triangles, but none with the wavy lines and
classification into 6 categories, as they appear in the illustration
on the page:
What are these 6 categories and what do they mean?
-- Thank you,
Warren Vidrine

From: Gemma.Woldendorp@brs.gov.au
Date: Fri, 18 Jun 2004

I too have never seen a texture triangle with the wavey lines and 6 categories. I would also like to know what they mean.

I am trying to find a soil texture triangle for Australia as I would like to estimate bulk density from it (roughly). I found a great US website http://www.pedosphere.com/resources/bulkdensity/triangle_us.cfm that allows you to do this, but the US soil types (by name) are slighty different in terms of % sand, silt, clay from the Australian versions I've seen (but then the Australian ones are not always consistent either).
Any suggestions?


Gemma Woldendorp
Landscape Sciences
Bureau of Rural Sciences

Thanks for your comments.
I will try to contact the original author about this. I'm afraid it's not something a web site editor can answer!
Best wishes, JH. [22/6/04]

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