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Soil ColourThe colour of your soil can tell you a lot about what’s happening down there. That is of course, if you haveremnant soil left in your garden. If you’ve imported a whole bunch of potting mix, or bought soil youmight not have much colour variation.There are 3 main components that affect the colour of your soil: 1. Minerals derived from the parent material. Iron is a good example. In well drained conditions, iron produces a red colour, so the more red your soil, the more iron it has. 2. Organic matter - Although it is difficult to assess the organic matter content of your soil, the colour of the soil will help you. There are two basic ‘rules of thumb’ to follow • The darker the soil is, the more organic matter it contains • Dry soil that leaves your hands ‘dirty and dusty’ has a high organic matter content 3. Moisture content. Pour water on a soil and it becomes darker. This we all know. But a soil that is often waterlogged produces long term colour changes. Waterlogged soil is often mottled, and a dull bluish-grey colour.Well drained soils usually are usually brighter in colour. One sign of a soil that has been waterlogged for a long time is the presence of manganese nodules or concre- tions. These nodules are black in colour.A bit about iron....The iron type and amount in your soil can lead to a varietyof different colours. When a soil is dry, iron oxides becomeyellow. Under normal (i.e. good drainage and moisture con-tent) conditions, it is red. But when a soil is waterlogged (alack of oxygen), the soil often becomes a green, grey or bluish-grey colour.Alisa Bryce www.organicsoilguide.com
The colour chart above, from DPI Vic, illustrates how iron content and organic matter (in this case car-bon) can affect the colour of soil. It’s not as simple as just red, brown and black - the proportion of ironand organic matter affects the degree of colour. For example, a soil with a high carbon content, but lowiron content could be pink, pale yellow or a grey colour.Black and brown soilThese soils often have higher levels of organic matter, although some minerals can also give a dark col-ouring. Sodium can also make a soil look darker. Sodium makes the organic matter particles disperse,making the soil appear blacker.Red soilA red soil indicate good drainage. Iron found within the soil is oxidised more readily due to the higheroxygen content. This causes the soil to develop a ‘rusty’ colour. Organic matter in the soil might make thesoil appear as a darker red. Alisa Bryce www.organicsoilguide.com
Yellow and yellowish-brown soilYellow and yellowish-brown soil often has poorer drainage than red soils. The iron compounds in thesesoils are in a hydrated form and therefore do not produce the red colour.Grey and blue grey soilGrey and blue grey soil is usually waterlogged or has poor drainage.Light grey and white soilA light grey or white soil indicates a high amount of rainfall and good drainage. Excessive rain can ‘washout’ the iron and manganese particles, which give the soil some of its colour.White soil can also indicate the presence of quartz or the presence of salts. In very dry regions, the min-eral calcite can also make a soil appear white.The following table from DERM, QLD will help to explain the different properties of coloured soil Soil Property Black Light Grey Red Brown Yellow Grey/Blue Drainage Often slow Well drained Well drained Well - Less well Poorly moderately drained drained Water-logging Medium Low Low Low Low to High potential medium Organic High Low Medium Medium - Medium - Low Matter High low Accumulation Leaching of Low High Medium Medium Medium Low nutrients Nitrogen loss Medium Low Low Low Low - High mediumAlisa Bryce www.organicsoilguide.com