Rock dust, also known as rock powders, rock minerals, rock flour, soil remineralization, and mineral fines, consists of finely crushed rock, processed by natural or mechanical means, containing minerals and trace elements widely used in organic farming practices.
The igneous rocks basalt and granite often contain the highest mineral content, whereas limestone, considered inferior in this consideration, is often deficient in the majority of essential macro-compounds, trace elements, and micronutrients.
Rockdust is also the limestone-based product sprayed on walls inside underground coal mines to keep coal dust levels down. This is to prevent coal dust explosions and also to prevent the incidence of black lung disease.
Soil remineralization creates fertile soils by returning minerals to the soil which have been lost by erosion, leaching, and or over-farming. It functions the same way that the Earth does: during an Ice Age, glaciers crush rock onto the Earth's soil mantle, and winds blow the dust in the form of loess all over the globe. Volcanoes erupt, spewing forth minerals from deep within the Earth, and rushing rivers form mineral-rich alluvial deposits.
Rock dust is added to soil to improve fertility and has been tested since 1993 at the Sustainable Ecological Earth Regeneration Centre (SEER Centre) in Straloch, near Pitlochry, in Perth and Kinross, Scotland. Further testing has been undertaken by James Cook University, Townsville, Far North Queensland.
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SEER's research claims that the benefits of adding rockdust to soil include increased moisture-holding properties in the soil, improved cation exchange capacity and better soil structure and drainage. Rockdust also provides calcium, iron, magnesium, phosphorus and potassium, plus trace elements and micronutrients. By replacing these leached minerals it is claimed that soil health is increased and that this produces healthier plants.
- Typical composition table of rockdust
Silicon is thought to be the major element effecting the strength of cell wall development. However it is the amount of available silica that has a dramatic effect on the plant strength and subsequent health. To highlight this, plants that are grown in very sandy soils, (being high in non available silica), often require a silica based fertiliser to provide available silicon.
Silicon comes in silicon multi-oxide molecules (e.g. SiO2, SiO4, SiO6, and SiO8). Each molecule shape is thought to pack in different ways to allow different levels of availability.
Often phosphorus is locked in soils due to many years of application of traditional fertilisers. The use of micronutrient-rich fertiliser enables plants to access locked phosphorus.
Spreading and applying dust in agriculture
Rockdust can be applied to soil by hand application, via broadcast spreader or by fertigation. Where possible the rockdust can be worked into the ground either physically or by using water to wash in.
Rate of application
In some soils which display poor levels of nutrients, application rates of 10 tonnes per hectare are required. In Australia, namely the Riverland, Riverina, Langhorne Creek, Barossa and Mclaren Vale regions, rates are 3–5 tonnes per hectare. In a garden application, this might equate to 400 grams per square metre.
Rockdusting, also known as soil remineralization, was mentioned in the 19th-century book Bread From Stones by chemist Julius Hensel.
In the 20th century, rockdusting was popularized by science writers John D. Hamaker, Larry Ephron, Alden Bryant, Don Weaver, Harvey Lisle, Arden Andersen and Lee Klinger. Klinger uses remineralization through rock dust to restore ailing trees, and "prefers a product called Azomite, a mined natural crushed rock that contains potash, calcium and more than 50 trace minerals". 
- Remineralization Might Save Us From Global Warming, The Independent, Paul Kelbie, 21 March 2005
- De Silva, Meragalge Swarna Damayanthi Luxmei. "The effects of soil amendments on selected properties of tea soils and tea plants (Camellia sinensis L.) in Australia and Sri Lanka.". James Cook University. James Cook University. Retrieved 25 April 2015.
- Klinger, Lee (29 December 2007). "Minerals for Aging Soils". Remineralize the Earth. Remineralize.org. Retrieved 11 March 2014.
- Rich, Deborah (29 October 2005). "OAK LORE: Preserving a heritage tree / Scientist takes holistic approach to sudden oak death". SF GATE. San Francisco Chronicle. Retrieved 11 March 2014.