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.
Rock dust is used in coal mines to help prevent coal dust explosions by acting as a heat sink. The dust is usually made of pulverized limestone. Rock dust has been used since the early 1900s, but there have been technological improvements since then.
U.S. federal regulations require that rock dust be applied in all underground areas of a coal mine to mitigate the propagation of a coal dust explosion. Prior to September 2010, U.S. federal regulation (30 C.F.R. 75.403) mandated that the nation's coal mines maintain a total incombustible content (TIC) of at least 65% in nonreturn entries and at least 80% in the return airways. In September 2010, U.S. Mine Safety and Heath Administration (MSHA) published an emergency temporary standard increasing the total incombustible requirement in intake airways to 80%, effective June 21, 2011. The 65% TIC requirement was based on an average particle size termed "mine-size dust," which was based on an average of representative samples collected from mines in the 1920s. To determine compliance with the federal regulation, mine inspectors systematically collect dust samples from sections of underground coal mines and send the samples to the MSHA National Air and Dust Laboratory at Mt. Hope, West Virginia, for analysis of incombustible content.
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.
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.
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.
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[where?], Barossa and Mclaren Vale[where?] regions, rates are 3-5 tonnes per hectare. In a garden application, this might equate to 400 grams per square metre.