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{{Short description|Glacier-generated sediment}}
{{Use British English|date=July 2018}}[[File:Rock flour from glacial melt enters headwaters at Lake Louise.jpg|thumb|Rock flour from glacial melt enters [[Lake Louise (Alberta)|Lake Louise]], Canada]]
[[File:TWC Hokitika Gorge • Stewart Nimmo • MRD 16.jpg|thumb|Rock flour intensifies the water's hue at [[Hokitika Gorge]] on the [[West Coast, New Zealand|West Coast]] of [[New Zealand]]|alt=]]
'''Rock flour''', or '''glacial flour''', consists of fine-grained, [[silt]]-sized particles of rock, generated by mechanical grinding of bedrock by [[Glacier#Glacial deposits|glacial erosion]] or by artificial grinding to a similar size. Because the material is very small, it becomes suspended in [[meltwater]] making the water appear cloudy, which is sometimes known as '''glacial milk'''.<ref>[http://www.physicalgeography.net/physgeoglos/g.html "Glossary of Terms: G: Glacial Milk"] PhysicalGeography.net</ref><ref>Gornitz, Vivien (editor) (2007) "Glacial Geomorphology" pp. 361–374 ''Encyclopedia of paleoclimatology and ancient environments'' Springer Netherland, Berlin, [https://books.google.com/books?hl=en&lr=&id=yRMgYc-8mTIC&oi=fnd&pg=PA365 page 365], {{ISBN|978-1-4020-4551-6}}</ref>

When the sediments enter a river, they turn it grey, light brown, iridescent blue-green, or milky white. If the river flows into a [[glacial lake]], the lake may appear [[Turquoise (colour)|turquoise]] in colour as a result. When appearing as any shade of blue, the colour is most commonly caused by the [[Tyndall Effect]], with the particular hue determined by suspended particle size. When flows of the flour are extensive, a distinct layer of a different colour flows into the lake and begins to dissipate and settle as the flow extends from the increase in water flow from the glacier during snow melts and heavy rain periods. Examples of this phenomenon may be seen at [[Lake Pukaki]] and [[Lake Tekapo]] in New Zealand, [[Lake Louise (Alberta)|Lake Louise]], [[Moraine Lake]], [[Emerald Lake (British Columbia)|Emerald Lake]], and [[Peyto Lake]] in Canada, [[Gjende]] lake in Norway, and several lakes (among others, [[Nordenskjöld Lake|Nordenskjöld]] and [[Lake Pehoé|Pehoé]]) in Chile's [[Torres del Paine National Park]], and many lakes in the [[Cascade Range]] of [[Washington (state)|Washington State]] (including [[Diablo Lake]], [[Gorge Dam|Gorge Lake]], and [[Blanca Lake]]).

==Formation==
{{Unreferenced section|date=May 2018}}
[[Image:Muru memurubu.jpg|thumb|[[Muru (Lom)|Muru]] river pours rock flour into [[Gjende]] lake, Norway]]Typically, natural rock flour is formed during [[glacial motion|glacial migration]], where the [[glacier]] grinds against the sides and bottom of the [[Rock (geology)|rock]] beneath it, but also is produced by [[Freeze-Thaw Cycle|freeze-and-thaw action]], where the act of water freezing and expanding in cracks helps break up [[rock formations]]. Multiple cycles create a greater amount.

Although clay-sized, the flour particles are not [[clay minerals]] but typically ground up [[quartz]] and [[feldspar]]. Rock flour is carried out from the system via [[meltwater]] streams, where the particles travel in [[Suspension (chemistry)|suspension]]. Rock flour particles may travel great distances either suspended in water or carried by the wind, in the latter case forming deposits called [[loess]].

==Agricultural and silvicultural use==
'''Rock flour''', artificial or natural, is a source of [[plant micronutrients]] ([[minerals]] [[trace elements]]) widely used in organic farming practices as a [[soil conditioner]]. Synonyms in this case include '''rock dust''', '''rock powders''', '''rock minerals''', and '''mineral fines'''.

The [[igneous rock]]s [[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 [[micronutrient]]s. 

===Background===
Soil remineralization (in the sense of re-incorporating minerals, different from [[remineralisation]] in biogeochemistry) creates fertile [[soil]]s 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 (soil)|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]].<ref>Remineralization Might Save Us From Global Warming, ''The Independent'', Paul Kelbie, 21 March 2005</ref> Further testing has been undertaken by James Cook University, Townsville, Far North [[Queensland]].<ref>{{cite thesis|last1=De Silva, Meragalge Swarna Damayanthi Luxmei|title=The effects of soil amendments on selected properties of tea soils and tea plants (Camellia sinensis L.) in Australia and Sri Lanka.|url=http://researchonline.jcu.edu.au/2016/|website=James Cook University|access-date=25 April 2015|date=March 2007|doi=10.25903/ns0d-1a64 }}</ref>

=== History ===
{{Unreferenced section|date=May 2018}}
[[Thomas J. Goreau]] who wrote the book ''Geotherapy'' believed that mafic/ultra-mafic rock flour had a powerful effect in restoring trace minerals to soils, which increases the health and vigour of the Microorganism, Plantae, Animalia pathway and also sequesters carbon.  An early experimenter was the German miller, [[Julius Hensel]], author of ''Bread from Stones'', who reported successful results with ''steinmehl'' (stonemeal) in the 1890s. His ideas were not taken up due to technical limitations and, according to proponents of his method, because of opposition from the champions of conventional [[fertiliser]]s.

[[John D. Hamaker]] argued that widespread remineralization of soils with [[rock dust]] would be necessary to reverse soil depletion by current agriculture and forestry practice.

While this originally was an [[Alternative culture|alternative]] concept, increasing mainstream research has been devoted to [[soil amendment]] and other benefits of rock flour application: for instance, a pilot project on the use of glacial rock, [[granite]] and [[basalt]]ic fines by the [[U.S. Department of Agriculture]] exists at the [[Henry A. Wallace Beltsville Agricultural Research Center]]. The SEER Centre in Scotland is a leading source of information on the use of rock dusts and mineral fines. The Soil Remineralization Forum was established with sponsorship from the Scottish Environment Protection Agency and has commissioned a portfolio of research into the benefits of using mineral fines. The Forum provides an interface among research, [[environmentalist]]s, and industry.

[[Image:Blanktjärn.jpg|thumb|Östra Blanktjärn Lake in [[Vålådalen Nature Reserve]], Sweden]]

===Research===
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 [[Soil drainage|drainage]]. Rockdust also provides [[calcium]], [[iron]], [[magnesium]], [[phosphorus]] and [[potassium]], plus trace elements and [[micronutrient]]s. By replacing these [[Leaching (agriculture)|leach]]ed minerals, [[soil health]] is increased and that this produces healthier plants.<ref>{{Cite journal
 | last1 = Swoboda
 | first1 = P.
 | last2 = Döring
 | first2 = T. F.
 | last3 = Hamer
 | first3 = M.
 | title = Remineralizing soils? The agricultural usage of silicate rock powders: A review
 | journal = Science of the Total Environment
 | volume = 807
 | article-number = 150976
 | year = 2022
 | issue = Pt 3
 | doi = 10.1016/j.scitotenv.2021.150976
 | pmid = 34662609
 | bibcode = 2022ScTEn.80750976S
 | issn = 0048-9697
}}</ref>

A 2022 study found that basalt dust improved soil fertility and increased available phosphorus, potassium, calcium and magnesium levels compared to soil without the basalt dust in a period of several months.<ref>{{cite journal|author=Conceição, Lucas Terto; Silva, Gutierres Nelson|year=2022|title=Potential of basalt dust to improve soil fertility and crop nutrition|journal=Journal of Agriculture and Food Research|volume=10|issue=|article-number=100443|doi=10.1016/j.jafr.2022.100443|doi-access=free}}</ref>

Greenland rock flour increased yields from corn fields in Ghana and barley fields by 30–50%. Melting ice is depositing one billion tons of rock flour annually; one ton can [[direct air capture]] {{convert|250|kg}} of carbon.<ref name="dyer20250116">{{Cite news |last=Dyer |first=Evan |date=2025-01-16 |title=Why acquiring Greenland is more than just a whim of Trump |url=https://www.cbc.ca/news/politics/trump-greenland-security-resources-1.7431319 |access-date=2025-01-16 |via=CBC}}</ref>

In forestry, rock flour can gradually increase the pH and buffering status of soils via its slow-release liming properties<ref>{{cite journal |last1=Van Der Bauwhede |first1=Robrecht |last2=Muys |first2=Bart |last3=Vancampenhout |first3=Karen |last4=Smolders |first4=Erik |title=Accelerated weathering of silicate rock dusts predicts the slow-release liming in soils depending on rock mineralogy, soil acidity, and test methodology |journal=Geoderma |date=2024 |volume=441 |issue=116734 |article-number=116734 |doi=10.1016/j.geoderma.2023.116734|doi-access=free |bibcode=2024Geode.44116734V }}</ref> and increase tree radial growth if nitrogen is not deficient, even up to 34 years after the application.<ref>{{cite journal |last1=Van Der Bauwhede |first1=Robrecht |last2=Troonbeeckx |first2=Jordi |last3=Serbest |first3=Iris |last4=Moens |first4=Claudia |last5=Desie |first5=Ellen |last6=Vancampenhout |first6=Karen |last7=Smolders |first7=Erik |last8=Muys |first8=Bart |title=Restoration rocks: The long-term impact of rock dust application on soil, tree foliar nutrition, tree radial growth, and understory biodiversity in Norway spruce forest stands |journal=Forest Ecology and Management |date=2024 |volume=568 |issue=122109 |article-number=122109 |doi=10.1016/j.foreco.2024.122109|bibcode=2024ForEM.56822109V }}</ref> Furthermore, rock flour amended in the planting pits of tree saplings can increase sapling survival and growth when reforesting acidified forest soils.<ref>{{cite journal |last1=Van Der Bauwhede |first1=Robrecht |last2=van den Berg |first2=Leon |last3=Vancampenhout |first3=Karen |last4=Smolders |first4=Erik |last5=Muys |first5=Bart |title=Field phytometers and lab tests demonstrate that rock dust can outperform dolomite and fertilisers for acid forest soil restoration |journal=Plant and Soil |date=2025 |volume=513 |issue=1 |pages=237–257 |doi=10.1007/s11104-024-07175-8|bibcode=2025PlSoi.513..237V }}</ref>

=== Composition ===
{| class="wikitable floatright" border="1"
|+{{cn|date=March 2022}}{{what|reason=What flour, made from what rock? Who did the tests?|date=March 2022}}
|-
! Element
! Unit
!
|-
| [[calcium]]
| %w/w
| 6.44
|-
| [[iron]]
| %w/w
| 10.5
|-
| [[magnesium]]
| %w/w
| 6.54
|-
| [[sulfur]]
| %w/w
| 0.21
|-
| [[potassium]]
| %w/w
| 1.25
|-
| [[phosphorus]]
| mg/kg
| 3030
|-
| [[cobalt]]
| mg/kg
| 35
|-
| [[copper]]
| mg/kg
| 43
|-
| [[manganese]]
| mg/kg
| 790
|-
| [[molybdenum]]
| mg/kg
| <5
|-
| [[zinc]]
| mg/kg
| 92
|-
| [[silicon]]
| %w/w
| 21.6
|}[[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.{{Citation needed|date=May 2015}}  Silicon comes in silicon multi-oxide molecules (e.g. SiO<sub>2</sub>, SiO<sub>4</sub>, SiO<sub>6</sub>, and SiO<sub>8</sub>). 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 [[fertiliser]]s. The use of micronutrient-rich fertiliser enables plants to access locked phosphorus.{{Citation needed|date=October 2025}}

The [[Chemical element|elements]] high in available 2+ valence electrons (particularly calcium, iron and magnesium) contribute to [[paramagnetism]] in soil which aids in [[cation]] exchange capacity.{{Citation needed|date=October 2025}}

The [[calcium]] and [[magnesium]] in high quality have the ability to neutralise [[pH]] in soils, in effect acting as a [[Agricultural lime|liming agent]].{{Citation needed|date=May 2015}}

===Application===
Rock dust can be applied to soil by hand application, via [[broadcast spreader]], [[helicopter]] or by [[fertigation]]. Where possible the rock dust 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 up to 50 tonnes per hectare are required. In [[Australia]], namely the [[Riverland]], [[Riverina]], [[Langhorne Creek, South Australia|Langhorne Creek]]{{where|date=December 2018}}, [[Barossa Valley|Barossa]] and [[McLaren Vale]]{{where|date=December 2018}} regions, rates are 3–5 tonnes per [[hectare]]. In a garden application, this might equate to 400&nbsp;grams per square metre.

== See also ==
{{div col|colwidth=30em}}
* [[Diatomaceous earth]]
* [[Greensand (geology)|Greensand]], used as a source of potassium
* [[Organic farming]]
* [[Organic fertilizer]]
* [[Phosphorite]], also called rock phosphate
* [[Rock butter]]
* [[Soil conditioner]]
{{div col end}}

==References==
{{reflist}}

==External links==
*[http://www.remineralize.org/ Remineralize the Earth] – "non-profit organization incorporated to disseminate ideas and practice about soil remineralization"
*[http://www.carbon-negative.us/soil/bread.htm Julius Hensel by John Mann]
*[https://pubs.usgs.gov/of/2004/1216/index2.html The USGS Glossary of Glacial Terminology]

{{sediment transport}}

{{DEFAULTSORT:Rock Flour}}
[[Category:Glaciology]]
[[Category:Soil]]
[[Category:Agronomy]]
[[Category:Organic fertilizers]]
[[Category:Soil improvers]]