Fertile soils occur in valleys and plains, where silt and organic matter from eroded hills settle down. The idea behind using rock dust as a soil amendment derives from this observation.
The information provided here has been gathered from various sources. Not all of them have controlled studies or science backing them. Any users of rock dust as soil amendment, will have to do their own observations and testing to make sure it works for them.
Agricultural soils have undergone heavy erosion from mechanised farming.
Previously most farm lands were small plots with bunds that kept rain water from flowing out. The bunds were themselves held together by the presence of grass, weeds and shrubs. These also helped in infiltration of water.
In some cases active erosion control measures were taken. One approach would be to create vetiver hedges on the bunds, or the borders of the farm. VETIVER GRASS A thin green line against erosion. The other is to plow on contour Contour Plowing, which makes the rain water stay in the furrows, so that it seeps in, or to create water harvesting swales, or use other Water harvesting techniques.
There is also no-till agriculture, where the land is not tilled. However the most common version of this practice depends on weed killers.
Unfortunately many of these practices make it inconvenient to use machinery, and most farms have abandoned these traditional water conservation and soil preservation techniques. Erosion not only leaches the nutrients, but also destroys the structure of the soil, removing the fine silt, and leaving behind sandy, gravelly soil.
In villages soil from lakes and ponds was extracted and used in farms. This brought back some of the eroded soils. Organic fertilizers like animal manure, and organic matter, composted, or left to compost in the fields, also helped. Nitrogen fixing green manure crops are grown, and then plowed into the soil.
Chemical fertilizers are convenient, and quick acting. They however destroy microbial populations in the soil. Fungi, bacteria (especially nitrogen-fixing) help plants with mineral absorption from soil.
Agricultural research with fine rock dust was started in the 19th century by Missoux (1853/54), Hensel (Bread from Stones, 1890, 1894) and others.  This is different from Von Liebig's salt fertilizer thesis, Law of Minimum, which states that plant growth is impacted by scarcest nutrient resource. Liebig went on identify the chemical elements nitrogen (N), phosphorus (P), and potassium (K) as major influencers of plant growth. Synthetic salts that provide these elements, were then used for agriculture.
Julius Hensel's view was much broader, and he found that fertilizing with stone dust, found in primeval rocks, yield healthy, wholesome and life sustaining food from plants that were healthy and resistant to pests. Although plants take bulk of a plant's needs come from air, and water, the nutrients in rocks are indispensable for healthy crops.
Fertile valleys were usually created from silt carried down from mountains from glacial and water erosion. Trees up in the mountains, have been able to thrive in extremely rocky conditions, by using micro-organisms, bacteria and fungi, to extract minerals from rocks, and the thin layer of soil. Synthetic fertilizers may be a quick fix, but in the long run, a more complete and slow release nutrition from rock dust, was found by Hensel to be better.
Agrogeology is broadly defined as 'geology in the service of agriculture', a study of geological processes that influence the distribution and formation of soils, and the application of geological materials in farming and forestry systems as means of maintaining and enhancing soil productivity for increased social, economic and environmental benefits (Chesworth and van Straaten 1993; van Straaten and Fernandes 1995). Once we understand the geological process that has created rich soils, we find that there is a reasonable justification in the belief that rock dust can remineralize soil.
Rock dust is most effective when mixed 50-50 with organic compost and a handful of soil to add some microorganisms. The microorganisms feed off the rock dust, taking only the nutrients they need while leaving the remainder in the sub-soil. The compost provides the medium for the microorganism growth. Optimally, the rock dust and compost mixture should be incorporated into the top few inches of soil if possible but may also be spread by broadcasting or spread by hand if you use a no-till method of gardening..
Since micro-organisms have a role in making nutrients available to plants, use of synthetic fertilizers with rock dust, make not be effective. Panchagavya or Jiwamrita as recommended by Subhash Palekarji  can increase soil microbial activity.
The amount of rock dust required is something that has to be determined by testing. But there are likely to be no negative consequences in using rock dust.An application of 2 tons per acre is the minimum amount, and as much as 20 tons per acre for exceptionally poor, dry soil. Use about 7 kg of rock dust per 100 square feet of garden bed (or 2 kg per square yard)..
Growth of carrot with and without rock dust is compared here. A
permaculture study found tree seedlings grew at five times the
normal rate in the nursery. .
This study also found
Quarry dust improves water infiltration rates to the benefit of non-wetting soils.
Quarry dust improves water retention in free draining soils.
Additional anecdotal evidence by Henry Homeyer is available here.