Eons ago the earth had a mantle of rock. Then the glaciers slowly and inexorably moved and ground the rocks into a thick layer we called dirt. The dirt supported plant life, which took in the minerals needed for growth, and gardens were born. Over thousands of years, essential micronutrients were continually taken up by plants, often to the point of exhaustion. We as gardeners attempt to add nutrients back to the soil by means of fertilizers, compost and/or other amendments that we purchase and apply in vast and expensive quantities. However, those materials rarely contain all the micronutrients eroded away or taken up by plants.
What are Micronutirents?
“Eight of the seventeen elements essential for plant growth are micronutrients. On soils deficient in these micronutrients, the application of small amounts of these nutrients can greatly enhance crop production. The micronutrients are Boron (B), chlorine (Cl), cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn). With the exception of nitrogen, all plant nutrients are of geological origin. Under natural climatic conditions the physical breakup, chemical weathering and release of nutrients from minerals is not fast enough to provide the nutrients for annual crop production.” 
There was an area of New Zealand where the sheep were dying for no obvious reasons – there was plenty of food. Then it was discovered that the soils were deficient in the element cobalt. It is well known that cobalt is needed for our immune systems. Another example is provided by E.I. Steifel, Science, 996, vol. 272, where he showed that the process that accounts for much of natural nitrogen fixation in soils requires molybdenum. In many soils adding a trace of Mo would reduce the need for nitrogen fertilizers. 
It has been suggested elsewhere that there are as many as 90 minerals needed by plants. Minerals are also essential for human health and the human body utilizes over 80 minerals for maximum function.  Because our plants and soils are so nutrient depleted, even if we eat the healthiest foods, we are not getting all the minerals we need.
There is one simple solution: rock dust. Rock dust is generally a by-product of the gravel industry and is available almost everywhere, often free for the taking. Rock ‘gravel’ or dust is found in the bottom of creek beds, and pond settlings. The finer the size, the easier microorganisms can access the minerals but if the rock dust has been ground into silt, it is too fine and will merely clog up the pores in the soil.
The use of rock dust is not new. Agricultural research with finely ground and chemically unprocessed rocks and minerals, based on the concept of “bread from stones” started in the 19th century by Missoux (1853/54), Hensel (1890, 1894) and others. The Hardin Brothers in Queensland Australia have been using rock dust more than 20 years. They have found less environmental damage, 25% higher yields, 20% increase in growth rates and 80% less fertilizer. 
One reason seaweed is so effective in the garden is the high trace mineral content. Greensand, a common soil amendment, is just dust from sandstone rock that was deposited in marine environments.
How does rock dust work best?
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.
It is not totally necessary to add the rock dust mixed with compost. The dust alone may be added and raked in, or tilled in. The addition of compost just gives the rock dust a head start as food for the microorganisms.
An interesting benefit of adding rock dust is that it will help create more organic matter, which in turn helps hold the soil in place and conserve water. Soil erosion is an effect of the shortage of minerals to support the soil organisms. “That shortage can be made up on any piece of land in the time it takes to work ground gravel dust into the topsoil. When that is done, the soil microorganisms begin to multiply and it is they who prevent soil erosion by granulating the soil and holding it against both wind and rain." 
What will Rock Dust Do for my Garden?
Applying rock dust is often called “remineralization” which actually revitalizes the soil by mimicking the process of glaciiation.
Provides slow release of trace minerals
Increases microorganism growth
Builds more soil
Increases nutrition in plants and crops
Cuts the need for chemical fertilizers
Increases water retention in the soil
Increases pest resistance
Increases crop yield
Reports of increased winter hardiness
There is a wonderful photo comparing carrots grown in, and grown without rock dust here: http://prorev.com/dust.htm. Unfortunately I have not been able to locate the copyright holder to get permission to use the photo here.
What Kind and Size Rock Dust and How Much?
If the mesh that screens the rock dust is of the size “minus 200” (gravel industry standard nomenclature) it will work in your garden. Masonry sand and sand made for sand-blasting procedures are too coarse and will not be as effective in your garden, taking longer to break down. Rock dust from a gravel pit is usually unscreened but about the right size.
Locally, I have granite dust available from the gravel pits, which is okay to use but not best. The mixed gravel dust from the local stream beds is better. A better yet rock dust comes from glacial gravel or volcanic rock like basalt. Another is montmorillonite. Montmorillonite is a very soft phyllosilicate mineral that typically forms in microscopic crystals, forming a clay. It is the main constituent of the volcanic ash weathering product, bentonite. 
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 14 pounds of rock dust per 100 square feet of garden bed (or 5 pounds per square yard). If this sounds too good to be true, try an experiment at home with potted plants: take a 6” pot filled with half soil and half peat. Add 3 tablespoons of rock dust. Do another pot just the same but leave out the rock dust. Put identical plants in both pots, and watch for astonishing differences!
Rock Dust photo ©iStockPhoto.com/prill, used by permission
Gravel Pit photo ©iStockPhoto.com/pixelbank, used by permission
 Rocks for Crops, ©Peter van Straaten, used by permission http://www.uoguelph.ca/~geology/rocks_for_crops/
 Aston, Bernard Cracroft http://rsnz.natlib.govt.nz/volume/rsnz_80/rsnz_80_01_002320.html
 W.S. Fyfe, Professor Emeritus, Dept. of Earth Sciences, University of Western Ontario, London, Ontario, Canada
The Survival of Civilization, John Hamaker
Additional information here:
Bread from Stones by Julius Hensel http://www.amazon.com/Bread-Stones-Acres-U-S-classic/dp/0911311300/ref=pd_bbs_sr_2?ie=UTF8&s=books&qid=1201968923&sr=1-2