Rock dust, rock fines and rock flour are powdered, grainy or sand-like materials originating from deposits caused by glacial activity, rivers, volcanoes and the sea. Basalt, granite, limestone and dolomite formations also offer up their own natural rock dust compositions.
Why might rock dust be good for the garden?
Despite its heavy weight, high shipping costs and limited evidence that it works in the backyard garden, many folks add it liberally to their soil annually or more often to replenish trace minerals.
Depending on the source, rock dust can contain varying amounts of calcium, iron, magnesium, potassium and trace minerals.
Where do I find rock dust?
Commercially available rock dust products contain different mixes. For example, Azomiteâ contains a combination of seawater minerals and other minerals found in volcanic ash. Greensandâ, a potassium-rich marine sediment, is valued for its ability to improve water retention. Gaia Greenâ offers calcium, iron, magnesium and potassium plus trace elements and micronutrients. (All of these brand names are registered trademarks – just in case the symbols don’t show up in this eBook.)
Finding My Own Rock Dust
Since I’m determined to find free sources of soil amendments, I’ll concentrate on showing you how I forage for small amounts of rock dust to amend my raised beds, containers and potting soil.
If you live near a quarry, uncontaminated river, lake or desert, inspect the sand and soil in that area and if allowed, take a small bag back home to test in your garden.
If you know a gravel pit operator and have permission to collect the leftovers from around the crusher and conveyers, you may have a fine source of rock (as long as it tests safe for agricultural use – ask for test results). Not all the tailings and piles of sand at a gravel site will benefit your garden.
When we go camping in the desert, especially in areas of prior volcanic activity, I might bag up a gallon of dust to bring back to my garden. As amateur rock hounds, our family likes to hunt for all kinds of interesting minerals and crystal specimens.
Which plants may benefit from rock dust?
Berry and potato farmers sometimes use rock dust to improve soils and supply plants with added potassium. I have also heard of great pepper harvests credited to applications of rock dust.
Combine rock dust with…
Rock dust is a good companion for other organic fertilizers, including compost. Add it to the compost pile or mixing it into the finished compost will fortify it additional nutrients and improved texture.
Rock Dust Recipes for Raised Beds and Containers
- LARGE GARDEN PLOTS: I have not used rock dust on a larger plot but have done research. Use 14 – 25 pounds of rock dust for every 100 square feet of garden area.
- RAISED BEDS: For a raised bed measuring, 8 feet x 4 feet, cover the bed with ¼ inch rock dust and work it into the top 4 inches of soil. Best time to do this is in the spring, before planting.
- CONTAINERS: Use ½ cup of rock dust for every gallon of soil. If I were growing in a 5 gallon container, 2.5 cups of rock dust would be included in the homemade mix or amended into the soil before planting.
Rock Dust Top Dressing Recipe
Sprinkle the dust around the plant at the drip line and water in. Use ¼ cup for every gallon of soil.
Recommend you treat only some of your plants to see just how well it works, especially if you are not sure of the mineral content. Success with rock dust will depend on lots of factors including watering consistency, temperature, plant type and existing nutrients in the soil.
Rock Dust Quick Tips
- Heavy metals and other contaminants could be present in the rock so be careful and ask first if you want to get free product from a local quarry or commercial stone company.
- Be careful not to add sand to heavy clay soil. It will not improve texture. Instead, it might make it more cement-like.