Answer: Rock powders (also called rock dusts) are just what they sound like -- very finely ground rock. There are different types for different uses, but in general they are used as a very slow release, long term source of nutrients, especially micronutrients.
The two most common rock powders are:
Greensand, used as a source of potassium.
Granite dust, used as a source of micronutrients.
(Technically speaking, lime, used for raising soil pH and consisting of ground up limestone, and gypsum, used as a source of calcium and sulfur, are rock powders. However, they aren't usually referred to by that name.)
Rock powders can be added at planting time to provide a long term source of micronutrients for trees and perennials. The finer the powder, the faster the nutrients will become available to the plants. In vegetable gardens, rock dust can be added any time, but it's easiest to apply it in the spring, along with compost and other organic matter, and till it in all at once.
Begin your search for a source at your local garden center. As you can imagine, rock powder is quite heavy and expensive to ship via mail order.
I hope this helps!
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