Garden Word of the Day
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Touted as a miracle soil amendment, what’s true about rock dust and what’s not?
Also known as mineral powder, rock flour, rock powder, rockdust, or stone powder, and soil remineralizer, rock dust is what’s leftover from mining and quarry work. This finely pulverized material claims to contain important plant nutrients, “enhance the ability of beneficial microbes to flourish”, improve plant structure, increases water retention and resistance to pests and disease, and create “intense flavor profiles for fruits and vegetables”.
Wow. That sounds pretty important and impressive, doesn’t it? While it’s certainly true that plants need nutrients to grow and thrive, let’s see what research says about those claims. We can start by learning what, exactly, is in that bag of rock dust.
What is rock dust?
The contents of your rock rust shipment will depend entirely on where it was mined. It may contain a lot of calcium. Or none. The same is true for other minerals. That being said, on average, rock dust contains significant amounts of aluminum, silicon, and sometimes iron. It may also contain copper, magnesium, manganese, potassium, sulfur, and zinc. But does your soil need these elements? Without a lab-based soil test, you really can’t be sure.
Your rock dust may also contain toxic levels of aluminum, arsenic, cadmium, chromium, copper, lead, nickel, and sodium. Believe me when I tell you that dealing with toxic levels of nutrients is a lot harder and takes a lot longer than adding missing nutrients. I’m speaking from experience. If you do nothing else for your garden this spring, get your soil tested before you add anything.
Big batch or specific sediments?
If your soil is low on a certain nutrient, rather than relying on luck, you can order specific types of rock dust. If you know your soil is low in something, you can apply crushed versions of that nutrient in the form of rock dust. Of course, it costs more that way. The nice thing is, it is an organic method of fertilizing your plants.
Making rock dust work for you
If you decide to apply rock dust, combining it with nutrient-rich organic matter, such as aged compost or manure, creates a slightly acidic environment more likely to break down the dust into bits small enough to be carried by irrigation water into your plants.
What rock dust is not
Because it does not contain significant levels of nitrogen, phosphorus, or potassium, rock dust is not a fertilizer.
Bottom line, in my opinion, rock dust is best left to commercial growers who regularly deplete their soils through heavy use and often have a fleet of chemists, soil experts, and lab technicians on hand to determine what’s needed and what’s in each particular load of rock dust.
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