New to Rockdust? Here’s a brief primer.

Rock Dust & Remineralization

If you’re coming into organic growing and perhaps done a bit of light reading, you’ve likely noticed the term remineralization in many of your sources. If you haven’t, remineralization is the utilization of rock dust in your soil to provide a slow release of minerals and trace elements. Using rock dust in your garden mimics the earths natural cycle of fertility, as soil and rock wear down and are leached into the water it accumulates in glaciers; during ice age these minerals are returned to earth. Over time volcanic activity pulls minerals and metals from their slumber deep in the earth, and volcanic ash helps spread them to the wind. But not all rock dust are equal, or optimal for any grow condition. Let’s take a look at their differences, and find which will work best for your garden. The major types of rock dust, or remineralization products, marketed to growers today are: Azomite, Glacial Rock Dust, Basalt, Gypsum, and Sea-90. That’s quite a selection, and some have uses that others do not, so rightfully some people may be confused about which to use, or which ones are okay to be used together.

Azomite®: The name itself stands for “A to Z of Minerals Including Trace Elements”. Azomite® is a combination of volcanic dust that filled a nearby seabed, creating hydrated sodium calcium aluminosilicate with the presence of other trace minerals. Azomite® contains a rather high amount of metal ions. In particular aluminum, some of which has already bound with silica to form aluminosilicate which will not be broken down by bacteria found in most soil; further any extra aluminum binds with silica you water-in, forming more non-soluble aluminosilicate. Containing the highest amount of lead and arsenic of any of the rock dust, which is a serious consideration for the average gardener not only in handling and safety, but when growing crops that could accumulate those compounds. It does however, contain the greatest amounts of rare elements and minerals, which is particularly useful when growing food crops. All in all, Azomite® is a suitable tool for mineralization if it’s what’s on hand locally, or if growing food, but perhaps not if growing cannabis.

Glacial Rock Dust: GRD (Glacial Rock Dust) as the name implies, is rock dust that is the result of weathering of rocks that has accrued in glacial formation; then as the glaciers recede they leave behind a glacial moraine. These glacial moraines are then mined, and the product is sold as GRD. GRD does not have quite as many rare elements as Azomite®, but it contains much lower aluminum, lead, and arsenic levels. It’s worth considering that Cannabis accumulates metals found in the soil, so if growing for smoke, GRD will contribute less impurities to the end product. The magnesium levels found in GRD is perfect for recycling soil, and contributing a slow but balanced level of magnesium.

Basalt: Basalt Rock Dust is made from igneous rock which means it hasn’t been processed or had any of nutrients leached, as consequence it has most of the rare elements found in Azomite®, while having incredibly low levels of the elements that are more toxic (lead, arsenic, etc.). Basalt has quite a high amount of silica, and weathers much quicker than any other rock dust, so it becomes available quicker. It’s also a good slow source of Phosphorus & Potassium, and will balance the pH of the soil. It provide some of the lowest sodium levels of any rock dust, and is found in great amounts, so generally speaking one of the cheapest of the rock dust. Glacial Rock Dust and Basalt together provide all the elements needed for mineralization of a medical garden, in my anecdotal experience.

Gypsum: Gypsum is Calcium Sulfate, so unlike the rest of the rock dust, it is not a composite of many different minerals and trace elements. However, it’s such a powerhouse it’s the one rock dust you absolutely shouldn’t omit. It helps with aeration in compacted soil, desalting the soil, provides an excellent source of calcium as well as sulfur, while balancing the soil pH. more. When trying to keep proper magnesium levels in your soil for recycling I’d recommend using gypsum for liming agent rather than dolomite, as not only will gypsum not add unneeded magnesium to the soil, it will provide more immediately available calcium than most other organic sources, such as oyster shell flour. Combining oyster shell flour and gypsum in your compost, top dressing, and soil will provide more than enough calcium to never use a product like cal-mag again. Topdressing gypsum as you begin bringing out the fade in your plant will provide a source of sulfur, increasing the terpene content before harvest.

Sea-90: Sea-90 is produced by solar dehydrating seawater trapped in retention ponds located in a secluded coastal area; in other words unrefined sea salt. Sea-90 is not a rock dust but since it’s used for remineralization I wanted to include it. Now, since Sea-90 is produced from seawater it contains a staggering amount of sodium and chloride, roughly 77% of each sea-90 crystal in fact. This massive amount of sodium and chlorine will definitely have an effect on microbiotic life, some of which can not handle the osmotic stress and toxicity. Sea-90 is water soluble though, and with the remaining 23% of each sea-90 crystal being minerals, metals, and elements it has the advantage of not having to wait for a rock dust to weather, but with the relatively toxic nature of chlorine and sodium, water solubility may not warrant it’s usage.

Now that you’re familiar with the differences between each product, you can start making your mineral mix confidently. For those of you curious about what I use personally, my mineral mix is:

2 part Oyster Shell Flour

2 part Gypsum

1 part Glacial Rock Dust

1 part Basalt

1 part Calcium Bentonite – I didn’t include more information about this, as it’s a clay and not used strictly for remineralization but instead for increasing the cation-ion exchange capacity of the soil.

This mineral mix is used at a rate of 3 cups per cuFt of soil