Micronutrients in the Garden

Soil in Wheelbarrow SG

Anyone who has worked in a garden for a season or two is familiar with the major soil nutrients – nitrogen, phosphorus, and potassium – or NPK. Fertilizer bags and bottles have the content of these nutrients written in numbers, such as 10-10-10 or 5-2-6. Many of us are also familiar with the secondary nutrients necessary for plant growth – calcium, magnesium, and sulfur. (See my post on Epsom Salt in the Garden for supplementing magnesium and sulfur organically.) Yet, few gardeners are aware that plants must also have eight micronutrients to grow and produce well, and most soil in the U.S. is deficient in many micronutrients.

If we were to apply the “weakest link” theory to gardening, your soil is only as fertile as its most deficient nutrient. So, we need to understand all the nutrients that are needed for optimum growth and production, and that includes micronutrients.


The eight micronutrients necessary for plant health are boron, chlorine, copper, iron, manganese, molybdenum, and zinc. These micronutrients assist the macronutrients, both primary and secondary, by being “activators” of many plant functions. They also increase disease and pest resistance, assist in growth and maturity, increase fruit yields, and add more flavor and nutrition to our food.


Modern farming techniques have been a major contributor to the depletion of micronutrients in our soil. Continual farming (or gardening) without supplementing the soil will lead to the depletion of most nutrients, both macro and micro. Farmers and gardeners supplement the soil with NPK on a regular basis, and even supplement the secondary nutrients on occasion. But micronutrient levels in the soil are often unaddressed. Also, growing more plants per acre to increase yields depletes the soil of micronutrients at an even greater rate.

Another reason plants are not getting enough micronutrients is that the soil is lacking sufficient organic matter. Many micronutrients are put into the soil by the decomposition of organic matter, especially grass clippings and leaves. (Compost those leaves and use grass clippings as garden mulch!)

Finally, the pH of the soil affects the plant’s ability to absorb the nutrients. The more alkaline (above 7.0) the soil, the more difficult it is for the nutrients to be bio-available for the plant.


The best way to know for sure the amount of micronutrients in your soil is to get it tested. Do-it-yourself kits are decent for testing the major nutrients (NPK) and the pH (also necessary to know), but they won’t test for micronutrients. For that you’ll have to have your soil professionally tested.

Last February was my last professional soil test, which was done by Clemson University. Here’s a look at the test results. The test includes a couple of micronutrients but not all of them. (I didn’t specify what I wanted tested, so this is the basic soil test.)

My Feb 2013 soil test results

My Feb 2013 soil test results


Unless your soil test shows a major depletion of a micronutrient (with instructions on how to supplement), it is best NOT to add one or two micronutrients to your soil. You could do more harm than good. Your local agricultural college/cooperative extension can help you with any major issues you have with your soil.

That said, what’s a common gardener to do? Year after year, we plant our seeds and grow our crops. Rotating them is great, but every plant uses micronutrients. Even if our soil had sufficient nutrients when we started, eventually the soil will become depleted.

  • Use Organic Material in the Garden: Augment your soil with good composted material every year. Use grass clippings for mulch. Fertilize with a quality organic fertilizer, which will have some secondary nutrients and micronutrients in addition to NPK.
  • Supplement with Rock Dust:
    I have just learned that rock dust contains many of the micronutrients that are necessary for plant growth, as well as trace elements that are needed for humans. (These trace elements aren’t necessary for plant growth but are absorbed into the leaves and fruits of plants that we eat and are necessary for our growth: iodine, fluorine, selenium, cobalt, arsenic, lithium, chromium, silicon, tin, and vanadium.)
Azomite - volcanic rock dust

Azomite – volcanic rock dust


When I first heard about rock dust, I bopped all over the internet to find out more about it. I found there are two types: glacial rock dust and volcanic rock dust. I chose the volcanic rock dust, which is called Azomite, because the company website was very informative and included a Certificate of Analysis.

The Azomite site also includes the various research studies which have been done, showing that application of Azomite rock dust produced healthier, larger plants with markedly increased yields.

The application rate varies, depending on whose site you look at. Generally, I’ve seen the application rate of 1-2 pounds of Azomite per 10 square feet. However, unlike over applying NPK, which can “burn” your plants, Azomite won’t harm your garden if you apply too much. (John Kohler of “Growing Your Greens” stated in a video that he’s used quite a bit more than that with no negative affects.)

Well, I got very excited about this product and wanted to buy it in time to apply it before my winter planting (which is today, as a matter of fact). However, I couldn’t find any retail outlet within 50 miles of my home that carried it. So, I had to order it online. And yes, the shipping was expensive. What I did was search for a source online that was closest to me geographically and had the best price. I bought three 44-pound bags of micronized (dust rather than granulated/pellets) from Countryside Organics in Virginia. That should be enough for me to apply 2 pounds per 10 square feet – about 6-7 pounds per raised bed.

I plan to keep track of my plants’ growth after applying Azomite and compare it to growth in 2013. I have hundreds of pictures of my plants as well as a detailed garden log that I can use to help me with my comparison. Although this won’t be a true scientific study, I hope you’ll join me on the journey to see the difference supplementing my garden with Azomite’s micronutrients will make in the growth, health, and yield of my garden.

Have you tried rock dust in your garden? What differences did you see?


2 comments on “Micronutrients in the Garden

  1. I also run to the asian markets and grab big bags of mushrooms and grind them in the food processor. Then I add those in for nutrients when I compost.

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