Is Copper Sulfate in Miracle-Gro Fertilizer Toxic?

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Robert Pavlis

Some types of Miracle-Gro fertilizer are a blue color due to the addition of copper sulfate. But copper sulfate is toxic to microbes and fungi. Several online sites and videos are now claiming that it makes the fertilizer toxic and should never be used because it kills soil microbes. Are they right?

box of Miracle-gro fertilizer with crystals of copper sulfate.
Copper sulfate crystals are blue giving the fertilizer its distinctive color, source: Depositphotos

Does Miracle-Gro Fertilizer Contain Copper?

The MSDS for Miracle-Gro All purpose fertilizer clearly states that the product includes copper sulfate between 0.1 and 1%. Copper sulfate is blue as is the fertilizer so there is no doubt it contains copper.

Is Copper Toxic?

Everything is toxic, even the water you drink. A lot of people ask this question but it does not make a lot of sense. Toxicity is based on dose. Given a high enough dose, everything is toxic. The right question to ask is, does fertilizer contain toxic amounts of copper?

Copper sulfate is a common fungicide and algicide. Copper in our water pipes helps kill bacteria. In a high enough dose it is toxic.

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Keep in mind that this pesticide is allowed in certified organic farming, so how toxic can it be? You will be surprised to learn that it is toxic enough to cause several countries to ban its use, even though it is acceptable for organic farming.

It is also important to realize that plants and animals need some copper to grow. It is a necessary micronutrient. In plants it is used in photosynthesis and enzyme function.

How Much Copper Does Miracle-Gro Add to Soil?

The important question really is about dose. Does the fertilizer add enough copper to our soil to be a concern?

I calculated the amount of copper that would be added to soil if you follow directions and fertilize 6 times in a season. You would increase the level of copper by 1.4 ppm, assuming it is all in the top 6″ of soil.

Now we have a number. To understand the number we can compare it to copper levels in soil and food.

Natural Copper Levels in Soil

The natural copper level in soil is between 2 and 100 ppm with an average of 30 ppm. Sufficient levels for plant growth are in the range of 5 to 30 ppm, with values over 50 ppm being excessive.

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Adding 1.4 ppm to this won’t really make much of a difference.

Remember the claim. The fertilizer has so much copper that it kills all the soil microbes. If that were true, soil would not have any microbes. The reality is that the natural levels in soil are not high enough to kill all the microbes. So clearly, adding a bit more by fertilizing will also not kill them.

Natural Copper Levels in Food

All living things, including plants, microbes and animals, need copper to live. It is found in everything you eat and most of that copper is from natural sources like soil.

Carrots contain 0.5 ppm copper. In fact three medium-sized carrots contain the same amount of copper as in the fertilizer spread over 10 sq ft. Since we consider carrots to be healthy and non-toxic, it clearly shows that the amount in fertilizer is not a concern.

What happens if you take 3 medium-sized rotting carrots, compost them and then add them to the garden? You have just added the same amount of copper to the garden as with fertilizer. Has anyone ever warned you about adding carrots to the garden because they contain toxic levels of copper? Of course not. They aren’t toxic and neither is the fertilizer.

You might think that carrots have a special high level of copper, but lettuce has almost the exact same amount. All of our food has similar levels.

Copper Sulfate is Used in Organic Farming

Organic farming is allowed to use synthetic chemicals when a comparable natural source is not available. Copper sulfate is a synthetic that is allowed in certified organic farming where it is used mostly as a fungicide, but it can also harm bacteria.

A common organic fungicide is the Bordeaux mix, which contains copper sulfate and lime. The concentration of copper in this mix is 4,000 ppm. Compare that to the fertilizer mixture which has a concentration of 400 ppm. The allowed organic pesticide is 10 times higher than Miracle-Gro fertilizer and yet, the latter is not allowed in organic farming and is considered to be toxic by some organic gardeners. Where is the logic in this?

Is Copper Sulfate Safe?

The answer depends on how it is used and the dose.

Copper is a heavy metal and can accumulate in soil. However, it is also an essential nutrient for plant growth and soils which are deficient need to have more added in order to grow plants.

two different leaves. left one showing spots the right one showing puckering
Signs of copper deficiency on plants, source, Lucidcentral

If too much copper is added to soil it will leach into rivers where it can cause environmental problems.

Too many people think that since copper sulfate is allowed in organic farming it is perfectly safe and natural. That simply is not true. Copper sulfate is not natural and every chemical, either natural or synthetic can be harmful at high doses.

Try not to use copper sulfate as a pesticide. There are much safer synthetic products on the market and they work a lot better. Copper is a “forever chemical”, while most synthetic options are not.

What About The Salts in Miracle-Gro?

Many people are concerned about the “salts” in fertilizer. These are not the same as table salt, but based on chemical definitions they are salts, as is copper sulfate. All of the nutrients plants need are salts, and they don’t harm the plant or microbes, provided they are used in reasonable amounts. In fact, without salts every living thing on earth would die.

Should You Use Miracle-Gro Fertilizer?

If you should stay away from copper sulfate, is it a good idea to use Miracle-Gro fertilizer?

It depends.

If your soil has a copper deficiency, then it is a good way to add copper to soil. Remember, when you fertilize you do not feed plants, you replace the missing nutrients in soil. If copper is missing, then add some.

In general, most garden soil is not deficient in copper so a fertilizer containing copper is not the best choice. A garden fertilizer does not need to contain the micronutrients.

What about potted houseplants and containers? These usually do not contain soil and therefore they contain no natural copper. You have to add the copper as part of the fertilizer and that is where Miracle-Gro and similar products come in. They include the micronutrients you need to add when you are fertilizing soilless mixes such as the ones you find in potted houseplants and containers.

Fertilizer containing copper is also essential for hydroponics, although Miracle-Gro is not the best option because it is based on urea. For more on this see, What is Hydroponic Fertilizer?

Understand the products and the chemicals in them. Then use them appropriately. Almost nothing is toxic when used in the right way and in the right amounts.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

5 thoughts on “Is Copper Sulfate in Miracle-Gro Fertilizer Toxic?”

  1. thanks for debunking another bit of silliness that was being spread across the internet. a little knowledge can be dangerous.

    Reply
  2. I am your devoted follower. I read your posts. I first found you on growing orchids on YouTube. You have so much knowledge and common sense in gardening & plants. I am so thankful for your honest opinions, very down-to-earth & simple & straight forward. You are wonderful! Thank you on your eBook on growing tomatoes. I am sure I will grow great tomatoes by following you valuable instructions.

    Reply
  3. The late Dr. Robin D Graham, a scientist I used to know, was studying the effects of Copper deficiency on plants.
    As part of his studies, he wanted to grow Copper deficient plants hydroponically.
    He found it was extremely difficult to do, and almost all water contains enough Copper for plants to grow hydroponically without any obvious Copper deficiency. Even double distilled water had enough Copper for plant growth. Eventually, he succeed in getting pure enough water, using a system of multiple purifications, including distillation, deionisation and reverse osmosis.

    Reply
      • metal content in distilled water is greatly influenced by the condenser tubing. if it was copper or an alloy that contains copper, the universal solvent (water) will pick up some.

        Reply

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