Closer look at popular organic fungicide copper sulfate–how does it compare to glyphosate?


Organic farming uses pesticides just as conventional farming does. The only difference is that (with a few exceptions) the pesticides used in organic agriculture have to be derived from “natural” sources. One such example is copper sulfate, which is used in temperate climes as a fungicide. Many anti-GMO activists have pointed to pesticides used in conventional agriculture, such as glyphosate, and 2,4-D, as dangerous to people, animals, and the environment. For example, anti-GMO organization, The Environmental Working Group, says this about 2,4-D:

Researchers have linked 2,4-D to hypothyroidism, suppressed immune function, Parkinson’s disease, cancer and other serious disorders. Farmworkers could inhale 2,4-D and get it on their skin while spraying it. The chemical could drift from sprayed fields into nearby neighborhoods. People would track it into their homes. The damage can reach beyond the farmers who live close to sprayed areas.

The International Agency for Research on Cancer (IARC), the cancer research arm of WHO, declared the popular synthetic pesticide glyphosate “probably carcinogenic to humans”. A ruling that is often used by organic activists to argue that glyphosate (and glyphosate resistant GE crops) should be eliminated from use.

Many activist organizations try to tie organic farming to pesticide-free farming and conventional (both GMO and non-GMO) to heavy pesticide use. The result is that many consumers have false perceptions about organic and conventionally grown foods. The Soils Association in the UK published poll results that showed 95 percent of consumers said they buy organic food because they believe they are avoiding all pesticides.

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While no pesticide is entirely safe, organic products are not necessarily less hazardous than their conventional counterparts. Nor are they pesticide free. Copper sulfate is one of many pesticides approved under the USDA National Organic Program and a number of studies show that it has a great deal of toxicity to both humans and the environment. For example it is toxic to honeybees when used as a fungicide and a study showed extreme toxicity to bees in tropical environments(it was carried out in Brazil), where copper sulfate is used as a sprayed fertilizer (to provide heavy metal nutrients). In addition, some wineries in France, the US and elsewhere have backed away from growing organic wine because of accumulation of copper in the soil.

What does copper sulfate do?


copper sulfate

Copper sulfate is one copper atom bound to one sulfur and four oxygen atoms (the sulfur and four oxygens make the “sulfate” part). It has been registered for use in the US since 1956. Copper sulfate works because the copper atom binds to proteins, altering the structure of those proteins. This can disrupt the membranes around cells, causing the cells to die. Copper sulfate is effective at killing fungi, algae, and even snails in this way.

In conventional agriculture as well as organic, it can be used as a fungicide. In organic applications, it’s used a lot. According to the National Center for Food and Agricultural Policy, copper, one of the top two organic fungicides (sulfur’s the other one), was used at a rate of 4 pounds per acre in 1971. In contrast, synthetic fungicides used on conventionally grown crops only required a rate of 1.6 lbs per acre.

It’s also used in non-agricultural applications, such as killing algae in swimming pools and even keeping algal populations down in natural lakes. And, like many other pesticides, some fungi and algae have developed a resistance to it, a characteristic that’s been known for nearly 60 years.

Spraying copper sulfate.

Copper sulfate also presents health hazards to humans and larger animals since its actions are not specific to just fungi. It has been associated with skin and eye irritation, and swallowing large volumes of it can cause nausea, vomiting and tissue damage. It has not been associated with cancer, but its long-term exposure effects are unknown.

Its manufacture is also done in a factory. There are several US patents that apply to making copper sulfate, which is usually done by combining other forms of copper (in this case, copper carbonate) with sulfuric acid and water in a commercial reactor, which is then ultimately centrifuged to create the final product. The largest manufacturer of copper sulfate is a company named Old Bridge Chemicals, based in New Jersey.

While much attention has been focused on glyphosate, 2,4-D and other conventional chemicals (which also are used on genetically modified crops), it’s important to understand what’s being used on organic fields, as well. Many of the criticism of synthetic pesticides can also be applied copper sulfate, but this doesn’t mean we should drop pesticides—organic and synthetic—altogether. The ways in which farmers use these beneficial crop protection agents are both safe for farmers and consumers, and they also provide major economic benefits to both groups too.

Andrew Porterfield is a writer and editor, and has worked with numerous academic institutions, companies and non-profits in the life sciences. BIO. Follow him on Twitter @AMPorterfield.


  • John Lejderman

    I was just researching the relative toxicity of glyphosate vs copper sulfate.
    Copper sulfate:

    Looking at the LD50 (lethal dose for 50% of the population) you find that copper sulfate is 187 times more acutely toxic than glyphosate in rats, 30mg/kg vs 5600mg/kg. I wonder if I have interpreted the data correctly.

    • ferkan

      Just came across your comment. The LD50 on it’s own is not interesting. What’s interesting is the LD50 as a multiple of the therapeutic dose (in pharma terms). The closer the LD50 to the dose that one needs to give to get a successful result, the riskier the product. Incidentally, LD50 is normally calculated from animal studies, which means that it may be somewhat out for humans. LD50 of chocolate is quite high for humans, but not for dogs!

  • mem_somerville

    Oh, I didn’t realize copper fungicides were banned in some countries. So if banned-in-Europe is our standard, organic growers better watch out.

    • Andrew Porterfield

      Copper fungicides are heavily restricted in Brazil, too. The research I talked to told me he looked at it’s use as a fertilizer because it’s less regulated (and–surprise–more common!).

  • I dont think its a fair comparison to compare on of the most benign conventional or synthetic HERBICIDEs with one of the nastiest FUNGICIDEs out there. Apples to apples.

  • Rob Wallbridge

    As previously mentioned, it’s rather disingenuous to compare copper sulfate, a fungicide registered for use on comparatively few crops (both organic and conventional) with widely-used, broad spectrum herbicides like glyphosate and 2,4-D. Comparing rates of application is also meaningless unless you consider the relative toxicity of each substance. And comparing acute toxicity doesn’t tell you much about risk unless you also consider the likelihood and rate of exposure.
    Copper sulfate is nasty stuff, no doubt. But it’s non-systemic, which means it is not absorbed by the plant – it washes off, which is good for risk of consumer exposure (organic standards prohibit visible residue on crops), and not so good for rate of application. Which is why organic certification standards require monitoring and restrict producers from using copper sulfate if copper is accumulating in the soil at excessive levels.
    With respect to human health, apart from farm workers, people are highly unlikely to be exposed to copper sulfate because it breaks down so easily. The EPA has concluded that people are more at risk of copper deficiency in their diets than from any negative effects associated with excess intake (
    This context and information would probably have been more useful to include in the article than the manufacturing details.

    • Good4U

      Your comments are well taken; that’s why we should defer to the regulatory agencies who are trained and experienced in risk assessment to set safe levels for putative exposures to the fungicide and herbicides that you mentioned. The U.S. EPA and their counterparts in other countries have the professional expertise to assess all the data, not only the intrinsic toxicity hazard but the magnitude of exposure data. The authentic regulatory agencies are very much unlike the IARC, which doesn’t even attempt to take into consideration exposure data, therefore is not a regulatory agency. The anti-pesticide railers & ranters are generally not at all aware of what factors constitute the risk assessment process. Fairness is not part of the anti-pesticide agenda.

    • Alokin

      Reasonable points, but unfortunately, those same points regarding “actual” exposure are ignored by most anti-glyphosate advocates who conflate risk with hazard.

    • Andrew Porterfield

      There might be a story just lining up everything everybody uses (organic and conventional), and comparing hazards and exposures. But Rob, I have another story idea to run by you; can I email or message you?

    • Any study done to back up the claim that copper is not systematically picked up crops? It’s well known copper stays in soil though.