Copper Octanoate vs Copper Sulfate: 7 Differences

Both products work the same way: they release the cupric ion (Cu2+), which damages the proteins of fungi and bacteria on the leaf surface. The difference is delivery. Copper sulfate pentahydrate dissolves readily, with about 32 g in 100 ml of water at 20 °C. Copper octanoate, a copper salt of an eight-carbon fatty acid sold as “copper soap”, has low solubility but disperses in water (OMRI report for USDA, 2022). On the US label of one 2.4% octanoate product the restricted-entry interval is 4 hours; EPA’s 2009 review sets 48 hours for copper sulfate pentahydrate. Always follow the label of the product in your tank.

This page compares the two on the points that decide a purchase: form, copper content, re-entry time, plant injury, organic rules and water risk. It includes a re-entry chart, a copper-content chart, a US usage chart and a calculator that turns a label rate into pounds of copper per acre. It is written for advisers, consultants and growers who read labels for a living. It does not recommend a dose, because the only legal dose is the one on your product’s label.

Copper Octanoate vs Copper Sulfate: 7 Differences at a Glance

The table sets out what the cited documents say. “Fixed copper” means a copper compound with low solubility that stays on the leaf and releases copper slowly. Bordeaux mixture is copper sulfate neutralised with lime.

Sources for the table: the EPA-stamped Cueva 2.4 Copper Soap label (2017), the EPA coppers reregistration decision (2009) and the USDA technical report on copper products (2022).

What Copper Octanoate Is and How It Is Used

Copper octanoate is the copper(II) salt of octanoic acid, also called caprylic acid. It is made by reacting a soluble copper compound with a fatty acid source such as palm kernel oil, which is why products call it “copper soap” (USDA, 2022). In the US it is sold for gardens and for commercial crops under the same product name. The Cueva 2.4 Copper Soap label, for example, carries EPA registration number 67702-39, and the registrant is W. Neudorff GmbH KG.

The label mixes 2.6 to 10.7 fluid ounces of concentrate per gallon of water. The low rate is for prevention or light disease pressure; the high rate is for heavy pressure. It asks for thorough cover of both sides of the leaf, a first spray about two weeks before the disease normally appears, and at least 12 dry hours afterwards if possible. Re-spray gaps differ by crop: not within 3 days on tomato and pepper, 5 days on potato, 7 days on beans and carrots, 10 days on beet and turnip, and 30 days on kiwi.

Use is small compared with the older coppers. A high-end estimate of US agricultural use put copper octanoate at about 12,400 kg in 2017, against about 1,271,000 kg of copper sulfate (see the chart below). The same estimate says octanoate first passed 1 kg of reported use in 2005.

What Copper Sulfate and Bordeaux Mixture Are

Copper sulfate, also called bluestone or blue vitriol, was one of the first copper fungicides (Purdue Extension, BP-69). Alexis Millardet published work in 1885 showing that copper sulfate plus lime controlled downy mildew on grapes; that mixture is Bordeaux mixture. Lime acts as a “safener”: it neutralises the acidic copper sulfate and slows the release of copper ions, which reduces plant damage.

Purdue describes a 4-4-50 recipe (4 lb copper sulfate, 4 lb hydrated lime, 50 gallons of water) as one of the most popular and least phytotoxic for gardens and orchards, and notes that some labels call for double strength (8-8-50) for hard cases. Bordeaux mixture controls bacteria as well as fungi, which is why it still appears on fire blight and bacterial leaf spot programmes. Sensitive plants may need half strength, so the label rate wins over any recipe in a book.

University of California IPM gives a different recipe, 10-10-100 (10 lb copper sulfate, 10 lb hydrated lime, 100 gallons of water), and says it works for many pathogens. It also sets the season: Bordeaux sticks well in rainy weather, so it suits fall and winter sprays on dormant, deciduous plants, and applying it after trees break dormancy generally is not recommended because it can injure leaves. UC rates fixed coppers as the better choice in spring on new growth, but less persistent through winter rain (UC IPM Pest Notes: Bordeaux Mixture). Two universities, two recipes: that is another reason to treat the label and your local extension office as the authority.

By the mid-1930s many growers had moved to fixed coppers, which are easier to handle and cause less leaf damage. Those include copper hydroxide, copper oxychloride, cuprous oxide and tribasic (basic) copper sulfate. Copper octanoate is often grouped with them in organic standards, although it is a soap salt rather than a mineral compound.

Re-Entry Intervals: How Long Before Workers Can Go Back

EPA sets the restricted-entry interval (REI), the time after spraying when workers may not enter without protective equipment, from the acute toxicity of each copper compound. The 2009 coppers decision lists 48 hours for copper sulfate pentahydrate, copper hydroxide and copper oxychloride, and 12 hours for copper octanoate and cuprous oxide. It allows 24 hours for greenhouse uses of the 48-hour group. The 2017 label for one octanoate product states 4 hours.

Chart

Restricted-entry interval by copper product

Hours before workers may enter without protective equipment

The gap between 12 and 4 hours is a reminder that the REI printed on your jug overrides any table. A product registered later, or reformulated, can carry a different interval. Check also the early-entry protective clothing: the same octanoate label asks for a long-sleeved shirt, long pants, shoes, socks and chemical-resistant gloves.

How Much Copper Each Product Puts on the Crop

Pure copper sulfate pentahydrate is 25.4% copper by weight and the anhydrous form is 39.8%. Pure copper octanoate is 18.2%. Fixed coppers vary widely: tribasic copper sulfate is 56.2%, copper hydroxide 65.1% and cuprous oxide 88.8% (USDA, 2022, Appendix A). Those are properties of the pure compound, not of what is in the jug, because commercial liquids are mostly water.

Chart

Copper share of the pure compound

Percent copper by weight; commercial liquids are mostly water

That is why a label comparison must be done in metallic copper, not in product weight. The 2.4% octanoate product is only 0.4% metallic copper by weight, about 0.035 lb per gallon (the label also prints 0.34 lb per gallon, which does not match its own 0.4% figure, so this page uses the percentage). A mix at the low label rate of 2.6 fl oz per gallon of spray puts about 0.32 g of copper into each gallon, or roughly 85 mg per litre. The high rate of 10.7 fl oz gives about 1.3 g per gallon, or roughly 350 mg per litre. A Bordeaux mix at Purdue’s 4-4-50 recipe contains about 1 lb of copper (4 lb x 25.4%) in 50 gallons, which is roughly 2,400 mg per litre. These are arithmetic from the cited figures, not dose advice.

Plant Injury: Which One Burns Leaves?

Copper in excess damages the crop. Effects listed in the USDA report include shorter root growth, odd root branching, leaf yellowing (chlorosis) and slower growth. Highly soluble copper gives the most dissolved copper at once, so injury is a larger concern with plain copper sulfate; that is the reason for the lime in Bordeaux mixture. Fixed coppers release copper slowly and, in the report’s words, supply enough copper to kill fungi and bacteria without injuring the host plant.

Octanoate is not injury-proof. The commercial label says the product may cause some copper toxicity on some plant species. Specific warnings include roses sprayed in damp, cool weather below 60 °F, copper-sensitive lettuce, and certain Vinifera and French hybrid grape varieties that can show marginal leaf burn. For grapes the label also says not to mix with lime, which is the opposite of the Bordeaux recipe.

Weather matters for sulfate-based sprays. Purdue warns that Bordeaux applied above 85 °F can turn leaves yellow and make them drop, and that leaves can burn if rain follows soon after spraying; adding a spray oil can reduce the burn. When in doubt, test a few plants first.

Organic Rules and Copper Build-Up in Soil

Copper does not break down. It binds to soil and stays near the surface, so every organic standard puts a condition on its use. In the US National List (7 CFR 205.601), fixed coppers and copper sulfate may be used for plant disease control only in a way that minimises copper accumulation in the soil, and fixed coppers may not be used as herbicides. Every regime the USDA report surveyed allows copper sulfate, and most allow copper oxide, hydroxide, oxychloride and octanoate. All require growers to try preventive and cultural methods first.

Chart

High-end US agricultural copper use by compound

Tonnes per year of copper-based pesticides (high-end estimate), continental US, all crops

Numeric caps exist outside the US. The report says EU rules in force when it was written allowed up to 6 kg of copper per hectare per year, with member states permitted to average over five years, and that the IFOAM standard sets 6 kg per hectare per year on a rolling average. Rules change, so check the current text for your certifier. To see how a label rate compares with that cap, use the calculator on this page.

Background soil copper is normally 2 to 50 ppm depending on geology, and the USDA’s older sulfate report estimated that each 10 lb per acre of copper sulfate pentahydrate adds about 6 ppm to the top 2 inches of soil. That soil-application figure is not a spray figure, but it shows how quickly repeated copper adds up. The report also gives 5 to 20 ppm as the normal range in plant tissue and above 30 ppm as potentially toxic (USDA, 2001).

The US usage record below shows the direction of travel. A high-end estimate of copper-based pesticide use in US agriculture fell from about 6,430 tonnes in 1992 to about 5,160 tonnes in 2017, a fall of roughly 20%. Copper sulfate use dropped by more than half, while tribasic copper sulfate and cuprous oxide rose. These are estimates for all crops, not a measure of any one farm.

▶ Farmonaut video on soil organic matter, which binds copper in soil · 7:33

Water, Fish and Runoff

Neither product is a water-safe material. The octanoate label says it is toxic to fish and aquatic invertebrates, may reach water in runoff, and can leave runoff potential for several months, especially on poorly draining soils or shallow water tables. It tells the user not to spray near surface water or let rinse water reach drains. EPA’s 2009 review of the whole copper group found acute and chronic risk to aquatic life, noting that copper binds to fish gills.

In practice, buffer strips, calm weather, no spraying before rain and careful disposal of rinse water matter with either product. USDA’s 2001 report adds that sulfate toxicity to fish is lower in hard water, but it still ranks the material as highly toxic to fish. If your fields drain to a creek, read the aquatic hazard section of the label before you read the efficacy section.

Which One Fits Which Job: Best for, Pros and Cons

Copper octanoate

Best for: small growers, nurseries, greenhouses and gardens that want a ready-to-mix liquid with a short re-entry time and a long list of label crops.

• Pros: short REI on current labels; wide tank-mix list on the label; listed for organic gardening; can be applied up to the day of harvest on the label studied.
• Cons: a low copper content (0.4% metallic copper by weight in the 2.4% product), so compare products per pound of copper; damp-cool injury on roses and leaf burn risk on some grape varieties; some disease claims carry a “Not for use in California” mark, so check state registration.

Copper sulfate and Bordeaux mixture

Best for: orchards and vineyards with a long programme and experienced mixers, where a lime-neutralised copper has decades of local use.

• Pros: long record (since 1885); kills bacteria as well as fungi; highly effective and long lasting in UC IPM’s comparison; sticks through rain once mixed with lime, which suits dormant-season sprays.
• Cons: longer REI (48 hours in EPA’s review); high pH, a salty deposit, blue-green stains on surfaces and a corrosive mix (UC IPM); not compatible with most pesticides; leaf burn in heat or after rain; mixing errors; long-term exposure in vineyard workers, where the USDA report describes “vineyard sprayer’s lung” in long-term Bordeaux workers, although EPA (2009) called the copper link “not definitive”.

Fixed coppers (hydroxide, oxychloride, cuprous oxide, tribasic sulfate)

Best for: programmes that need a copper film that lasts through rain and does not burn as easily. Their re-entry intervals are 48 hours for hydroxide and oxychloride, and 12 hours for cuprous oxide, in EPA’s 2009 table.

What the Cornell Tomato Trial Can and Cannot Tell You

Cornell’s Margaret McGrath compared four organic copper fungicides on tomato in New York in 2014 (Plant Disease Management Reports, vol. 9). The products were Cueva (copper octanoate), Badge X2 (copper hydroxide plus copper oxychloride), Basic Copper 53 (basic copper sulfate) and Nordox 75 (cuprous oxide). The diseases were powdery mildew, late blight and bacterial speck. Plain copper sulfate pentahydrate was not in that trial.

The entry lists the products and diseases but this page does not quote efficacy numbers. What the entry does show is how a fair comparison is run: one crop, one site, one season, named products and named diseases. Treat any single trial as a hint. A result on tomato in New York says little about grapes in a wet valley, and a different season could reorder the products.

Do Resistance and Weather Change the Choice?

Copper acts on many targets in the cell, and EPA’s 2009 review, as quoted in a USDA technical report, found little evidence of significant resistance problems after centuries of use. The same report notes that coppers are alternated in some programmes with systemic fungicides that carry a high resistance risk. That is a point for rotation, not a licence for heavy copper use.

Copper is a contact material that does not enter plant tissue. It protects only the tissue it covers, so timing matters: early in infection, or ahead of rain when plants are vulnerable, works best. New growth after a spray is not covered. Label re-spray gaps for the octanoate product studied run from 3 to 30 days depending on the crop.

▶ Farmonaut video on spotting crop stress from satellite maps before it is visible on the ground · 4:09

How to Read the Label: 6 Checks Before You Buy

1. Metallic copper per gallon or per pound. Compare products on pounds of copper, not on price per jug.
2. Crop and disease on the label. If your crop or disease is not listed, the product is not legal for that use.
3. REI and protective clothing. Confirm both on the jug, not on a web table.
4. Minimum days between sprays and the seasonal maximum. Check the annual cap for your crop.
5. Water and bee warnings. Look for buffer and runoff statements.
6. State registration. A product legal in one state may be restricted in another.

When the Copper Comes from a Mine, Not a Sprayer

Copper in farm products starts as mined metal. The USGS puts US copper mine output at about 1.0 million tonnes in 2025 (estimate), with Arizona producing about 70% (USGS Mineral Commodity Summaries 2026). For consultants who advise on land near mines, soil copper from tailings or dust is a separate question from spray copper. If you need background on where the metal is found, our guides to copper deposit types and countries and porphyry copper deposits cover the geology, and this page on copper mining and water covers the water side.

If your site is a mining property and you want to map its surface from satellite imagery, Farmonaut offers satellite-based mineral detection. You can send the boundary through the mining query form or reach the team on the contact page. That work is separate from fungicide choice, and satellite images cannot tell you which copper product to spray.

If this topic matters to you, read Copper Sulfate for Roots: Pipe and Soil Doses next.

Frequently asked questions

Is copper octanoate the same as copper sulfate?

No. Copper octanoate is a copper salt of a fatty acid (octanoic acid) that disperses in water. Copper sulfate is an inorganic salt that dissolves readily. Both release the cupric ion, which is the active part.

Which has less copper: octanoate or sulfate?

Pure octanoate is 18.2% copper and pure copper sulfate pentahydrate is 25.4%. In a bottle, the figure depends on the product; one 2.4% octanoate product is 0.4% metallic copper by weight, about 0.035 lb per gallon.

Is copper octanoate organic?

It appears among the copper substances allowed by the organic regimes surveyed by USDA, including NOP, Canada’s CAN/CGSB list, the EU rules cited and IFOAM, but each requires you to limit soil copper. Check your certifier and the product’s OMRI listing.

Is copper sulfate allowed in organic farming?

Yes, in all the regimes the USDA report surveyed. In the US, 7 CFR 205.601 permits it for plant disease control if the grower minimises copper build-up in the soil.

How long should I stay out of a field after spraying?

Follow the REI on the product label. EPA’s 2009 review sets 12 hours for octanoate and 48 for copper sulfate pentahydrate; one 2017 octanoate label states 4 hours.

Can I mix copper octanoate with lime?

The label studied says not to mix it with lime on grapes. Lime belongs in Bordeaux mixture, where it neutralises copper sulfate. Do not combine the two ideas.

Which one is safer for plants?

Neither is harmless. Plain copper sulfate carries the higher risk of leaf burn, which is why it is used with lime. Octanoate labels still warn of injury on roses in cool damp weather and on some grape varieties.

Does copper cause resistance?

EPA’s 2009 review, quoted by USDA, found little evidence of significant resistance problems after long use. Rotating with other modes of action is still standard practice.

How do I work out how much copper I am applying?

Multiply the product volume per acre by the metallic copper per gallon on the label. Convert pounds per acre to kilograms per hectare by multiplying by 1.121. The calculator above does this for octanoate and for copper sulfate pentahydrate.

Is copper harmful to fish?

Yes. The octanoate label calls the product toxic to fish and aquatic invertebrates, and EPA’s review of coppers found acute and chronic risk to aquatic life. Keep sprays away from water.