Copper Requirements for Organic Growing

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In winegrapes, keeping copper levels above 2 ppm will prevent losses from skins splitting at the stem of each grape (all photos courtesy N. Kinsey.)

Every organically farmed soil requires adequate copper to produce the most nutritious food from organically grown plants. Consequently, each soil sample received for analysis and recommendations to be used for organic production should be tested for copper availability. Considering soils analyzed from thousands of growers in over 75 countries and all continents except Antarctica, the great majority of them are deficient in copper. In fact, on almost half of the soils that are tested, the copper content could be doubled and they would still be deficient in copper. And many are found to be far worse than that.

So, if copper levels are that bad and the crops are still growing, why worry about trying to build up copper levels in organic production? What does copper do for the soil and the crops and how does that translate to beneficial results for livestock, people in general and organic growers in particular?

And while keeping all the points above in mind, how much copper should be considered as adequate on a soil test? There are many useful indicators of copper deficiency in growing organic crops. This article will point out some of those as good reasons to consider testing for copper when soils have not been sufficiently analyzed to correctly determine whether copper is adequate or not. Especially note that the reported desired level can vary greatly depending on the testing procedures used and how those tests are expressed in terms of numbers on each soil test.

Copper is necessary to grow stronger and more resilient plants. And along with adequate boron, copper is needed to naturally ward off rust and fungus diseases. For nutrition, sufficient copper is needed for the proper conversion of protein in livestock. And as needed in the same way for people, nutrient dense food will not be continually assured until copper deficiency in the soil is correctly eliminated. Furthermore, combined with adequate boron it helps the body fight against all types of inflammation.

Plants need copper, combined with enough potassium and manganese, to build strong stalks. Copper also provides more resilience so stalks and limbs can bend and straighten back up instead of breaking. For green snap in corn or broken limbs on windy days in newly planted tree crops, correcting the copper level in the soil is a vital part of putting an end to such problems.

 

Copper Deficiency

Tomatoes are a good indicator crop regarding whether a soil test is measuring the minimum amount of copper needed by each soil. When tomatoes have cracks near the stem, this indicates they are not able to acquire enough copper. Specifically based on the laboratory analysis still being used that was developed by Dr. William A. Albrecht in the mid-1900s, when the soil analysis shows 2 ppm, the copper level is sufficient to solve this problem. This is also the minimum amount any soil should have based on Dr. Albrecht’s work. That required amount may be represented as a quite different number on soil tests done by other soil laboratories.

But even when nutrient-available copper is sufficiently supplied to the soil, tomatoes can still have a copper deficiency as indicated by splits around the stem. That is because just making sure enough copper in an available form is there is not all that is required. Soils that do not contain at least 60% base saturation of calcium (which can even include soils with a high pH) can still cause plants to be deficient in copper as well as any other of the needed nutrient under such conditions. This is one reason why so much “research” done on copper and other micronutrients conclude that adding them is not necessary.

Another similar example of copper deficiency is when gray spots can be seen on boiled potatoes as the peeling is being removed. Some believe that this problem is due to a calcium deficiency, and if calcium is too low in the soil and there is sufficient copper there, then adding the needed calcium will solve the problem. But if copper is barely sufficient or deficient where more calcium is added, it does not solve the problem until the true Albrecht test measures enough available copper, which is at least 2 ppm for potatoes grown in such soils.

Using this test, where the soil contains sufficient calcium and the copper level is barely above 2 ppm, potatoes from some areas may still have the gray spots while those from other parts of the field do not. Check the copper levels from both areas and notice the difference between sufficient (no gray spots in the potatoes) versus deficient (gray spots are still evident) levels of copper.

Some vegetable crops that are considered as most susceptible to copper deficiency include carrots and onions. Lettuce also suffers when copper is too low, but it can require much higher amounts to be successful in the control of troublesome rust and fungal diseases.

 

Common Applications

Wheat is a good example of how important copper is and responds well at the minimum level shown to be required for any soil. When levels are only moderately below 2 ppm, just applying five pounds per acre of 26% pure copper sulfate the previous autumn can be expected to control rust in wheat, again considering that there must be sufficient calcium in the soil for the plants to take it in. When that is the case, experimental field applications show no rust right to the line if copper has been sufficiently applied on some portion of the field.

Just like lettuce mentioned above, not all types of plants are able to take up enough copper for resistance to rust and fungal disease when a soil analysis shows the soil is just above the minimum requirement of 2 ppm. Soybeans need close to 2.5 times that much along with adequate boron to ward off sudden death syndrome and Brazilian soybean rust. For the more perishable fruits, such as blackberries and raspberries, the copper levels in the soil should be between 12 to 15 ppm for maximum resistance to rust type diseases.
There are those who maintain that since copper sulfate is a powerful fungicide, it should not be used on the soil. Before making such a judgment, consider what tends to be the end result. In their book, Soil and the Microbe, Dr. Selman Waksman and Dr. Robert Starkey, both at the time being professor and assistant professor of soil microbiology at Rutgers University, studied the effects of soil sterilization on microbes. The bacteria recover quickly, but the fungi and protozoa are frequently almost all destroyed and require an “extended interval of time” to recover.

Initially, when copper sulfate is applied, depending on how that is done, it can be a powerful fungicide. When liquified and sprayed uniformly on plants and soils, copper sulfate can be highly effective for that purpose. A safer form of foliar copper for soil organisms is copper chelate, which is not harmful to the fungi in the soil.

But even though highly effective as a foliar, chelates do not contain enough copper at such rates to correct any significant amount of measurable copper needed in the soil. However, to encourage and maximize beneficial organisms in the soil and allow the crops being grown there to properly fight off rust and fungal diseases, the needed copper level must be attained.

Therefore, the recommended form of material application for building copper in the soil is to use an available form of copper as part of a dry broadcast application. And the only form of copper that will work well enough to do this is pure copper sulfate.

Consider a dry blend of materials such as 10 pounds per acre of copper sulfate (2.3 pounds of actual copper per acre) blended with enough other fertilizers to be accurately spread over the soil, such as potassium sulfate. Though easily seen in the mix due to its blue-green color, one can readily observe that, accordingly, there are very few of those particles in the mix. How much soil would each of those particles have to cover to adversely affect the fungi on a per acre basis? Even at three times the 10 pounds per acre rate mentioned above, when shown to be needed to reach necessary levels to provide for crops on organic farms, that does not happen.

Furthermore, copper sulfate becomes stabilized when added to the soil, thus losing its toxic properties. There are several implications that this happens rather quickly when copper sulfate is added to the soil. And once there is enough stabilized copper available as shown by a proper soil analysis, no more is needed until less than the minimum desired level shows up again on the soil test.

A number of clients who have applied the needed amount of copper sulfate to reach just above 2 ppm in the late 1970s still have a sufficient amount and have not had to apply any additional copper to fight rust and fungal diseases in wheat crops since that time. So, the use of copper sulfate is not a constant need. Once enough is there to supply what each growing crop needs, it can be years if not decades before just a small amount for maintenance is required.

Although other micronutrients such as iron, manganese and zinc provide a pound for pound response in the soil when being supplied as sulfates, this is not the case even with a pure source of copper sulfate. Like iron and manganese, the measured response should not be expected until at least 12 months after being applied. But the difference is that the maximum available copper response will only be about 25% of the total pounds of elemental copper that is applied.

In other words, applying five pounds of 26% copper sulfate should only be expected to raise the soil’s available copper by a maximum of 0.3 ppm. The other 75% never suddenly becomes available, and its slow release over time is likely the reason copper levels remain so stable for years once they have been attained.

Moderate use of manure or compost, depending on the content of the material and the total copper content of the soil, can help build copper levels when used over a span of several years. For example, using four tons per acre of composted turkey manure (which is normally expected to contain the highest copper levels of all animal manures for building available copper in the soil) has proven to be sufficient to build copper levels above the minimum requirement of 2 ppm.

Thoroughly decomposed organic matter, measured and reported as the soil’s colloidal humus content, when present in moderate to good amounts of 4% to 5%, can help reduce the need for copper in growing crops. But it still requires just as much to diminish any significant need, and normally there is not enough in soil organic matter content, compost or manures to do so.

As measured by laboratory tests utilized by Dr. Albrecht, 5% to 7.5% colloidal humus is considered as most beneficial for growing crops. But in that regard, especially in the case of copper availability, more is not better. When the actual measurable soil humus is above 7.5% it is detrimental to copper availability and its uptake from the soil. Once that level is exceeded, the use of foliar copper becomes extremely important to avoid problems from copper deficiency. Just be careful when relying on a tissue or plant analysis alone to show whether there is enough.

One positive indication that soils and crops contain sufficient copper is the condition of livestock on the farm. Adequate copper is needed in the feed for conversion of protein in the animals. Animals have shiny hair coat with adequate copper in grass.

 

Tissue and Soil Analyses

Many growers or their consultants rely on leaf or tissue analyses to determine if crops are getting enough copper. Such testing can be misleading. The first rule to consider in such cases is to use a plant test to treat the plants and use a soil test to treat the soil. Even then, the two should correlate to show if there is a problem. Just be aware that plant or leaf analysis will often indicate good levels when the soil and crop are still showing that is not the case.

As an example, consider a group of clients where testing showed their soil would benefit from the extra moisture provided from higher potassium levels for growing grapes without irrigation in an area that usually tended to be quite dry. They were warned that the soil test not only indicated a need for potassium, but most of those soils also had deficient copper. An additional caution was that if the soils received more than the normal rainfall, then this would result in larger grapes. Then, in those soils with deficient copper, the grapes would split at the stem.

Note that splits can also happen when there is adequate copper if calcium saturation levels in the soil are too low. However, this was not the case in the high-calcium soils where these European vineyards were located.

The very first year this was done, that area received an inordinate amount of rainfall. In August, calls began coming in that many of the vineyards were suffering significant losses due to the skins splitting where they added the needed potassium. Even though all had acknowledged and agreed to apply any needed copper based on the soil tests, none of them had done it. But in each case, it was because they had to have a leaf blade analysis to prove they needed to apply copper. And in each case, the leaf blade analysis came back as sufficient.

Based on actual field results, the leaf analysis was wrong because the vineyards that had above the minimum recommended requirement of 2 ppm copper on the soil test had no problem with splits, but all those below that recommended minimum level were the grapes that suffered with great losses from the skins splitting at the stem of each grape. This happened even though the leaf blade analysis indicated good levels of copper in both grapes that did have the problem and those that did not.

How soon is copper taken up by the crop when added to the soil? A potato farm of 1,250 acres of land was sampled and found to be deficient in copper. All but 365 acres also showed a need for calcium lime. But the lime was not applied due to the grower’s fear of it causing common potato scab.

All other recommended fertilizer was applied, including 10 pounds of 23% copper sulfate just before planting, except for 40 acres that did not receive any copper sulfate and another 40 that received only five pounds per acre.

Leaf samples were taken at bloom on all fields. A lack of copper was the greatest deficiency on the 40 acres where the rest of the fertilizer was applied, but no copper sulfate. The 40 acres that received five pounds of copper sulfate showed calcium as the most limiting factor and copper as next most limiting. On all the rest of the fields which had received the recommended amount needed, copper was not shown to be a limiting factor at all.
Based on detailed soil testing, the copper content was shown to be just as deficient on all the other fields as on the one where no copper was applied. Yet, when applied at planting, the copper was already getting into the plants at bloom. The sooner needed micronutrients are applied, the sooner the crops can benefit. Even copper, which is considered far less soluble than the other trace element products, is able to be taken up by plants when applied in the right form at planting time.

 

Excess N and Copper

Excessive applications of nitrogen can cause copper deficiencies in soils that show to have enough. The results are even worse when soils are already deficient in copper. Dr. Andre Voisin, in his book Fertilizer Application: Soil, Plant, Animal, points out how exceeding the needed amount of nitrogen has been shown to cause copper deficiencies in crops.
This is one of the greatest reasons for stalk lodging in wheat and corn. Until 2 ppm of copper is achieved in the soil, even normally needed amounts of nitrogen can cause lodging. But once that level is achieved and the soil also has sufficient calcium, potassium and manganese, the problem with lodging can be overcome unless excessive amounts of nitrogen (generally 50% or greater than the yields being made requires) is being applied.
Excessive use of phosphate can also cause a copper deficiency in crops. Normally, this is only found where continued use of phosphate-containing materials is occurring and P levels there are extremely excessive. This is, at times, a problem when exorbitant amounts of compost and manure are being applied by those who feel you cannot apply too much of such materials.

One positive indication that soils and crops contain sufficient copper is the condition of livestock on the farm. Adequate copper is needed in the feed for conversion of protein in the animals. Adequate conversion of protein shows up quickly in the hair coat. A slick, shiny hair coat in livestock on pasture is a good indicator of at least the minimum requirement of copper in the soils producing the crops being used for feed. Once the minimum level of 2 ppm copper in the soil is reached, this type of result can be seen. Note that due to the extra time required to perform the analysis and the higher cost of the extractants used for the method of analysis, the recommended 2 ppm level for copper in the soil based on the Albrecht-type testing used here will usually be much different on those soil tests performed by other laboratories.

 

Adequate Micronutrients Are a Must

True nutrient-dense food production will never be achieved without adequate copper, and most soils tested, even for certified organic growers, do not even meet the very minimum level of copper needed to provide for the crops and solve the disease problems such deficiencies can cause from not enough being present in the soil.

To build the most fertile and productive soils without sacrificing top quality from the disregard of natural laws of nutrient uptake requires an understandable program for all growers in order to educate and consider the long-term outcome and expected results. Using micro-nutrients properly will not be achieved just by adopting a supposedly simple yet deceptive plan for better looking crops and higher yields.

 

Resources

The Soil and the Microbe: An Introduction to the Study of the Microscopic Population of the Soil and its Role in Soil Processes and Plant Growth by Robert Lyman Starkey and Selman A. Waksman. New York: John Wiley & Sons, Inc. 1931.

Fertilizer Application: Soil, Plant, Animal by Andre Voisin, published by Crosby Lockwood. 1965.