Copper and molybdenum are easy to manage if you know how to identify deficiencies
When you think about crop nutrition, copper and molybdenum might not be the first nutrients that pop into your mind. But that doesn’t mean their impact on yield isn’t significant.
“Copper deficiencies can be severe enough to kill plants and significantly reduce stand and ear count,” says Farm Journal Field Agronomist Ken Ferrie. “In soybeans, studies have found yield reductions of 3 bu. to 5 bu. per acre from molybdenum deficiency.”
Fortunately, deficiency—or, less frequently, toxicity—of copper and molybdenum are easy to prevent and manage, if you know what to look for.
Let’s start with copper. “Copper’s biggest role in plants is triggering enzymes that are involved with photosynthesis,” Ferrie explains. “It also helps plants resist disease. It plays a big part in the development, survival and release of pollen. So it is most important to self-pollinating crops such as wheat, barley and corn.
“In corn plants, copper helps stimulate protein formation. It aids in moving carbohydrates throughout the plant and into the kernels when the plant is finishing grain fill,” he adds.
Whew! That’s a heavy workload for a nutrient making up only 0.4 ppm to 1.5 ppm in most soils.
Because copper is involved with photosynthesis, a deficiency results in yellowing of the leaves. Because copper does not move inside the plant, you’ll see yellowing, or a pale color, show up in the youngest leaves at the top of the plant, Ferrie says.
Copper deficiency can resemble the symptoms of potassium deficiency, but potassium deficiency shows up in older leaves near the bottom of the plant.
In corn, the yellowing from a copper deficiency starts at the edge of the leaves, and the entire leaf might die. This dying of leaf tissue somewhat mimics nitrogen deficiency but the difference is nitrogen deficiency signs start in the middle of the leaf.
In wheat, when the leaves die, they take on a twisted appearance similar to minor frost damage. If damage is severe, the entire wheat plant might die.
Wheat and barely are most likely to show copper deficiency, followed by corn and soybeans. Alfalfa, oats and wheat are most likely to respond to a copper application. Corn, sorghum and sugar beets are medium in response. Soybeans, rye and potatoes are least likely to respond.
Plants require only a small amount of copper. Most crops remove less than 1 lb. per acre and many remove less than half a pound.
You can predict where copper deficiency is likely to occur. “Watch for deficiencies in high-organic matter soils—peats and mucks,” Ferrie says. “With 6% or more organic matter, copper is likely to become tied up and unavailable to plants.
“Crops planted on heavily manured soils might become copper-deficient because the copper in the soil gets tied up with organic matter in the bedding,” he continues. “This is likely in poorly drained areas because high temperatures and excess moisture create unfavorable conditions for the release of copper from organic matter.”
Ferrie has observed very high soil levels of phosphorus and nitrogen put pressure on copper uptake. “These conditions can occur when high rates of manure are applied,” he says. “Deep sandy soils might become deficient if the parent material that formed the soil was low in copper.”
Copper becomes less available in alkaline soil conditions. “A movement of 1 point in soil pH, as from 7.0 to 8.0, reduces copper availability 100-fold,” Ferrie says. In some soils, pH is naturally high; elsewhere, it can result from overliming.
When you spot deficiency symptoms, back up your observations with soil and tissue analysis. “Of the two, tissue testing is the most accurate,” Ferrie says. “Soil testing is difficult because the amount of copper in the soil is so small. But, taken together, soil and tissue tests will give you an accurate picture of what’s going on.”
Once you’ve identified a copper deficiency, apply copper sulfate to the soil. “One challenge of managing copper is the element doesn’t move very far in the soil,” Ferrie says. “So if you apply it on the surface and don’t get rain, it might stay on the surface. Other options include foliar application and banding. If you apply starter fertilizer in a 2"x2" band, you can include copper chelate in the blend.
“If soil levels are low, making a copper application every two or three years might be sufficient. But if a deficiency results from high soil pH or high organic matter content, you might have to apply copper every year. In wheat, which is highly responsive to copper, you might need to make two applications per year.”
In corn, copper deficiency causes yellowing that starts at the edges of leaves, killing the entire leaf.
If you foliar-apply copper, read the product label carefully and match the timing of the application to the size of the crop. “Timing is very important because copper will not move inside the plant,” Ferrie says. “With wheat, apply copper when the crop is tillering; don’t delay the application until the plants are heading.”
Man-caused deficiencies, such as overliming, are easiest to correct. “Base lime rates not just on soil pH but also on soil texture,” Ferrie says. “It’s easy to create a copper deficiency by overliming deep sandy soils that are already low in copper availability. If you have light sands and heavy soils in the same field, you must use variable-rate technology to apply lime.”
Avoiding saturated soil also reduces the risk of a copper deficiency. Install drainage, if necessary, and eliminate compaction so water can percolate through the profile.
Copper toxicity, caused by excess copper in the soil, is rare, Ferrie says. If it occurs, it will affect iron uptake.
Molybdenum is required by legumes because it helps drive the process in which rhizoctonia bacteria fix nitrogen from the air. It also plays an important role in converting nitrate-nitrogen to organic nitrogen or amino acids inside the plant.
Molybdenum deficiency symptoms resemble nitrogen deficiency. Molybdenum is immobile in the plant, so symptoms show up in new growth. In soybeans, the leaves of molybdenum-deficient plants turn pale green, the trifoliate leaves appear cupped and dead tissue is on the edges of leaves.
“If you split the nitrogen nodules of a molybdenum-deficient soybean plant during the vegetative phase, they will be an off-tan color rather than a healthy pink,” Ferrie says. “And there will be fewer nodules on the plant.”
Most crops need less than 1 ppm of molybdenum. Soil contains 0.2 ppm to 6 ppm. Alfalfa and soybeans show a medium response to molybdenum applications. Corn has a low response.
Deficiencies most often occur in acid sandy soils. “Molybdenum availability increases as soil pH increases,” Ferrie says. “You are most likely to see deficiency problems in soil with a pH reading below 6.2. Molybdenum will definitely become an issue if soil pH reaches the low- to mid-5 range.”
If you suspect molybdenum deficiency, follow up with soil and tissue testing. “As with copper, testing for molybdenum is difficult because of the small amount in soil,” Ferrie says. “So be sure to back up your soil tests with tissue analysis.”
If acid soil is causing molybdenum problems, liming will help. “When you raise soil pH above 6.2, it raises the odds against a molybdenum deficiency quite a bit,” Ferrie says.
You might be able to prevent molybdenum deficiency by being careful with sulfur applications. “Applying excessive amounts of sulfate can suppress molybdenum uptake,” Ferrie says.
Watch for molybdenum deficiency in acid peat soils, where the normal pH is in the 5.5 range (a reading that would be acid in most soils). “On these soils, you probably can’t correct a deficiency with a soil application,” Ferrie says. “The most efficient way to fix a problem in these soils is by applying a seed treatment. Read and follow the label because if molybdenum remains mixed with other seed treatments too long before planting, it might cause a problem with the other treatments.”
With acid peat soils, other options include foliar application and banding molybdenum in starter fertilizer.
If crops will be consumed by livestock, be especially careful to base molybdenum rates on soil and tissue tests results and follow product labels. Excessive levels of molybdenum in a crop can create health issues for livestock, especially dairy cows.
In summary, confirm deficiencies with soil and tissue tests and apply the appropriate rate to keep these mighty micronutrients from standing between you and maximum yield.
The Nutrient Navigator series focuses on efficient, environmentally sound management of nutrients. The goal is to provide practical knowledge that helps drive yields and profits higher.