Andy Walsh got a 6- to 8-bu. increase in yield when he added zinc to corn in field trials. Researchers who added copper and zinc to cornfields in southeast Indiana saw a 10-bu. yield increase.

Micronutrients carry the “micro” prefix because only a relatively small quantity of them is required for plant growth, but researchers are finding they are no less important to yield than are other nutrients.

However, the researchers also have discovered that the micronutients needed in one region of the Midwest may not be required in another. “You can't paint in broad brush strokes when you're working with micronutrients,” says Dave Franzen, extension soil scientist at North Dakota State University, Fargo.

More than soil. A soil test is used to determine if soil lacks any micronutrients, which include boron, chlorine, copper, iron, manganese, molybdenum and zinc. But you need to look at more than just soil. The crop and hybrid or variety planted also play a role in micronutrient needs.

“Certain crops are better able to pull nutrients out of the soil, even at low test levels, while others cannot,” Franzen says, noting that wheat grown in a field that tests low for zinc may produce 50 to 70 bu., whereas a dry bean in that same plot would display deficiency symptoms.

Yield potential and genetics. According to Don Huber, a soil microbiologist and plant pathologist at Purdue University, when a plant is starved, or any nutrient is very inefficient at utilizing other nutrients that may be in full sufficiency, both yield and quality will be affected. “Quality, especially, can be affected directly from the lack of the nutrient, but also from an increased disease susceptibility,” Huber says.

Plant genetics also play a role in the need for micronutrients. “In looking at genotypes, or the genetics of the different varieties and hybrids, we find that there is a wide range of differences in both their efficiency in uptake of the nutrients from the soils as well as their efficient utilization of them in their own physiology, then, in the plant,” says Huber, who has been studying plant nutrient research for 30 years, with an emphasis on micronutrients for the past four years.

With corn and wheat as their primary emphasis, Huber and his colleagues are looking at four soil types in four different Indiana locations. “The [soil types] represent both the general nutrient deficiency areas, as well as environmental conditions that we find are pretty much characteristic of the Midwest,” Huber says.

In one recent micronutrient research trial in a southeast Indiana location known for its borderline soil copper values, the researchers reported a significant yield response in 11 of 14 corn hybrids tested when the micronutrients zinc and copper were added to the fields.

“Overall, we saw about a 10-bu. yield response using ⅛ lb./acre of copper and ¼ lb./acre of zinc,” Huber says. “It didn't take much of the micronutrients for a very significant yield and quality response.”

Huber notes that his region's soils can be very low on a nutrient without producers being aware of it. Often because the symptoms aren't very distinct, the loss isn't obvious. “We're just not getting the yield and quality we know we have the potential to get,” he explains. “With very limited input of most of these micronutrients, there is anywhere from a four- to tenfold return to the producer.”

At Purdue, they've found that the heavier soils, the silt clay loams, tend to be marginal for copper and zinc on corn. The fine, sandy loams tend to be marginal for copper and manganese. The northern silt loams and sandy loams are marginal for manganese. “And the west-central silt loams are generally considered adequate for all of the micronutrients,” Huber says. “Yet we find that as we've increased our yield potentials, we also increase the need to provide micronutrients. In order to maximize that full genetic potential for yield and quality, we need to make sure we have the micronutrients and the other nutrients balanced and available for the plants. A 120-bu. yield potential didn't require nearly as much zinc, manganese or macronutrients as a hybrid with a 200- to 250-bu. yield potential.”

Huber reports seeing a much greater need for the micronutrients and a wider variation in genetic response or genetic potential of hybrids than he had initially anticipated. “So, it's important that a grower look at his yield potential or the genetics of the hybrids as well as the soil potential available to produce those crops, then make necessary adjustments in nutrient levels so he can achieve the full potential and economic efficiency and production,” he concludes.

Copper. In North Dakota, although well-established research has shown yield increases in corn, dry edible beans, potatoes and flax with the application of zinc, researchers continue to look at the effects of other micronutrients. Three years ago, after reviewing Canadian research that reported wheat yield increases in response to copper, Franzen of North Dakota State University decided to conduct similar tests. He applied 5 lbs. of copper sulfate/acre to sites that had low organic matter, sandier soils and low copper tests around .2 to .5 parts per million. The result was a 15-bu. yield increase in one of six plots the first year, with no response in the other plots.

Franzen reports continued inconsistent results in subsequent years, with a total of three responsive plots over the course of the three-year study. “Because of these results and the expense of the copper, we don't think it's something people should get too excited about,” Franzen concludes, noting that copper may prove beneficial in some areas, such as sandy, low-organic-matter sites that yield poorly even in wet years.

Zinc. Three years ago, Randy Killorn, professor of soil fertility at Iowa State University, started looking at spatial responses to zinc in Iowa. “As we move across our fields here, we have spots that have quite high pH. And right next to them are spots where it's at least slightly acid to quite acid, depending on the soil we're talking about,” Killorn says, noting that it had been about 20 years since they'd done any work with zinc. “We know that the pH affects zinc solubility, and there's reason to suspect that that would carry over to the availability to the plant. So we wanted to see if we got differential spot responses to zinc as we moved in and out of those high pH spots.”

Their results were very surprising. “We did get some positive responses to zinc, but we actually got more negative responses to the way we were doing it,” Killorn says, adding that they're still working on determining the reason for the yield decrease.

Based on preliminary results, Killorn says he would advise Iowa producers to conduct a soil test. “Our data show clearly that if that test is below the critical value, they should be putting some zinc on because they are going to get a positive response,” he adds, noting that in some cases, producers view a zinc application as a type of “just in case” insurance. “That may not be a very good decision, because there is some likelihood that it would actually cause a yield decrease,” he says.

In Wisconsin, Andy Walsh, agronomy sales manager, Kettle-Lakes Cooperative, Random Lake, has conducted a number of zinc trials on corn. “When half the corn planter contains a starter with zinc and the other half contains just starter without zinc, we get a 6- to 8-bu. yield difference. That's pretty meaningful,” Walsh says. He blames the area's extremely high pH soil and cool, wet, early-planted soil conditions for making it difficult to keep zinc available. “Even though our soil tests may show zinc to be available, it isn't always, because of tie-up conditions,” he says.

Copper, iron, boron and zinc. In Minnesota, University of Minnesota Extension Soil Scientist George Rehm recommends zinc for corn, and boron for alfalfa as standard. But last year, he and his colleagues also began looking at copper for spring wheat in the Red River Valley, as well as preliminary studies of iron chlorosis in soybeans. “We have had a copper recommendation for small grain grown on organic soils for a number of years, but we focused this year on the mineral soils,” Rehm explains.

They compared copper applied as a chelate or copper sulfate source to nontreated areas. The result was a small increase in yield in wheat grown on a very sandy soil with low organic content. “That's different than what we've experienced in the past, and we'll follow up on that study,” Rehm adds, noting that additional work on iron chlorosis also is planned.

Water solubility. For producers whose fields need zinc, Dwayne Westfall, soil science professor at Colorado State University, suggests another step in the micronutrient decision-making process. He urges farmers to have their dry zinc sources analyzed.

According to Westfall, many tons of zinc fertilizers are derived from industrial by-products in the form of a zinc oxide. The zinc oxide, which is not water soluble and not very available to plants, is treated with sulfuric acid to form a zinc sulfate. “The amount of reaction that occurs is directly proportional to the amount of sulfuric acid that they put into the system,” Westfall says. Therefore, using a small amount of sulfuric acid results in a small amount of water-soluble zinc sulfate, and a large amount of non — water-soluble zinc oxide.

His tests of materials ranging from zinc sulfate, which is 100% water soluble, down to some zinc oxysulfate materials, which are less than 1% water soluble, showed a direct relationship between water solubility and the ability of the zinc fertilizer to meet the demands of the plant. “Zinc fertilizers should be about 50% water soluble or greater in order to be a reliable source of zinc for plants,” Westfall reports, adding that the same type of relationship held for both acid or alkaline soil.

Another study looked at various organic fertilizer materials, including zinc EDTA, zinc sulfate, zinc oxysulfate, zinc lignosulfonate, derived from the paper-mill industry, and zinc sucrate, which is zinc oxide mixed with sugarcane molasses. “It didn't matter if the product was an organic or an inorganic compound, the availability was still related to the water solubility,” Westfall says.

“The only way you can determine its plant availability is to send [the zinc material] in for an analysis to an analytical laboratory and ask them to run a percent water solubility analysis,” Westfall continues, adding that dry zinc fertilizer that's not pure white has been manufactured from an industrial by-product. “Otherwise, you might be spending your money on a material that is inert when applied to the soil.”

He also underscores the importance of soil testing to determine if zinc is truly needed. He says, “In today's fertilizer cost squeeze, farmers are going to be looking for ways to save money, and [soil testing] is one way to capitalize on their fertilizer investment.”