Anticipate deficiencies and then fix them with fertilizer timing and placement
Zinc deficiency—which can easily cost 15 bu. to 20 bu. of corn per acre—is not something you can walk into a field and diagnose. Confirming a zinc deficiency is more like circling around the problem, then zeroing in.
“Of all the soil testing we do, micronutrient analysis is the least precise,” says Farm Journal Field Agronomist Ken Ferrie. “That’s partly because we have 2 million pounds of soil in a 6" acre slice, and we are trying to measure the volume of something that may be as low as 1 to 2 ppm [parts per million]. The margin of error for soil tests, from one test to the next, is greater than the actual value we’re looking for.”
Classic symptoms of zinc deficiency (interveinal yellowing or white streaks between leaf veins on the lower half of the leaf) were visible in corn plants throughout this field. Seed was planted into cold, wet soil with a high pH.
That’s not to say soil testing isn’t useful with zinc. “Soil tests give us insight into whether soil zinc values are high, medium or low,” Ferrie says. “To diagnose zinc deficiency, I prefer a three-pronged approach, including visual symptoms, tissue testing and soil testing. After you confirm a zinc deficiency, you can formulate a strategy to correct it.”
If you think you might have a zinc deficiency, look for symptoms in corn plants because corn shows the most obvious visual symptoms and produces the greatest response to zinc fertilizer. Corn is followed by soybeans and wheat. “If there is no deficiency in corn, it’s highly unlikely you’ll get a yield response to zinc fertilizer in wheat,” Ferrie adds.
Expect zinc issues when soil is cold and the weather is wet, and in mucks, sandy soils and soils high in pH or phosphorus. Your standard scouting tools—such as thermal imagery, NDVI maps and past years’ yield maps—can help identify potential problem areas, but with zinc, on-the-ground scouting is particularly important.
“In corn, you usually see symptoms of zinc deficiency in the first 20 to 30 days of the plant’s life,” Ferrie says. “As the plant gets bigger, soil micro-organisms usually become more active; they release more zinc from the soil, and the problem goes away. In the plant, zinc levels might rebound by tasseling time.”
Zinc deficiency symptoms first appear as interveinal yellowing or white streaks between leaf veins on the lower half of the leaf. If the deficiency becomes severe, the striping grows into wide bands between the midrib and the edge of the leaf.
If you spot zinc deficiency symptoms as late as tasseling time, the problem is severe, Ferrie says. Deficiencies later in the season usually are caused by high-organic, or muck, soils, alkalinity (pH above 7.1) or with very high phosphorus levels.
Once you confirm visual symptoms, have plant tissue samples analyzed. “Take a uniform tissue sample,” Ferrie advises. “Although all the plants in an area may be deficient, if a few plants are noticeably smaller and weaker, take your samples from healthier looking plants because they are more typical and uniform. Don’t mix tissue samples from different management zones, or even from different hybrids because hybrids vary in their ability to take up zinc.”
The sampling procedure is critical. Ask your lab for protocol on collecting and shipping tissue samples. Procedures might vary between labs, but they likely will include using a clean container, preferably a plastic pail or paper bag because metal containers can contaminate samples. Avoid dust, fertilizer and spray residues, or remove those residues with distilled or de-ionized water (but don’t prolong washing because it can leach nutrients out of plant tissues).
You can raise the zinc levels in your soil by broadcasting zinc with dry fertilizer. However, timing is tricky. If you wait until the V5 or V6 stage to apply zinc, the plant won’t be able to benefit from the zinc fast enough.
Air dry samples in the shade rather than the sun. Place them in clean paper bags or envelopes and mail to the lab. (Never mail them in plastic bags.)
If your tissue test confirms a zinc deficiency in your plants, it’s time for a soil test. Understanding your soil test results requires knowing which zinc extraction test—DTPA, Mehlich 3 or HCL—your lab uses. Never compare numbers obtained by different extraction methods.
“Here’s what could happen if you mix results from different extraction methods,” Ferrie says. “Say your soil test reads 4.0 ppm zinc. In the DTPA test, 4.0 ppm would be considered high, but in the HCL test 4.0 is
“A reading of 2.5 ppm would be considered low in the HCL extraction method, but medium with the DTPA test. In fact, the range of readings between 1.1 ppm and 2.9 ppm is considered low with the HCL extraction method but medium with the DTPA method. Confusing numbers from those two methods could lead you to believe the broadcast application of zinc you made two years ago has had no effect, based on the results of this year’s test,” he adds.
Plan to manage zinc issues early in the season because, as mentioned, deficiency usually occurs in the first three weeks to month. “The easiest way is to apply zinc at planting is in your starter fertilizer,” Ferrie explains. “This approach illustrates two of the principles of efficient fertilizer management: timing and placement. You apply the zinc when the crop needs it and band it close to the seed.”
If starter fertilizer is not an option, you might have to raise the zinc level in the soil by broadcasting zinc with your dry fertilizer.
“If you don’t find a zinc deficiency problem until the corn is at the five- or six-leaf (V5 or V6) stage, there is no good way to fix it,” Ferrie explains. “Applying zinc to the soil won’t get it into the plant fast enough. That limits you to foliar feeding.
“But if you broadcast a foliar application at the V5 or V6 stage, two-thirds of the fertilizer will hit the ground, rather than the plant, so it won’t correct the deficiency. Direct-banding on the plant will be more efficient. Zinc is not mobile in the plant though, so you may need to make multiple applications.”
In severe situations, with alkaline soils or soils containing high or low organic matter, you might need to make a broadcast soil application, a planting application in your starter fertilizer and one or two foliar feedings, Ferrie says.
For broadcast application, the most widely used source of zinc is zinc sulfate. In starter fertilizer and foliar fertilizer, the most widely used source is chelated zinc.
“When choosing a source of zinc, the most important quality is water solubility,” Ferrie says. “The more water soluble the zinc, the quicker the crop response. Look for a zinc source that is at least 50% water soluble, and the higher the better. Most zinc chelates are 100% water soluble. Zinc sulfate is 98% water soluble.”
The cheapest source of zinc might not be the most efficient. “A cheaper form of zinc is zinc oxysulfate. It comes in two forms, which are only 26% and 55% water soluble,” Ferrie says. “If you’re unsure about the solubility of a zinc product, most labs can run a water solubility test.”
With zinc management, the most important step is to watch for deficiency symptoms in corn, early in the season when they are visible and yield is affected, Ferrie summarizes.
“I recommend not applying starter fertilizer without zinc,” he adds. “The starter fertilizer creates a high-phosphate condition next to the small plants, which is the same as a high-phosphorus soil. To minimize cost, cut down on the amount of starter fertilizer but leave in the zinc.”
Learn and Profit from Nutrient Navigator
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.
Zinc Extraction Methods
Zinc extraction methods used by various soil testing labs can cause confusion because they use different scales to determine soil levels. The three most commonly used extraction methods are DTPA (diethylene triamine pentaacetic acid), Mehlich 3 (a strong acid) and HCL (hydrochloric acid). DTPA and Mehlich 3 can be used on acid, neutral or alkaline soils; HCL extraction is used only on acid and neutral soils.
Confusion can arise if you compare one extraction method’s scale against another’s. To illustrate the difference, here are the zinc readings by one lab for three soil samples using each method:
Here are the values for low, medium and high soil zinc levels based on the HCL and DTPA extraction methods:
A reading of 2.5 ppm would be considered low in the HCL test but medium with the DTPA extraction method. Readings between 1.1 ppm and 2.9 ppm would be considered low with the HCL extraction method but medium with the DTPA extraction method.
“You can avoid confusion by knowing which extraction method your lab uses and not comparing readings from one extraction method with those of another,” explains Farm Journal Field
Agronomist Ken Ferrie.
With the Mehlich 3 extraction method, readings for high, medium and low ranges depend on whether the lab uses a regression equation in its numerical scale. “So if your lab uses Mehlich, ask for their numerical scale for high, medium and low ranges,” Ferrie adds.