The Basics of Zinc
It may be a minor element, but it can bring big potential for trouble. Fortunately, zinc deficiency occurs only in certain soil conditions and environments, however in the right circumstances—for example, calcaric soil with a high pH reading—zinc deficiency can easily rob 15 bu. to 20 bu. of corn per acre, says Farm Journal Field Agronomist Ken Ferrie.
“Zinc and other minor elements aren’t called micronutrients because their role in crop production is minor,” Ferrie says. “On the contrary, they are micronutrients because plants only need small amounts, compared with macronutrients such as nitrogen, phosphate and potash.
Micronutrient problems can often be fixed with small applications—from 1 lb. to 3 lb. per acre. But if deficiencies are not corrected, they can seriously impact yield.
“Micronutrients are sort of like vitamins,” Ferrie continues. “Even if you consume a healthy balanced diet, you can run into trouble if you are missing just one vitamin. It’s the same with crops and micronutrients.”
In plants, zinc plays several important roles, all of which help drive yield.
“Zinc is involved in protein synthesis,” Ferrie says. “It plays a big role in grain and seed production, especially silk emergence in corn, because it
affects the flowering parts of the plant. Zinc helps drive metabolic processes and helps the plant maintain its hormonal balance.”
Problem-Prone Soils. Since zinc deficiency is not common, there’s usually an underlying reason when it occurs, Ferrie says. Understanding those reasons will tell you where and when to anticipate problems.
“Zinc deficiency is common in
calcaric soils because they tend to be
alkaline,” Ferrie explains. “In high-pH conditions, zinc becomes immobilized and unavailable to plants. Expect zinc problems if soil pH gets above 7.2.”
Sandy soils are also prone to zinc issues. “Because they are highly leachable, those soils can’t hold the amount of zinc plants need,” Ferrie says. “Abused, eroded soils also might not contain sufficient levels of zinc.”
Organic matter content is a factor. “You’re more likely to have zinc issues in soils with 1% organic matter than in soils with 3% to 4%,” he says. “At the other extreme, organic soils with 40% to 50% organic matter content also are prone to zinc problems.”
Problem Environments. Certain environmental conditions set the stage for zinc deficiency.
“The most common zinc deficiency situation I run into is when a farmer has cold, wet conditions at planting, the corn emerges and then the temperature drops,” Ferrie says. In this situation, a soil test would show adequate levels of zinc. But reduced
microbial activity, resulting from the lower temperature, reduces the soil’s ability to release zinc for the plants.
“Keep this in mind if farmers push the planting window, planting corn as early as they can,” he adds. “In that case, consider a hybrid that’s good at taking up zinc because some hybrids are more efficient than others.”
Soils that have been overlimed might have trouble taking up zinc. “Overliming is most common in
no-till and strip-till,” Ferrie notes. “It happens when farmers apply too much lime compared with how much they incorporate it. Two or three tons of lime per acre is fine if you incorporate it with tillage, but without tillage, the surface pH might become high enough to trigger a zinc deficiency.”
A surplus of a major nutrient can trigger problems with micronutrients, Ferrie says. With zinc, problems might occur in soils that are excessively high in phosphorus.
“One role of zinc is to move phosphorus from the root system into the plant,” Ferrie explains. “If the proper ratio of phosphorus to zinc is maintained, then the process works as it should. But if the ratio gets out of
balance, it leads to a higher concentration of phosphorus in the root system, which limits the ability of the plant to take up zinc, as well as sulfur.”
Fields where manure has been over-applied are candidates for zinc deficiency. “If farmers apply enough manure to supply all of the nitrogen needs, you may end up over-applying phosphorus,” Ferrie says.
If you suspect a zinc deficiency, there are three ways to confirm it:
- soil testing
- tissue testing
- visual symptoms
Soil testing might be the least effective. “With zinc and all other micronutrients, soil testing is subject to a range of possible errors,” Ferrie says. “To begin with, it only tells you whether the soil levels are low, medium or high.”
Soil testing laboratories also use different extraction procedures, which produce varying numbers. “If you want to track soil zinc levels over time, be sure to use the same laboratory for all your testing,” he advises.
Tissue analysis will reveal zinc deficiency in plants. “With corn, you need to do tissue testing early in the season,” Ferrie says. “A mild zinc deficiency will disappear by the time corn gets knee-high, having had its impact during the first three to five weeks of growth. Pulling a tissue test at pollination or ear fill won’t reveal that early season deficiency.”
Visual Symptoms. Identifying zinc deficiency symptoms is easier in corn than in other crops. “In corn, you usually see deficiency symptoms from emergence to knee-high,” Ferrie says. “Because zinc is not a mobile nutrient, look for symptoms in the new growth at the top of the plant.
“In new corn leaves, the symptoms begin as interveinal yellowing or white streaks. They are similar to symptoms of manganese and iron deficiency. But with iron and manganese, the yellow or white stripes run the full length of the leaf, while zinc stripes are primarily in the lower half.
“If a zinc deficiency continues to show up after corn is knee-high, it’s pretty severe,” Ferrie continues. “With severe deficiencies, the slight striping grows into wide bands between the midrib and the edge of the leaf. Normally, the symptoms disappear with age, but if they don’t, you will see a reddish caste in the area where the leaves turn white.”
In addition to leaf symptoms, zinc deficiency causes shortening of the
internodes. So shorter plants might be a clue to a zinc problem.
Zinc deficiency is less common in soybeans, where it reduces flowering and delays pod growth. Compared with corn, it is more difficult to diagnose. “In soybeans, a zinc deficiency has to be pretty severe before you can see symptoms,” Ferrie says.
With soybeans, symptoms include stunting of internodes and yellowing, which eventually turns brown,
between the leaf veins. The leaves will be a little crinkled. Both of these symptoms can be mistaken for a herbicide reaction.
In small grains such as wheat and barley, zinc deficiencies are hard to visually diagnose. In these crops, tissue analysis is very helpful.
In a rotation of soybeans, corn and small grain, and if you have a zinc deficiency in corn, pull tissue tests when the field is in wheat.
One good thing about zinc is that there are many ways to manage it and prevent deficiencies. “In environments where you know you have zinc deficiencies, you can put zinc in starter or broadcast fertilizer or you can foliar feed zinc,” Ferrie says.