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Don't neglect your corn's zinc needs

Jeff Caldwell 11/29/2010 @ 2:39pm Multimedia Editor for Agriculture.com and Successful Farming magazine.

When you think of nutrients important to your corn crop, zinc's probably not too high on the list.

But, if you're seeing yields drag, don't rule it out; research shows zinc is the most frequently deficient micronutrient in most corn fields, especially those in sandy or low-organic soils. And when you're short on zinc, the symptoms can persist from planting to harvest, ultimately leading to trimmed yields, according to Pioneer Hi-Bred agronomy information manager Steve Butzen.

"Of all micronutrients, zinc is the one most often deficient in corn production and most likely to elicit a yield response when applied as fertilizer," he says. "Fields showing zinc deficiency are seldom affected uniformly. Zinc deficiency symptoms may also vary from field to field, depending primarily on the timing and severity of the deficiency."

Starting out, if you're planting into cooler, wet soils, you can get any zinc deficiency off to a roaring start. Such conditions make any zinc present even tougher for the plant to absorb, and that can inhibit early root growth. Plant leaf color is a good indicator for whether or not you're facing a zinc shortage.

"Moderate deficiencies in seedlings may result in white to pale yellow longitudinal areas in the newest leaves that are usually more pronounced in the lower half of the leaf," Butzen says. "Severe deficiencies at this time may result in entire plants being pale yellow to white in color and stunted."

Later on, look for "interveinal chlorosis," or white or yellow streaks in leaves for signs of zinc deficiencies as the plant develops, typically in or around the V12 growth stage. If it's a moderate shortage, the streaks may be hit-and-miss. If the deficiency's more severe, the streaks will be more defined and present on both sides of the leaf.

IDing & treating zinc deficiencies

Identifying any potential zinc shortage can be done, Butzen says, via soil sampling or plant analysis. The former's usually the best option alone, but he recommends both to get the clearest picture. "Because soil tests for zinc are considered among the most reliable of the micronutrients, this method is recommended most often. Using both tests together can help to determine a firm recommendation for zinc application," he says.

Once you've got results, it's just as important to know how much supplemental zinc you need. When making this decision, it's also key to know the general soil characteristics, like pH, and any possible history the ground may have with a zinc shortage.

"If a deficiency is found, recommendations are generally to apply 1 to 2 pounds actual zinc per acre as a starter, or 5 to 10 pounds as a broadcast application. This assumes that a soluble inorganic form of zinc (e.g., zinc sulfate) is used in the starter and that soil pH is below 7," Butzen says. "If an organic chelate is used, rates (on a zinc content basis) may be one-fifth that of inorganic sources. To determine the pounds of actual zinc, multiply pounds of material by the percent actual or 'elemental' zinc in the material. For example, to apply 1 to 2 pounds of actual zinc when using zinc sulfate (33% zinc content) as the fertilizer source, 3 to 6 pounds of material should be applied."

Finally, picking the right type of zinc to apply is highly important. Butzen says zinc sulfate is typically the most cost-efficient and effective source regardless of whether applied as a granular or blended with solid or liquid fertilizer. But, that's not always the best choice.

"The form of zinc chosen often is dependent on how it will be applied. For band application with dry starter fertilizer at planting, zinc sulfate is used most often. However, many other zinc fertilizers including finely ground zinc oxide, also are used with dry starter," Butzen says. "If liquid starter is used, chelated zinc forms or zinc-ammonia complex easily can be included. For foliar applications, zinc sulfate or chelates are chosen most often."

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