Zinc may be a micronutrient, but a lack of it can have a significant impact on your corn crop. Zinc plays a key role in chlorophyll production, carbohydrate metabolism and cell elongation, which impacts leaf size and early ear development.
In many cases, the soil test recommendations for nutrients are dated and haven’t been revised since the 1960s. But so much has changed in row crop production over the last 50 years. If your soil test says 2 ppm of zinc is sufficient for your corn crop, you should read that with skepticism. I've seen a corn crop respond positively to a zinc application even when the soil test said the 2 ppm zinc supply was adequate.
Spotting a Zinc Shortfall in Your Corn Crop
Zinc deficiencies in corn will normally appear as yellow, white or beige streaks between the veins of the uppermost leaves. Zinc deficiency is common to see in corn prior to V8, and by tassel, many deficiencies are no longer apparent because soil microbes have mineralized more zinc in the root zone. On soils with high phosphorus levels, the deficiency can become even more apparent.
When a deficiency results in symptoms we can see, some yield has probably already been lost. However, it’s not too late to make some corrections to this year’s crop by adding a zinc source to a broadcast herbicide or fungicide application. Leaves from the upper portion of the plant can be sampled and sent to the lab to determine if zinc deficiency is present. Collect a leaf from each of 15-20 plants, air dry in the shade, and wrap in paper prior to sending to lab. The youngest collared leaf is the best choice, since zinc isn’t mobile in plants and the uppermost leaves will have the lowest zinc levels.
Going Foliar with Zinc
In corn, there can be short-term benefits to correcting for deficiencies when a micronutrient is applied to crop foliage.
- Foliar-applied sources of zinc need to be water soluble.
- They should be applied to leaf surfaces in such a way as to minimize runoff; once runoff occurs, you’ll have soil-applied zinc - which won’t help this year’s crop.
- Zinc should create a greening effect within 5-10 days after application. This effect will only be on tissue that received the zinc application, so foliar coverage without runoff is best.
- A 20 gpa spray volume is a good goal.
Foliar sources of zinc generally come as a sulfate, lignosulfonate or a chelate form.
- All three of these forms are water soluble and readily release zinc to the plant.
- Zinc sulfate or zinc lignosulfonate forms will work best in a foliar application . The chelated forms will generally cost more and are more suitable for a soil application.
- Avoid zinc oxides and zinc sucrates. They are not water soluble, and the zinc won’t be available to this year’s crop.
- A foliar application should apply 0.5 to 1 lb zinc per acre in the late vegetative or early reproductive growth stages.
For future crops, a good strategy is to map deficiencies in this year’s crop and take corrective action with soil applied zinc at much greater application rates this fall or early next spring. Since zinc isn’t mobile in soils, a banded application is also a good practice.
Read this next: Micronutrients are Actually No Small Thing
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