By Darren Hefty
Zinc is likely the most frequently applied micronutrient. Many farmers we know use 1 quart of chelated zinc
in-furrow for corn. Is this enough zinc for your crop? Do I need zinc for other crops, too? Is there a cheaper way to feed the crop? Why is zinc so important?
Functions of Zinc
- Key for moisture absorption
- Important for flower formation
- Frost protection
- Germination and vigor
- Assists plant growth hormones
The Truth About Zinc
- If your 9% zinc chelate weighs 11 pounds per gallon, 1 quart of liquid contains just one-quarter pound of actual zinc. As you can see in the chart below, that may not be as much as you are removing from the field each year, and especially in a two-year period, corn followed by soybeans.
- Using liquid zinc in corn and then nothing the following year in soybeans is almost certainly depleting the soil if you are getting high yields.
- High levels of phosphorus can easily tie up your soil’s zinc.
- Zinc does not move much in soil.
How Much Zinc Do You Need?
Our typical recommended level on a 6” Midwest Labs soil test (using a DTPA extraction method) is 1.8 to 3.5 parts per million (ppm). With soils high in phosphorus, we’d shoot for no more than a 10:1 ratio of available phosphorus to zinc. For example, if the Bray P1 phosphorus level was 50 ppm, we’d recommend having at least 5 ppm of zinc. Here’s a list of three crops and how much zinc it takes to produce the grain and the stover. The amount in the Grain column leaves the field when the grain is hauled away.
Cheaper Way to Get Zinc
If you are on a one-year rental contract or if you just need fast uptake of zinc, using a liquid zinc product is great. 9% liquid zinc chelate is about $4 to $5 per acre for 1 quart, which delivers about 0.25 pounds of zinc. Even where our zinc levels are great, we still use products like Micro EX that contain low rates of zinc and other micronutrients. That way even when the ground is cold, we still get good levels in the plant early in the season. If you need to build your soil and increase by several parts per million, consider dry zinc sulfate (35.5% zinc and 17% sulfur) at a cost of about $0.75 per pound. On our farm, we use variable rate application to target areas in our fields that fail to meet the 10:1 phosphorus to zinc ratio and also have at least 1.8 ppm. We broadcast zinc sulfate in the fall and normally see it show up on the soil test by the next fall or earlier. Keep in mind that like phosphorus, zinc doesn’t leach in most soils, so as long as you prevent soil erosion, your zinc should be there until your crop eventually uses it.
This year, consider building your soil to get your zinc levels up. Grid or zone sample to set up variable rate application as the zinc level may vary greatly across your fields. Then, consider applying a low rate of a liquid product that contains zinc for quick, early availability in any crop. Use plant tissue analysis throughout the season to see if the zinc you applied got into the plant, and check your yields to see if additional zinc helped your bottom line.