Plant and soil test reveal zinc level
As we round the corner on winter and head into spring, there are many final decisions that growers need to make to prepare for the upcoming season.
One of those decisions is a fertility program that will maximize yields and profitability. An important nutrient I have been seeing deficient in Minnesota fields in the last couple of years is zinc, an element used by crops in small quantities.
• Zinc deficiency may be induced by cold, wet soil conditions.
• Soil test will best determine if there is a zinc deficiency.
• For corn, sample the ear leaf at initial silking.
As a micronutrient, zinc is the one most often deficient in corn production and most likely to provide a yield response when applied as fertilizer.
Zinc deficiency symptoms
Cornfields showing zinc deficiency are seldom affected uniformly. Zinc deficiency symptoms also may vary from field to field, depending primarily on the timing and severity of the deficiency.
Moderate deficiencies may result in interveinal chlorosis (white or yellow) in the newest growth. Areas of the leaf near the stalk may develop a general white to yellow discoloration.
Very early zinc deficiency may be induced by cold, wet soil conditions that limit corn root growth and available zinc. In such cases, zinc deficiency may be exhibited on early leaves, but not on the later leaves that develop when the soil begins supplying and the roots begin extracting more zinc.
As plants grow beyond the seedling stage, the demand for zinc becomes greater and deficient soils may be unable to supply the need.
Soil and plant analysis
Both soil and plant analysis can be used to determine if zinc application is needed. Because soil tests for zinc are considered among the most reliable, this method is most often recommended. However, using both tests together can help to arrive at a firm recommendation for zinc application.
Soil test results will be reported in parts per million zinc. If a deficiency is found, recommendations are generally to apply one to 2 pounds actual zinc per acre as starter, or 5 to 10 pounds as a broadcast application.
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), 3 to 6 pounds of material should be applied.
The standard plant sampling technique for corn is to sample the ear leaf at initial silking.
Tissue test results will be reported as ppm zinc contained in the plant tissue. Recommendations will vary somewhat by state. Generally, within the range of 20 to 70 ppm, zinc content is considered sufficient; values of over 300 ppm are considered toxic.
However, because sufficiency levels vary by state, obtain and follow your local Extension recommendations.
The addition of zinc fertilizer to starter fertilizer banded 2 inches to the side of and 2 inches below the seed at planting is the most common approach to zinc fertilization of corn. This method places the nutrient near the plant roots for immediate uptake as seedling development begins.
If starter fertilizer is not used, or if growers wish to add enough zinc for several years of crops, then zinc fertilizers are typically broadcast and incorporated before planting.
Larson is a Pioneer area agronomist based in North Dakota. E-mail is firstname.lastname@example.org.
This article published in the February, 2011 edition of THE FARMER.