John Kruse | 8/8/2019 5:34:16 PM
Figure 1. Apparent zinc deficiency in young corn. Note interveinal striping. As the condition worsens, striping may appear white and become broader.
Several producers and consultants have contacted me this spring with photographs and reports of yellow-striped corn in the two to three leaf stage. ]
In many instances, these symptoms appear to be zinc deficiency, and what is so interesting is how widespread it was in the corn planting areas of Louisiana. Zinc is a trace element, meaning the corn plant does not require very much of it (compared to nitrogen or potassium), but it is very much needed in small amounts, and the lack of it can result in measurable yield reductions. Zinc is absorbed by the plant as a positively charged ion (cation: Zn2+), and is important in the synthesis of tryptophan – a building block of certain proteins that are needed for the production of auxins (growth hormones). Zinc is generally more available in acid soils and less available in neutral to alkaline soils.
Zinc can also react with phosphate to the point that it is bound up and less available to the plant. Many soils in the Red River Valley, with high pH and sometimes high phosphorus, often need supplemental applications of zinc to optimize yields.
However, apparent zinc deficiency has been observed on the Macon Ridge (generally acid soils) and in the Delta (generally slightly acidic soils) this year, as well. The causes can be varied, but certainly repeated corn production can result in less than ideal soil levels of zinc. Also, if a producer has historically planted cotton and/or soybeans for a number of years and has not had to pay close attention to zinc levels, a switch to corn may reveal the need for supplemental zinc. If soil test zinc is less than 1 ppm, supplemental zinc should be applied.
If soil test zinc is between 1 and 3 ppm, it may be needed, and if it is above 3 ppm it should not need to be applied under most circumstances. An ideal time to apply zinc is at planting in a band across the surface of the planting zone.
Recent research suggests between 2.5 to 5 pounds of actual zinc per acre is a good rate. If it is too late for a zinc application at planting, a second choice would be to include the zinc in the nitrogen sidedress application at 2.5 lbs per acre actual zinc. If a foliar application is desired, apply 0.1 to 0.25 pounds of actual zinc per acre in 20 gallons of solution.
The high volume of water is needed to prevent foliar burn. Repeat this application 10 to 14 days later, if possible. Mixing zinc with phosphorus fertilizer is not recommended due to the potential for nutrient binding. Chelated zinc, particularly EDTA-chelated zinc is a very good source of zinc. Zinc sulfate granules can also be dissolved in solution and applied as a spray.
Spreading granular zinc is not an ideal method due to the fact that such a small amount is spread over such a large area that many corn plants will not come into contact with it. Zinc oxides are relatively insoluble and slow to break down and become available, and are not recommended sources of zinc.
A consultant recently asked if grain sorghum needs supplemental zinc. It turns out little research has been done in this area, but several University-authored sorghum production manuals did not emphasize sensitivity to zinc as a major issue. Texas producers are cautioned to maintain optimal levels of iron (Fe) in grain sorghum due to the nature of their soils.