By Daniel Kaiser and John Lamb
Extension Soil Fertility Specialists
Many of our earlier planted fields in Minnesota have been exhibiting some significant variation in plant growth and yellowing this spring. Our conditions in May and early June have been less than favorable for corn growth and for the release of nutrients from organic matter. Due to the heavy rains nitrogen loss is being increasingly questioned and the decision of whether to side-dress or not will need to be made sooner or later. There are a few considerations to make when deciding if more nitrogen should be applied.
Second, some of the issues currently seen with corn could be due to a lack of oxygen to the roots. Oxygen is needed for normal root development and for efficient uptake of nutrients by the roots. Oxygen levels will be depleted in flooded soils and foliar symptoms can be exhibited that may look similar to some nutrient deficiencies. If problems within fields are due to lack of oxygen there is not much that can be done other than wait until conditions improve. As temperatures increase hopefully the appearance of the crop may improve.
Third, do not discount other sources of nutrient deficiencies. Specifically sulfur deficiencies may also show up on plants. While we have no direct evidence of widespread sulfur deficiencies, conditions are somewhat similar to what was seen in early 2009 where soils were wet and temperatures were cool. Some of our largest responses to sulfur came in 2009. If sulfur was applied and the crop still looks deficient the problem may be associated with lack of uptake and no fertilizer is likely needed. An exception would elemental sulfur applications which require higher soil temperatures for oxidization. It is likely that very little of the elemental sulfur applied last fall would have oxidized at this time. Sulfur can still be applied as an early side-dress if a deficiency is expected around the V5 growth stage. Dry fertilizer sources of sulfate sulfur can be broadcast applied at low rates. In most instances 10 lbs of S per acre should be adequate for an in-season application. Some leaf burning may occur but generally has not been found to reduce yields. Liquid sources containing thiosulfate should not be sprayed over the top of growing corn or severe crop injury may results. Coulter injection or dribbling ammonium thiosulfate is the best method for application.
How do I assess the amount of N that is available to the corn plant?
There are really only two tools left at this time of the growing season to determine whether to apply more N to a growing corn crop under non-irrigated conditions. The first is the pre-side dress nitrate-N test. This soil test was developed at Iowa State University in the 1990's. The soil test was for a sample taken to a depth of one foot. In Iowa, the researchers were able to calibrate it to an amount of N to apply. Similar research was conducted in Minnesota on many sites. A good calibration could not be developed in Minnesota. The only interpretation in Minnesota from the pre-sidedress N test is if the nitrate-N concentration is greater than 25 ppm then you do not need to apply extra N to the crop. This tool can not be used to determine the amount of N fertilizer to apply!
The second tool is the supplemental N decision tool. It can be found at:
This simple worksheet was developed in 1992 and has been modified and tested over the years as a means of helping people decide if supplemental, or extra, N is needed. This decision aid is for situations when all of the N fertilizer was applied pre-plant, either in the fall or spring. It was not developed for determining N rates in a split N program. Keep in mind that good judgment is still important when using this decision aid. The worksheet should be used in June while you have side dress application options available. The worksheet outcome is based on the answers to three questions. Each answer is weighted on how it affected nitrogen in the soil.
--Question 1. When was the N applied? The more points the greater the chances of N fertilizer loss. Nitrogen fertilizer applied in the fall when soil temperatures were higher than 50 degrees has a greater chance for loss than a spring application of N.
--Question 2. What was the predominant spring (May) soil condition? The wetter the soil conditions are the greater the score. It takes into account if the soil is dry, moist, or if water has been standing. The more water in the soil the greater the score.
--Question 3. How does the crop look? The more stress the crop is showing the greater the score. The stress is evaluated by the color of the corn and height.
With a score of 7 points or less, your current nitrogen program is doing fine. With a score of 10 or more, supplemental fertilizer is recommended at a rate of 40 to 70 lbs of N per acre, depending on the situation. In most cases 40 to 50 lbs N per acre is plenty. A score of 8 or 9 falls into a gray area and it is recommended that you recalculate the worksheet in a week - the corn height/color will most likely change. The "re-evaluation" option is only viable as long as you have side-dressing options.
The use of the U of MN Supplemental Nitrogen Worksheet for Corn is a useful tool to determine if there is a need for addition N application to corn. If addition N is needed, 40 to 50 lb N per acre will do the job.