Bt Steve Culman, Peter Thomison et.al.
The persistent rain this year may force many growers to sidedress their nitrogen in corn much later than what is considered normal. Other growers may be supplementing their earlier N applications to replace N lost from denitrification and leaching. The following are some suggestions based on common questions we’ve been hearing.
Do I need additional N?
Nitrogen is one the most dynamic crop nutrients in the soil and has many pathways for loss. It’s leaky nature plus the fact that crops need it in such large quantities makes the task of knowing exactly how much N to apply very challenging. The excessive water this spring has clearly driven losses in many fields, but how much? Recent research at Ohio State has shown that ear leaf N, soil nitrate and grain yields were significantly reduced after just 2 days of standing water in the field. So N losses can occur quickly with excessive water.
What tools can help me determine if I need additional N?
A perfect indicator of N need does not exist, but some tools can help. Crop sensing tools like NDVI meters or crop sensing aerial imagery can provide insight if they are used routinely. Soil-based tests can also monitor N availability. Soil nitrate is the most widely-available and vetted test. A value of 25 ppm or higher indicates that there is sufficient N. More information can be found here: https://agcrops.osu.edu/newsletter/corn-newsletter/2017-20/manure-psnt-and-n-recommendations Ongoing research at Ohio State is looking to develop soil health indicators that can provide insight into how much N will be available over the growing season.
How late can N be applied?
Corn typically takes up less than 1 pound/ acre of N before the V4 stage, but N uptake rates will increase dramatically through tasseling. N uptake does continue beyond tasseling and into grain fill, but at much lower rates. Research at Ohio State and Purdue has shown that if sidedress applications are not made due to saturated conditions, rescue N fertilizer applications can increase yields and reduce the negative impact of flooding. Note these responses are much more likely to occur in fields that had high N loss conditions (excessive water).
How much N should be applied?
This is a difficult question to answer, it’s important to keep in mind that yield potential of corn can be severely restricted by excessive stress in the early phases. But corn that has simply grown too tall to sidedress might not have been severely stressed and yield potential could still be good. The potential of N loss and the extent of stress should be considered when determining N rates. It’s also important to consider the likelihood of economic return to invested N fertilizer. This economic model is used to maximize farmer profitability: http://go.osu.edu/corn-n-rate
What is the best N source to use?
This choice will likely be driven by application equipment, but best practices for minimizing N losses should be considered and practiced if at all possible. Consider that N losses can increase as the growing season progresses and soil and air temperatures rise. For example, if broadcasting with urea, consider a stabilizer such as Agrotain to minimize volatilization losses.