Is It Time For More Nitrogen? 

The world of nitrogen application in agriculture is changing very quickly. The number of tools to assess the amount of N available to crops and the options for making late-season applications are growing tremendously while the environmental pressure is completely different and escalating. To add to that many of today’s hybrids respond more strongly to late-season N than previous hybrids, and it’s no surprise the way farmers approach their N programs is evolving rapidly. 

Sometimes late season new is planned, while other times poor weather pushed back applications so it is more of a “rescue” approach.

Late season N has its place. It is important to take care of corn on the front end as well as the backend of the growing season. You do not want your corn to run out of N towards the end of the season. This goes with the saying “never let corn have a bad day”.

Tissue samples, nitrate tests, aerial imagery, including satellites, planes and drones, soil mineralizable, nitrogen sensor technology, chlorophyll readers, and computer modeling like FFT are some of the tools that can be used to determine how much N is available for the crop and by extension, how much N should be applied. At the same time, be sure to scout the trouble spots. Scouting is still important because N deficiency has some look-alikes in today’s tools. Those look a likes include sulfur deficiency and saturated soils, among others. 

Planned programs that automatically build in a late application are good for fields that are at high risk of running short of N. High-risk fields often have heavy clay soils, sands or highly variable soil with poor drainage. 

Fields with high clay content soils run into trouble after heavy rains, when the clay hangs onto the water, leading to denitrification. On the other end of the spectrum, sandy soils are at high risk for leaching, especially during heavy rain+ events. 

Late-season N applications can be made with fertigation through center pivot systems. This is especially effective with sandy soils. Putting 20 lb. of N on every other turn of the pivot will make a big difference.

With a planned program, N applications are often staggered to give corn a steady dose of N. For instance, put a portion of your N down pre-emergent, sidedress another application at V5 to V7, and add a last dose between V10 and tasseling. The later the final application is made, the more N needs to be put on in the front end versus the back end. 

A well-rounded N program begins with split-applying N throughout the growing season. This planned approach should be on a field-by-field basis. When farmers split apply N, they pick up 10% efficiency by applying closer to the time when the plant will use the nutrients, making it easier to manage loss. The key to splitting N applications is to sidedress green corn.

Many often like to have half to two-thirds of N up front to get through the vegetative stages, then sidedress for ear fill. When corn shows a strong visual response right after sidedressing, this is a sign there wasn’t enough N for the vegetative stages and growth was slowed. In an ideal program, you’d sidedress dark green corn that doesn’t need that supply of N for weeks out. We always want corn to have a good day, and we always want the plant to have all the N it needs.

Planning a variable-rate nitrogen program should begin after harvest. Use yield maps to pick out high-risk areas within management zones. Yield zones and management zones, you created after years of collecting data for each field make variable-rate N possible and effective. Using variable-rate N can have a cost savings of around 8% compared to flat rate applications.

Soil nitrate testing results can be made into a map. The samples for this map should be taken about four days before the planned application. The nitrate test should be your baseline, but depending on cost and the turnaround time, you can also get the lab to measure the ammonium level in the soil. Nitrate and ammonium are the two mineral forms of N the plant is able to uptake and use. Tissue tests can also be useful by providing a snapshot of the concentration of N in the plant. 

To figure out the supply and demand of N in the field, consider things like: soil availability, plant uptake, hybrid, planting population, stand counts and past field performance.

A rescue program can be used with fields that only occasionally run out of N. These are often loam soils with good drainage and water holding capacity. In cases like this, N loss is minimal and the additional application cost of a late season dose might be hard to pay for. 
If you only encounter a rescue situation, one of every five years or even one in every 10 years, you’re better off staying with your existing N program and monitoring the crop for problems. A big rain or a prolonged wet season can create the need for another dose of N. A timely response will save the most bushels.

Keep in mind nitrates move up and down with water, especially after a rain or irrigation. Also, the plant can be pick up as much of 10 lb. of N per day from the soil.

To see what tools are available to help you plan your nitrogen applications contact your local Centra Sota Crop Advisor.

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