The change in weather this spring has allowed for earlier fieldwork to begin. Questions that come to mind include what kind of tillage to do and how do these condition affect a nitrogen-management program for corn.
Last fall tillage was difficult at best because of the dry conditions. Large, hard clods were created. It looks like the moisture we got this winter did mellow the clods enough that a tillage operation will be able to create a good seedbed this spring. One thing to keep in mind: Unless we get a large amount of rain this spring, do not over-till fields. Tillage will dry the soil and cause a dry seedbed. This will cause problems with plant stands and thus affect grain yield even if we receive adequate amounts of moisture during the growing season.
With field conditions rapidly improving there likely will be nitrogen (N) applicators getting into fields. What about the N-management problem for corn production in 2012?
It's true that field conditions were less than optimum for N application last fall. The dry soil conditions made it difficult for fall anhydrous ammonia application and the incorporation of urea.
Will you need to make a supplemental N application this spring? At this point there is no good way to assess the amount of N in the soil. Soil sampling for soil nitrate-N at this time is unreliable and will likely not give an accurate picture of what is still contained within the soil. The soils have started to warm. At this point in time (March 18), we would not expect a large amount of conversion of ammonium to nitrate within the soil if the fall applications were applied according to NBMPs, such as application when soil temperatures are less than 50° F and the anhydrous band was sealed and did not volatilize out of the soil after application. The best option at this point would be to wait and plan on a sidedress application. This sidedress application would be planned based on the supplemental N application decision tool developed by the University of Minnesota. Check www.extension.umn.edu/nutrient-management for more information.
I did not apply N last fall. Should I be applying N this early?
Soil conditions do appear to be ideal for application but there still is some risk associated with application this early in the season. First, if you have sandy soils you should not consider early application. Nitrogen applied as part of the starter and a sidedress application is the best practice on sandy soils.
For heavy-textured soils that did not have a fall N application, a grower should consider using a soil nitrate-N soil test for prediction of N application rate. Since we had a dry fall and winter, there is good chance that there is a significant amount of soil nitrate left in the soil that the corn crop could use. This is particularly true if the previous crop was corn or small grains. If you are west of highway 71, the soil sample for nitrate-N can be taken anytime before planting. This soil sample should be to a depth of 2 ft. and analyzed for nitrate-N in pounds per acre. Sixty percent of this soil test value will be used as a credit on the corn N guideline. If you live east of highway 71, it is recommended that you take the soil sample to a depth of 2 ft. close to planting time or just at planting. The result should be reported in parts per million (ppm). The soil test value will be used to determine the adjustment for the suggested nitrogen rate. For more details on the use a soil test nitrate-N value, read "Fertilizing Corn in Minnesota" (pdf).
For heavier-textured soils, the risk of loss is less for spring N application compared to fall N application. But if optimum conditions exist for conversion of ammonium to nitrate, there still could be a serious risk for N loss if heavy rainfall events occur in April or May. The majority of the crop uptake tends to occur after the V5 growth stage or when the plant is about a foot tall. Having most of N in nitrate form when the plant is not actively growing does present some risk. While the weather is warm it is hard to tell what may occur a month of more later.
Thinking of an N inhibitor?
An N inhibitor may be considered if there is some worry about N loss from early application either through tile drainage or denitrification. Research in Minnesota has shown a benefit from inhibitors when used with spring-applied N. Two chemicals that have been shown to slow nitrification are Nitrapyrin, the active ingredient in N-serve, and Instinct, or DCD. These are the only two inhibitors shown to consistently slow the conversion of ammonium to nitrate. The length of these chemicals' effectiveness is governed by soil moisture and temperature. This is because Nitrapyrin and DCD reduce the microbial populations of the bacteria that convert ammonium to nitrate.
If the temperatures are warm and the soil is dry, there is concern of urea volatilize before it can convert to ammonium. Agrotain is a product used to slow the activity of urease, which converts urea to ammonia, and the best use for this product is when urea is surface applied and not incorporated. Normally it takes an average of 0.25 in. of rainfall to effectively incorporate urea into the soil. Agrotain extends the amount of time for this to occur.
In summary, keep your tillage operations few in number and shallow to reduce soil moisture loss. If you made fall N applications and are concerned about how much N is left, you should seriously consider using the supplemental N decision tool to time a sidedress N application. If you did not apply N in the fall on heavier textured soils, you may want to consider using a soil test for nitrate this spring. If you have sandy soils, follow established best management practices for nitrogen application. These practices include side-dress application of N. Finally, the use of a nitrification inhibitor or urease inhibitor will depend on the local soil situations and the weather this spring.