Brown Co. KS normally has a monthly rainfall total of 4.8 inches for May. Month to date, we are sitting at 5.8 inches, above normal but not nearly as high as last years impressive total of 11.84 inches for the month. While it is not nearly as big of a concern as last year, nitrogen loss is still an issue that we want to discuss. But even more specific than “nitrogen loss”, we want to talk about the specific places that your nitrogen might be going, and conditions that might lead to those problems.
The three main reasons for nitrogen loss are leaching, denitrification, and volatilization. Other reasons can also include things like crop residue removal, runoff and erosion.
Leaching is a physical removal of nitrogen. In high rainfall situations, soluble NO3- is washed out of the root zone. This nitrogen will possibly reach the groundwater, or move via tile drainage to a nearby pond, lake, or stream. This is much more common in coarser textured soils but is still possible for our soils in Northeast Kansas.
Volatilization is one of our more common forms of nitrogen loss. This is the conversion of ammonium to ammonia, which is lost to the atmosphere. We can see a larger amount of volatilization when we apply anhydrous. If we aren’t getting it placed deep enough or sealing the slot behind the applicator, we can lose an increased amount to volatilization. Subsurface injections help reduce volatilization. Consequently, applications that occur on the surface like urea or manure will generally have higher rates of volatilization.
Our bigger concern this spring, and any wet spring, is denitrification. Denitrification is the conversion of nitrate to a gas form of N. When soils are wet, bacteria turn to nitrate for a source of oxygen, resulting in N2 gas. This will be common across all our soils if we have excessive rainfall. However, with the current amount of rain we have received we probably only need to focus on select spots in our fields. What areas of your field have a higher clay content and are usually the last ready to plant? What areas of your field have a higher water table or poor drainage? Do you have portions of your field that sit under water after a rain, or collect a lot of runoff from surrounding areas of the field like in terrace channels? These are the portions of the field to keep an eye out for. They will be much more susceptible to denitrification.
So what can you do?
First of all, it is very difficult to estimate the exact amount of N lost. This chart from UNL shows expected loss due to denitrification.
Your first step is to take some detailed observations of your field. Note the spots that are saturated and for how long. This will help you plan for a possible sidedress/rescue application in the future. Remember, the majority of nitrogen uptake by corn is during tasseling. This gives us time to make good observations and plans, but it is important to get the ball rolling now.
Soil tests are one way to track nitrogen in the soil throughout the growing season. We will be tracking soil nitrate from a specific location in several fields this growing season to track how nitrogen amounts change with rainfall and plant maturity. Testing will occur twice a month. Our first two samples show no level changes in the 0-12 inch depth or the 12-24 inch depth between the two sample dates. (First collection date was May 13th, second on May 26th) This series approach to soil test data should give us a better picture than a standalone soil sample will. Be on the lookout for more data soon.
Another way to check for nitrogen deficiency is to use a chlorophyll meter. Often times you will need to calibrate against a high rate N strip in the field as a way to compare against a crop that would be non limiting for nitrogen. A chlorophyll meter can detect changes much sooner than the naked eye. Similarly, using drone collected imagery and vegetative indices like NDVI or NDRE can also detect changes in leaf color before the naked eye can.
Tools like Granular Nitrogen track your nitrogen applications, soil type response, and rainfall to give you an up to date look at the nitrogen status in your fields. It can even use hybrid specific parameters to make the most informed calculation. This tool can point out locations where nitrogen deficits at VT may occur and will create Rx maps to help target applications.
Nitrogen loss can be hard to target. However, a broad range of tools are available to help you make some in season decisions. Talk to one of our agronomists today to get more info!
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