By: Brad Carlson, Extension educator & Fabian Fernandez, Extension nitrogen management specialist
This year’s planting season went off without a hitch for most farmers in Minnesota. With abnormally dry conditions, most of the state’s corn was in the ground by early May. However, cool weather has led to delayed emergence and slow growth. Observation shows that very little field work has occurred in the last couple of weeks as farmers wait for the weather to warm up and crops to grow. The trend over the past few years towards split application of nitrogen (N) fertilizer means that there are still significant amounts of N to be applied.
Some who split-apply N believe it is better to “spoon-feed” the crop. In reality, this is not necessary, as needed nitrogen is either present or it isn’t. In other words, if you applied N fertilizer pre-plant and it wasn’t lost (thanks to recent dry conditions), it is likely still available. This means it doesn’t matter if you applied all of your N fertilizer pre-plant or if you are planning on split-applying.
So why split apply? First, if conditions (soil and/or weather) are right for N loss, split-applying helps minimize the risk of N loss. Additionally, a split system allows one to adjust rates based on conditions. For example, if there is heavy rainfall early in the growing season, it might be a good idea to sidedress a higher rate of N fertilizer in June.
This year, there has been an overall deficit of soil moisture, meaning that most of the N applied last fall or this spring, as well as some carryover N from the previous growing season, is still there. While it has been a bit wetter lately, the risk of loss is still relatively low. This is because most soil (other than a few isolated spots) has not reached saturation – the point necessary to drive leaching and denitrification. In fact, just last week while collecting soil samples down to 3 feet in Southwest Minnesota, the recent rain was all retained within the top foot or so of the soil profile and the tiles had no flow. Furthermore, denitrification is temperature-dependent. Cool conditions slow denitrification rates, meaning loss in this way has been minimal, even is some fields where there was some ponding.
Based on all of these factors, N rates probably do not need to be adjusted either up or down this year. If you are waiting to apply sidedress N until corn reaches V4-V8, this is probably not necessary. In most situations, this is only a couple of weeks away. The risk of losing N applied today in the next two weeks is very minimal. The soil has substantial capacity to retain rain right now and very soon the crop will start to use a lot of the moisture in the soil, creating more water storage capacity in the soil. So if field conditions are good, now is a good time for a sidedress application.
A few final notes of caution
If you are applying liquid N fertilizer products, avoid direct contact with small plants. If you are planning to use inhibitors, remember what they are used for. Nitrification inhibitors rarely pay off during sidedress time unless it is excessively wet. Since the potential for nitrogen loss, either through leaching or denitrification, is very low this year, a nitrification inhibitor is not necessary. Urease inhibitors can be important for urea-containing fertilizers that are broadcast and left on the soil surface for a few days, especially where there is a lot of crop residue. A urease inhibitor will keep urea from volatilizing for a while, and in that time frame hopefully there is at least a quarter inch of rain to move the fertilizer into the soil.
Listen to a recording of a June 2, 2021 Field Notes webinar on sidedressing nitrogen in corn featuring four U of M experts: z.umn.edu/6xgb
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Support for Minnesota Crop News nutrient management blog posts is provided in part by the Agricultural Fertilizer Research & Education Council (AFREC).
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