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Field Notes: Soil moisture and nitrogen management

Angie Peltier, UMN Extension educator, Vasudha Sharma, Extension irrigation specialist and Fabián Fernández, Extension nutrient management specialist

Photo: Tom Peters, UMN Extension
The following information was provided during a 2023 Strategic Farming: Field Notes session. Read further to learn more about this free program that takes place each Wednesday morning throughout the growing season.

Soil moisture conditions

Soil moisture conditions throughout the state have tended toward the extremes in 2023. Heavy spring rains in much of southern Minnesota led to soils being too wet early on, and lack of spring rains in central Minnesota have led to very dry soil conditions.

One of the University of Minnesota’s irrigation research locations in Becker, MN (Sherburne County) received 3 inches less rain in May and the first week of June 2023 compared to the same period in what was a very dry 2022. At this location, the first 10 inches of soil is very dry, but there is moisture lower in the profile. 

After a crop’s stand is established, not only do the dry soil conditions favor roots growing deeper in the profile to mine water, crops can tolerate some early-season stress. Despite the lack of rainfall, at both the Becker and the Westport, MN (Pope County) UMN irrigation research locations, established crops have not yet been irrigated for these reasons. If hot, dry conditions continue, irrigation will need to take place in the next couple of weeks or producers risk the crop experiencing enough stress to impact yield potential.

In soils that were too wet early on, stand establishment can be reduced and waterlogged roots can reduce yield potential. For these reasons some farmers have replanted or at least gone through to break the surface crust. The moisture that is banked in the soil profile may be welcomed later in the growing season if it turns out to be a dry one again this year.

Deciding when to irrigate

The data generated by monitoring soil moisture sensors placed in irrigated fields helps in timing irrigation frequency and amount to one that is both most cost effective and beneficial for maximizing a crop’s yield potential and resiliency. As with any other input, there is a cost to irrigate, so crop producers are urged to dig a bit deeper into the soil profile than the top 6 inches to determine whether there is moisture in the profile that one could reasonably assume the crop will access.

Crop water demand changes throughout the growing season. In corn, the period of highest water demand is when the crop is getting close to tasseling (when there are more than 10 leaf collars); it is during this time that 0.7-1 inches of irrigation water (also depending on soil type and irrigation system capacity) is recommended when irrigation is warranted.

How are water tables looking after 3 years of very dry or droughty conditions in some areas of the state? 

We are unaware of news that water tables are declining in irrigated areas, outside of some of the groundwater management areas designated by the Minnesota Department of Natural Resources. Some of these areas have had declining water tables and stream water levels for some time, declining more in the last couple of years. While water tables in other irrigated regions can get depleted when irrigators start running, which was the case in 2021 and 2022 when well levels during the irrigation season declined substantially and had well interference issues. However, those sitting under sand are quickly replenished due to the quick infiltration of spring snow melt.

Very wet, very dry soils and nitrogen availability 

Both very wet soil conditions and very dry soil conditions can lead to issues regarding crop nitrogen availability. Corn plants in the earliest growth stages not only do not have a high water demand, they also do not have a high nutrient demand. Plants need nitrogen to be dissolved in water in order to take it up. So when there is very little soil moisture, there will be very little nitrogen available to be taken up. As the crop grows and develops, both water and nitrogen demand increase.

Very dry, sandy soils have a low potential for nitrogen loss. Any nitrogen that has been applied with fertilizers or mineralized by soil bacteria from organic matter - a form that plants are unable to take up - to nitrate, a form that plants can readily take up when dissolved in water and that is easily lost to the environment by leaching downward in the soil profile, particularly in sandy soils. UMN research has found that the majority of nitrogen lost from our farm fields is lost in early spring. This is because at the same time that we have no crop or a very young crop in the field, we often have excess water. The fact that it has been dry in sandy soils is actually a good thing because nitrogen has been retained in the soil.

In the more southern regions of the state where soils have been much wetter, nitrogen loss from fertilizers or mineralized from the organic matter had some potential for loss, but not as much as one might suspect. Spring 2023 was unseasonably cool, which slowed down the activity of the bacteria that catalyze the chemical reactions that convert ammonium into nitrate. This is important because nitrate can be lost by leaching or denitrification (conversion of nitrate by bacteria to a gas form that can be lost to the atmosphere in waterlogged soil). Much of the rain we had came fast and didn’t move deep in the profile, but rather ponded in low areas in the fields. So while there may have been some losses due to denitrification, there was likely very little substantial nitrogen lost through leaching early on this spring. Fall application of nitrogen provides the greatest risk of environmental losses during wet springs. This is especially true for fall urea applications. The research is pretty clear and consistent showing that fall urea applications are not a good practice; you simply have too much potential for nitrogen loss.

Nitrogen credits from irrigation water? 

In areas of the state with sandier soils that warrant irrigation, nitrogen losses occur when nitrate leaches into the water table. If one gets their irrigation water tested for nitrate content and there is nitrate present (more than 10 ppm), the applied nitrogen rate may be able to be adjusted lower to account for that already present in the water. The nitrogen credit from irrigation water means you can save money on nitrogen fertilizer costs.

Split-applying nitrogen

Applying nitrogen more than once for a particular growing season, or a split application, is typically either planned or unplanned. An unplanned sidedress application may occur because weather conditions resulted in too much of the applied nitrogen to be lost. A producer may decide ahead of time to reduce the chances of nitrogen losses to the environment by splitting their nitrogen application to provide a small amount to the crop early on and the rest between the four and eight leaf-collar growth stage (V4-V8). Waiting to either side- or top-dress nitrogen later than around V8 in the growing season does not add any additional benefit to what you might see with an earlier sidedress application, and in fact, you can actually reduce yield if there is not enough rain to move nitrogen from the later application into the root zone.

Pre side-dress nitrate test (PSNT)? 

The University of Minnesota Extension nutrient management team is currently working on a large-scale project testing PSNT as a nitrogen management tool. Preliminary results suggest that the PSNT is a useful tool, particularly in those fields and situations where one suspects that there is a lot of residual nitrate in the soil. Even if the test shows low concentrations that would indicate the need to apply additional nitrogen, make sure to look at the condition of the crop. For example, in low parts of the field where nitrogen loss through denitrification is most likely, if the crop is struggling because it was underwater, chances are that adding more nitrogen is not going to solve the problem. You can’t regain lost yield potential by throwing more nitrogen on the crop.

What about sulfur, loss situations and ammonium sulfate? 

Similar to mineralization of nitrogen, under favorable warm and moist soils, sulfur is mineralized by bacteria in the soil from a form that is not available for crop uptake (organic matter) to one that is (sulfate). Also, similar to nitrogen that can be leached once transformed to nitrate, the same is true once sulfur is converted to sulfate. As mentioned for nitrate, the wet conditions this year are unlikely a reason to worry too much about substantial sulfur loss. Even if the crop is showing some sulfur deficiency, it is likely going to be short-lived as roots are growing and soil conditions are favorable for mineralization.

However, if you need to apply sulfur, ammonium sulfate is a great source if you also need some nitrogen because it provides both nutrients, and unlike urea, nitrogen isn’t lost to the atmosphere through volatilization when left on the soil surface. However, ammonium sulfate application is best for small plants to decrease the risk of it getting trapped in the whorl and causing leaf burn.

Do biologicals add value? What about micronutrients? 

There are many native bacteria and fungi and considerable biological activity in Minnesota soils. Recent research results have shown that it is difficult to predict when and under which circumstances biological inputs may bring value to a crop and so commercially available biological products are not recommended at this time. Those crops that have a micronutrient deficiency will clearly reveal themselves through abnormal growth and development. Routine application of micronutrients is not recommended as this practice is unlikely to pay.

Do you want to attend Field Notes next week? 

This program will run from 8:00-8:30 a.m. on Wednesdays through the 2023 growing season. Topics will be announced the week of the program and may include issues related to soil fertility, agronomics, pest management, equipment, and more.

Learn more and register

Can’t make the live session? No problem. The discussion-based series will be posted immediately following the webinar to your favorite podcast-streaming service to listen at your convenience. Listen here online.

Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.

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