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Fall nitrogen fertilizer application: The what, where, when and how

By: Fabian Fernandez, Extension nitrogen management specialist Crop harvest is progressing quickly in Minnesota due to the dry conditions. As soon as the crops are out, it is time to start field activities to prepare fields for next year. One of these activities might include nitrogen fertilization. What to apply? In my recent blog post on nitrogen best management practices , I discussed the fact that, for fall applications, the only source that is reliable enough is anhydrous ammonia. This is because all other commercial nitrogen fertilizers have a high potential for loss when applied so far in advance of crop uptake. The reason anhydrous ammonia works for fall applications is that, when ammonia (NH3) is injected, it disperses in the soil, reacting with soil moisture (H2O) within a retention zone, which, depending on soil moisture, can be around three to six inches in diameter. As NH3 reacts with H2O, it forms NH4 and OH-. The OH- creates a temporary increase in soil pH that inhibits
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No P, no problem? Skipping phosphorus fertilizer application may make agronomic, economic sense

By: Daniel Kaiser, Extension soil fertility specialist With fluctuating fertilizer prices, it is a good time to review a few principles related to phosphorus (P) availability in soils. We know that P is a critical element required for crop growth and development, and if deficient, the lack of P can significantly reduce crop yield. While application of P fertilizer is common, it is not always required to achieve maximum yield.   Phosphorus exists in the soil in many forms, which vary in crop availability. In fact, crop available P only represents a small fraction of the total P in the soil. When crops take up P, nutrients in less available pools in the soil can move into a more available form, replacing what was taken up by the crop. Crop producers sometimes feel that it is most economical to re-supply the soil with the amount of phosphorus taken up by the crop. However, the soil supply of phosphorus is more complex, and soils testing Very High in phosphorus may be able to fully supply
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Ragweeds seed samples requested for herbicide resistance screening

Datta Chiruvelli, Graduate student; and Debalin Sarangi, Extension weed scientist Herbicide-resistant weeds are causing crop yield loss and reducing farm profitability in Minnesota. Confirming the presence of herbicide-resistant weed populations will help the farmers to plan for an effective weed management strategy. Currently, we are screening suspected herbicide-resistant waterhemp populations collected from corn, soybean, and sugar beet fields in Minnesota in 2020 and 2021.  Giant and common ragweeds are also troublesome weeds in row crop production fields in Minnesota (Photo 1). Previous research showed that giant and common ragweeds resistant to FirstRate (ALS inhibitor, Group 2) and glyphosate (EPSPS inhibitor, Group 9) are present in Minnesota. However, the distribution of herbicide-resistant ragweeds and the development of any new resistance cases are unknown. Photo 1. Giant ragweed (left) and common ragweed (right) plants. Previous research showed that giant and common ragweed
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Maximize your cover crop benefits by selecting the best species

Eric Yu, Graduate research assistant; Debalin Sarangi, Extension weed specialist; Liz Stahl, Extension educator – crops; and Axel Garcia y Garcia, Sustainable cropping systems specialist Photo 1. Cereal rye cover crop (plant residue in between the soybean rows) planted in late September and terminated in early May of the following year at SWROC near Lamberton, MN. When planting cover crops in the fall, it's important to identify your goals for using a cover crop in rotation. Your expected outcomes will influence the cover crop species selection, seeding rate, planting date, and termination timing.  Cover crops may increase soil organic matter, improve soil health, reduce erosion, suppress weed growth, scavenge residual nitrogen, and conserve soil moisture. Although many cover crop species provide these benefits, some species or mixes are better than others depending on the geographic location and objectives (Table 1). This article mostly focuses on cereal rye and oats, two fall-pl
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Updated soil management resources available

University of Minnesota Extension has updated many useful soil management resources. Whether you enjoy listening to podcasts, watching webinars, or reading in-depth articles, we're sure to have it all. Publications Soil organic matter does matter Soil Organic Matter Does Matter is an upper Midwest resource for producers, beginning-level college students, and agronomic personnel who are interested in understanding the role organic matter plays in our agricultural soils. z.umn.edu/SOMpub Upper Midwest Tillage Guide Upper Midwest Tillage Guide is a regional resource for producers and agronomic personnel who are interested in reducing tillage, but who may not feel comfortable choosing the best options for their specific operation. The guide lays out the benefits of various equipment types and tillage options and is conveniently broken into four chapters that may be read consecutively or individually. z.umn.edu/TillageGuide Upper Midwest Compaction Guide Upper Midwest Compaction Gu
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Scout for tar spot of corn

Dean Malvick, Extension plant pathologist Tar spot at low level of infection. Photo: Dean Malvick Tar spot of corn has continued to develop in many fields across southeastern and into central Minnesota. Fortunately, with the exception of some areas in southeastern MN, tar spot has been detected at low levels that are not affecting yields. In the next week or two is a great time to scout for tar spot to determine where this disease has spread. Based on observations in many states and fields, the fields at greatest risk of yield loss due to tar spot are those fields where rains have been frequent and tar spot has developed in previous years.  Yes, tar spot can spread between fields. However, yield loss often occurs in fields where the disease starts to develop in July, which are also often those fields where tar spot has developed previously. Thus, it is important to determine when and where tar spot is spreading. The map of known distribution of tar spot in MN can be found at thi
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‘Stick to what you know for sure’: High fertilizer prices call for greater awareness of nitrogen BMPs

By: Fabian Fernandez, Extension nitrogen management specialist With high fertilizer prices, farmers are looking for alternatives to make every pound of nitrogen (N) fertilizer count. To be as efficient as possible with nitrogen, the most important thing to do is to rely on best management practices (BMPs) that have been proven through years of unbiased research. While the discussion is often centered around nitrogen rate, it is important to recognize that nitrogen BMPs encompass much more than just rate. Applying the correct rate of nitrogen while being careless with which nitrogen source you use, fertilizer placement, or timing can lower your nitrogen use efficiency, and therefore your profitability, especially when fertilizer prices are high. In fact, the entire database that we use to calculate the maximum return to nitrogen (MRTN) rate was generated with research trials that use several N rates applied using BMPs. For example, there are no data points from fall nitrogen applicati
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