John Breker

Soil Sampling for Nitrogen in a Delayed Spring

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Spring planting is clipping along in some parts of the region, while other parts are still waiting to hit the field, as excessive rainfall and cold temperatures have delayed spring field work and planting. Who would have thought last fall that this is what spring 2022 would look like, after the worst region-wide drought in 30 years? Mother Nature always reminds us to stay prepared for anything.

A delayed spring start means that every day in the field is important. AGVISE delivers next-day turnaround on processing soil samples. The soil samples are analyzed and reported the next business day after arrival at the laboratory. Soil test results are posted to our online AGVISOR portal for quick and easy access. If you need any soil sampling supplies for spring, please let us know and we will send them to you right away.

So, what is the best strategy for spring soil testing and assessing soil nitrogen losses after the rain? The compressed fertilizer and planting window might not leave enough time to adjust preplant fertilizer rates, especially if the field is just barely dry enough to plant. If soil nitrogen losses have occurred following spring rains, a spring soil test collected now will be helpful to create a split-applied nitrogen plan or to direct a supplemental nitrogen application later. In the AGVISE Spring 2022 Newsletter, we answered some questions on split-applied nitrogen application strategies, so please take a look at those options for applying nitrogen during the growing season.

Short-season crops develop quickly, so additional nitrogen should be applied in the upcoming weeks. A soil sample collected before or shortly after planting will provide the best assessment of preplant soil nitrogen supply and losses. Do not wait too long to collect the soil sample because, as we move into June, plant nitrogen uptake and nitrogen mineralization from soil organic matter will make the soil nitrogen result more difficult to decipher. To maximize yield in small grains, apply all topdress nitrogen before jointing (5-leaf stage). Any nitrogen applied after jointing will mostly go to grain protein. In canola, apply nitrogen during the rosette stage, before the 6-leaf stage.

Long-season crops like corn offer more flexibility and time for in-season soil sampling and nitrogen application. Rapid nitrogen uptake in corn does not begin until after the V6 growth stage. The Pre-sidedress Soil Nitrate Test (PSNT) can help you decide the appropriate sidedress nitrogen rate. For more details, take a look at the PSNT article link for instructions on collecting and submitting PSNT soil samples. The PSNT requires a 0-12 inch depth soil sample taken when corn plants are 6 to 12 inches tall (at the whorl), usually in late May or early June. Late-planted corn may not reach that height before mid-June, but PSNT soil samples should still be collected during the first two weeks of June. If spring rainfall was above normal, Iowa State University guidelines provide additional PSNT interpretation criteria for excessive rainfall, manured soils, and corn after alfalfa.

If you have any questions on the best strategies for spring soil sampling and in-season nitrogen application options, please call our technical support team and we will be happy to answer any questions you may have.

John Breker

Sidedress Nitrogen for Corn Using the Pre-Sidedress Soil Nitrate Test (PSNT)

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As the corn crop begins to emerge, it is time to prepare for sidedress nitrogen applications. Sidedress nitrogen for corn can be applied any time after planting, but the target application window is generally between growth stages V4 and V8 before rapid plant nitrogen uptake occurs. Split nitrogen applications have become a standard practice in corn to reduce in-season nitrogen losses on vulnerable soils, such as sandy and clayey soils. More and more farmers now include topdress or sidedress nitrogen as part of their standard nitrogen management plan. These farmers have witnessed too many years of high in-season nitrogen losses through nitrate leaching or denitrification.

The target timing for PSNT sampling is when corn is 6 to 12 inches tall at the whorl, often near V4 or V5 growth stage (pictured above). Do not hesitate in collecting soil samples for the PSNT; the target window for sidedress nitrogen applications in corn is between the V4 and V8 growth stages.

Whether your nitrogen management plan includes a planned sidedress nitrogen application, or if this is now your first opportunity to apply nitrogen to corn this spring, the Pre-Sidedress Soil Nitrate Test (PSNT) is one tool to help make decisions about in-season nitrogen. You may also hear this called the Late-Spring Soil Nitrate Test (LSNT) in Iowa. PSNT is an in-season soil nitrate test taken during the early growing season to determine if additional nitrogen fertilizer is needed. PSNT helps assess available soil nitrate-nitrogen prior to rapid plant nitrogen uptake and the likelihood of crop yield response to additional nitrogen.

The PSNT can also help you decide the appropriate sidedress nitrogen rate. The PSNT requires a 0-12 inch depth soil sample taken when corn plants are 6 to 12 inches tall (at the whorl), usually in late May or early June. The soil sample is analyzed for nitrate-N. Late-planted corn may not reach that height before mid-June, but PSNT soil samples should still be collected during the first two weeks of June. The recommended soil sampling procedure requires 16 to 24 soil cores taken randomly through the field, staggering your soil cores across the row as you go. All soil cores should be placed in the soil sample bag and submitted to the laboratory within 24 hours or stored in the refrigerator.

You can submit PSNT soil samples using the online AGVISOR portal by choosing the “Corn Sidedress N” crop choice and submitting a 0-12 inch soil sample for nitrogen (nitrate-N) analysis. AGVISOR will generate sidedress nitrogen fertilizer guidelines, using the Iowa State University PSNT critical level of 25 ppm nitrate-N (0-12 inch depth). If PSNT is greater than 25 ppm nitrate-N, then the probability of any corn yield response to additional nitrogen is low. If spring rainfall was above normal, then the PSNT critical level of 20 to 22 ppm nitrate-N (0-12 inch depth) should be used. Iowa State University provides additional PSNT interpretation criteria for excessive rainfall, manured soils, and corn after alfalfa.

If the PSNT is taken after excessive rainfall, the soil cores can be very wet and difficult to mix in the field. Therefore, it is best to send all soil cores to the laboratory to be dried and ground, ensuring a well-homogenized soil sample for analysis. Although in-field soil nitrate analyzers have improved over the years, the difficult task of blending wet, sticky soil cores in the field still remains. The only way to get accurate, repeatable soil analysis results is to dry, grind, and homogenize the entire soil sample in the laboratory before analysis. AGVISE provides next-day turnaround on PSNT soil samples. The soil samples are analyzed and reported the next business day after arrival at the laboratory. The soil test results will be posted to the online AGVISOR portal for quick and easy access.

More resources on the Pre-Sidedress Soil Nitrate Test (PSNT)

John Breker

Split the Risk with In-season Nitrogen

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For some farmers, applying fertilizer in the fall is a standard practice. You can often take advantage of lower fertilizer prices, reduce the spring workload, and guarantee that fertilizer is applied before planting. As you work on developing your crop nutrition plan, you may want to consider saving a portion of the nitrogen budget for in-season nitrogen topdress or sidedress application.

Some farmers always include topdressing or sidedressing nitrogen as part of their crop nutrition plan. These farmers have witnessed too many years with high in-season nitrogen losses, usually on sandy or clayey soils, through nitrate leaching or denitrification. Split-applied nitrogen is one way to reduce early season nitrogen loss, but do not delay too long before rapid crop nitrogen uptake begins.

Short-season crops, like small grains or canola, develop quickly. Your window for topdress nitrogen is short, so earlier is better than later. To maximize yield in small grains, apply all topdress nitrogen before jointing (5-leaf stage). Any nitrogen applied after jointing will mostly go to grain protein. In canola, apply nitrogen during the rosette stage, before the 6-leaf stage. For topdressing, the most effective nitrogen sources are broadcast NBPT-treated urea (46-0-0) or urea-ammonium nitrate (UAN, 28-0-0) applied through streamer bar (limits leaf burn). Like any surface-applied urea or UAN, ammonia volatilization is a concern. An effective urease inhibitor (e.g. Agrotain, generic NBPT) offers about 7 to 10 days of protection before rain can hopefully incorporate the urea or UAN into soil.

Long-season crops, like corn or sunflower, offer more time. Rapid nitrogen uptake in corn does not begin until after V6 growth stage. The Pre-sidedress Soil Nitrate Test (PSNT), taken when corn is 6 to 12 inches tall, can help you decide the appropriate sidedress nitrogen rate. Topdress NBPT-treated urea is a quick and easy option when corn is small (before V6 growth stage). After corn reaches V10 growth stage, you should limit the topdress urea rate to less than 60 lb/acre (28 lb/acre nitrogen) to prevent whorl burn.

Sidedress nitrogen provides great flexibility in nitrogen sources and rates in row crops like corn, sugarbeet, or sunflower. Sidedress anhydrous ammonia can be safely injected between 30-inch rows. Anhydrous ammonia is not recommended in wet clay soils because the injection trenches do not seal well. Surface-dribbled or coulter-injected UAN can be applied on any soil texture. Surface-dribbled UAN is vulnerable to ammonia volatilization until you receive sufficient rain, so injecting UAN below the soil surface helps reduce ammonia loss. Injecting anhydrous ammonia or UAN below the soil surface also reduces contact with crop residue and potential nitrogen immobilization.

An effective in-season nitrogen program starts with planning. In years with substantial nitrogen loss, a planned in-season nitrogen application is usually more successful than a rescue application. If you are considering split-applied nitrogen for the first time, consider your options for nitrogen sources, application timing and workload, and application equipment. Split-applied nitrogen is another tool to reduce nitrogen loss risk and maximize yield potential.