John Breker

Fall-applied Nitrogen Fertilizer: A Couple Simple Rules

in , /by

This article is shared annually to help answer frequently asked questions about nitrogen fertilizer applications and nitrogen losses in the fall.

October is here, and many people are preparing for fall nitrogen fertilizer applications. Before you hit the field, we want to share these important reminders about fall nitrogen application timing and placement to help you reduce potential soil nitrogen losses through fall and winter.

It is important to wait until soil temperatures reach 50 °F (10 °C) before applying fall nitrogen to reduce the risk of soil nitrogen loss. Once nitrogen fertilizer is applied, soil microbes begin converting ammonium-nitrogen (NH4+) to nitrate-nitrogen (NO3-), a process called nitrification. In the nitrate form, nitrogen is vulnerable to loss through nitrate leaching or denitrification. Soil temperatures cooler than 50 °F help slow microbial activity and keep nitrogen in the safer ammonium-nitrogen form longer. This applies to any ammoniacal nitrogen fertilizer source, which includes anhydrous ammonia, urea, UAN, and ammonium sulfate.

Quick rules for fall-applied nitrogen timing

  1. Wait until after October 1 because cool soil temperatures are more consistent and reliable after this date. After October 1, measure soil temperature in the early morning (6 a.m. to 8 a.m.) at the 4-inch soil depth.
  2. When the 4-inch soil temperature has reached 50 °F (10 °C), it is relatively safe to start applying anhydrous ammonia.
  3. Wait one week after the anhydrous ammonia-safe date to apply banded urea.
  4. Wait two weeks after the anhydrous ammonia-safe date to apply broadcast urea.

Soil temperature map from the North Dakota Agricultural Weather Network (NDAWN) from 21 September 2025. You can find an updated daily soil temperature map at the NDAWN website.

It is a good idea to keep a soil thermometer with you to measure the current soil temperature in the field. In addition to NDAWN, a number of regional climate mesonets have online tools to search for local and regional soil temperatures.

The 50 °F soil temperature rule of thumb is particularly important for soils prone to nitrogen loss: well-drained, coarse-textured soils are prone to nitrate leaching and poorly-drained, fine-textured soils are prone to denitrification. If such soils receive excess precipitation or become saturated (waterlogged) through fall or spring, soil nitrate can be lost through leaching or denitrification. In general, it might be better to apply nitrogen fertilizer on such soils in spring. But, if you must apply nitrogen fertilizer in the fall, make sure you wait until soil temperatures are cold enough to keep it in the ammonium-nitrogen form for a longer period of time to reduce potential soil nitrogen losses.

For fall-applied nitrogen, subsurface banding or incorporation is also important to reduce ammonia volatilization, another potential nitrogen loss mechanism. Fall precipitation (rain or snow) is often too sporadic and unreliable to be considered an effective incorporation “strategy” for broadcast applications. Fall-applied urea should be banded below the soil surface (3 inches or deeper) or incorporated with tillage (at least 3-4 inches) to ensure complete coverage.

Shallow fertilizer bands or shallow incorporation with vertical tillage does not provide adequate soil coverage to prevent ammonia volatilization. If soils are very dry, successful incorporation may not be possible because tillage can produce large, uneven clods that leave nitrogen fertilizer exposed to the atmosphere and vulnerable to ammonia volatilization. Although dry soil poses a lower risk of ammonia volatilization than moist soil, soil moisture is not the only factor that contributes to ammonia volatilization risk (Table 1).

Fall-applied anhydrous ammonia should be banded 5 to 6 inches deep. Ensure that anhydrous ammonia trenches are sealing properly to prevent gaseous ammonia losses from the trench. In addition, the nitrification inhibitor nitrapyrin (brand name N-Serve) can be added to anhydrous ammonia to slow nitrification, offering additional insurance to keep nitrogen in the safer ammonium-nitrogen form for longer. Nitrapyrin is also available in formulations for dry and liquid nitrogen products. Please note that nitrapyrin degrades faster and loses its effectiveness at warmer soil temperatures, so it is no substitute for cooler soil temperatures (<50 °F).

Fall-applied nitrogen is a convenient way to allocate time and labor resources, leaving one less thing to do in the spring. But, you must be smart and consider fertilizer source, timing, and placement options to make sure that the nitrogen applied in the fall will still be there next spring. With fertilizer prices still remaining high, now is not the time to risk soil and fertilizer nitrogen loss.

John Breker

Sodic Soil Problems? Try the NDSU Gypsum Requirement Calculator

in , /by

This article originally appeared in the AGVISE Laboratories Spring 2025 Newsletter.

Salinity and sodicity are two related but distinct terms to describe salt-affected soils. Salinity is the overall abundance of soluble salts, which compete with plant water uptake and reduce crop productivity. Salinity is measured as soluble salts (mmhos/cm or dS/m) on soil test reports. Sodicity specifically refers to high sodium in soil that destroys soil structure, resulting in poor water movement, poor trafficability, and soil compaction. Sodicity is measured as extractable sodium percentage (%Na) or sodium adsorption ratio (SAR) on soil test reports.

Saline soils have an overall abundance of soluble salts, which must be managed with salt-tolerant plant species or improved soil water management (tile drainage). There is nothing you can add to make the salts disappear, such as the mistaken suggestion to apply gypsum to saline soils. Gypsum, however, can be an effective amendment for sodic soils (those with low salinity yet high sodium). A soluble calcium source, like gypsum, can help reduce soil swelling and dispersion and help improve soil structure and water movement on troublesome sodic soils.

The amount of gypsum required is often in tons per acre. This is no task accomplished with a few hundred pounds of gypsum. To calculate the amount of gypsum needed, North Dakota State University has released a gypsum requirement calculator, available online: https://www.ndsu.edu/pubweb/soils/GypsumRequirementWebApp/ The calculator will ask for the soil depth to amend, soil bulk density, CEC, gypsum purity, and initial/target SAR values.

John Breker

AGVISE Demonstration Project: Lowering Soil pH with Elemental Sulfur

in , , , /by

This article originally appeared in the AGVISE Laboratories Spring 2025 Newsletter.

There may not be silly questions, but there are silly answers. Every so often, we get questions about unusual solutions to manage calcareous soils in the northern Great Plains and Canadian Prairies. The most frequent oddball “solutions” involve lowering soil pH with elemental sulfur on calcareous soils. Such suggestions might work on acidic soils; however, the dominant calcareous soils in the region have high pH (>7.3) and tons of natural calcium carbonate that make such attempts impractical and expensive. To put the nail in the coffin, AGVISE Laboratories started some long-term demonstration projects to show plainly why such ideas do not work or may cost way too much!

If possible, we’d like an easy and cheap solution to lower soil pH, like applying only 100 to 200 lb/acre elemental sulfur (S). In soil, elemental S oxidizes to sulfuric acid, which can lower soil pH. However, the large amount of calcium carbonate (free lime) keeps our soils buffered at high pH. To lower soil pH permanently, you must first react and neutralize the carbonate with elemental S before the soil pH can budge. With 100 lb/acre elemental S applied each year, that does not sound too difficult, right?

Elemental sulfur project with rates ranging from 0 to 40,000 lb/acre elemental sulfur. Can you identify the 20 ton/acre rate?

Not so fast. A soil with only 1% calcium carbonate equivalent (CCE) takes 3.2 ton/acre elemental S (6,400 lb/acre) to neutralize the carbonate alone in the 0-6 inch soil depth. In 2020, we started an elemental S project at Northwood, ND on soil containing 4.5% CCE, which would require literal tons of elemental S to lower soil pH. A previous project started in 2005 had used 10,000 lb/acre elemental S, but it was not enough to lower soil pH beyond pH 7.8 after 15 years. This time, we decided to get serious and use elemental S rates from 0 to 40,000 lb/acre (Figure 1). The elemental S rates were intended to hit above and below the target 30,000 lb/acre elemental S rate required to react and neutralize 4.5% CCE.

For the first three years of the project, we saw little to no change in soil pH, regardless of elemental S rate. The oxidation process that converts elemental S to sulfuric acid is a slow, biological process that can take a long time. In Fall 2024, we finally saw real changes in soil pH following elemental S application. The 16,000 lb/acre elemental sulfur rate reached pH 7.5. The 24,000 lb/acre elemental sulfur rate reached pH 7.0. The 40,000 lb/acre elemental sulfur rate reached pH 6.0, a dramatic change! The lowest 8,000 lb/acre elemental sulfur rate, however, was no different than the control.

There is still some unoxidized elemental sulfur and unreacted calcium carbonate in the soil, and we will continue to monitor these long-term demonstration plots in future years. The project demonstrates that elemental sulfur can lower soil pH, but it also shows that the very high amounts of elemental sulfur required are both impractical and expensive. A few hundred pounds of elemental sulfur applied each year will get you nowhere. In contrast, the very high elemental sulfur rates will break the bank. This is why we consider such “solutions” as either ineffective attempts or downright silly wastes.

Brent Jaenisch

Start Strong, Finish Strong with Starter Phosphorus

in , , /by

This article originally appeared in the AGVISE Laboratories Spring 2025 Newsletter.

Cool and wet soil conditions can limit root growth and phosphorus uptake during the early growing growing season. This is why starter phosphorus placed with or near the seed can be so effective in enhancing early plant growth and development. In small grains, starter phosphorus helps improve tiller initiation, even on soil with high soil test P. Faster development in spring can also help flowering wheat or canola beat the summer heat, or advance the corn silking date and maturity to help save grain drying expenses in fall.

Research at the University of Minnesota has found that starter phosphorus applied to corn can promote 10-15% more early season corn biomass and advance corn silking date by 1-2 days, across a range of planting dates and hybrid maturities. This can be achieved with starter phosphorus rates as low as 2.5 gal/acre 10-34-0. It is important to choose a phosphorus source and rate that can provide at least 10 lb/acre P2O5 with or near the seed. There are many phosphorus products available. Liquid orthophosphate and polyphosphate sources are equally effective at supplying phosphorus (in spite of what you may read in marketing materials). Compare products based on the cost per total amount of phosphorus applied; simply multiply the phosphorus content (% P2O5), product density (lb/gal), and intended application rate to calculate the total phosphorus rate in lb/acre P2O5.

Starter phosphorus increased early corn plant biomass (growth stage V5) when applied as 10-34-0 with seed, with and without broadcast phosphorus fertilizer. Summarized across multiple soils with pH ranges including 6.0 to 8.5. Reference: Kaiser, D.E, and J.A. Lamb. 2023. Banding fertilizer with corn seed. UMN Ext. Circ., Univ. Minnesota, St. Paul, MN. https://extension.umn.edu/crop-specific-needs/banding-fertilizer-corn-seed

John Breker

Soil Sensors: Helpful Gadgets or Hapless Gimmicks?

in , /by

This article originally appeared in the AGVISE Laboratories Winter 2024 Newsletter.

A number of new handheld sensors have hit the market, claiming to accurately and precisely measure soil nutrient content in the field, similar to traditional wet chemistry analysis at a soil testing laboratory. The draw for any person soil sampling is the ability to receive soil analysis results right in the field in real time. We know that our clients have a lot of questions about these types of sensors because we are getting these questions too. For almost 50 years, AGVISE has been an early adopter and innovator of new technologies in soil and plant analysis, and these new soil sensors are among the newest to gain popular attention in agriculture.

First, handheld sensors in general are nothing new for soil analysis. There are a number of handheld pH and electrical conductivity (EC) sensors available on the market that are often used for assessing and mapping environmental sites for reclamation and remediation projects. The environmental consultants still need to collect field soil samples and send them to the laboratory for calibration and validation in their official reports. The handheld sensors are used to help them assess the site size and variability.

Second, the type of sensor for the intended soil nutrient or property for measurement is important. After all, you should not try to measure something that the sensor cannot detect, right? The new handheld soil nutrient sensors often rely on near-infrared (NIR), mid-infrared (MIR), or X-ray fluorescence (XRF) spectroscopy methods. These technologies have long existed as benchtop instruments in analytical laboratories for various applications, and each method has its strengths and limitations.

For example, NIR spectroscopy is widely used in feed and forage analysis, food processing, and even meat science. The American Society of Agronomy compiled an 800-page book on NIR applications in agriculture (https://doi.org/10.2134/agronmonogr44.c10). There is one chapter on soil analysis at the end of the book. The strengths of NIR for soil analysis include soil organic matter, total carbon, organic carbon, organic nitrogen, and even pH. However, it does not perform well for nitrate-N, P, K, sulfate-S, Ca, Mg, Na, Cu, Fe, Mn, Zn, or soluble salts (EC). Simply put, NIR fails at measuring the actual soil nutrients we are trying to manage! This is why we do not use benchtop NIR for any soil analyses at AGVISE, let alone a handheld unit with less accuracy or precision. You might see handheld NIR sensors being used for some things, but you will not see them replace soil sampling or soil nutrient analysis soon.

Third, the handheld sensor outputs are often correlated and converted, in the end, to traditional wet chemistry analysis methods, like Bray P, Olsen P, or ammonium acetate K. These are the plant-available soil test methods that we are all familiar with and have decades of soil test calibration research behind them, which allow us to make fertilizer guideline calculations from the soil test result. Whenever a correlation and conversion step takes place, this introduces error for any subsequent calculations, like fertilizer rates. It is important to know what is actually being measured versus what is being reported.

As new sensors hit the market, a person thinking about trying them should be asking a lot of questions. AGVISE is always evaluating new analysis technologies, which can help us do a better or faster job while providing high-quality data to our clients. The questions outlined above are those that we use when we evaluate new analysis technologies for our own operation, and we hope the same questions can help guide you through the gamut of new soil sensors too.

John Breker

Sticky Wet Soils? Try Adding a WD-40 Holster

in /by

This article originally appeared in the AGVISE Laboratories Fall 2024 Newsletter.

Do you have challenges collecting good quality soil cores in sticky wet soils? You are not the only one! WD-40 has been the soil probe lubricant of choice for over 30 years to help obtain better quality soil samples. University researchers have also tested WD-40 and found it does not contaminate soil samples.

Spraying WD-40 on your soil probes with the spray cans can get messy inside the pickup cab. A smart idea to make the WD-40 application process simpler and cleaner is making a WD-40 holster with some PVC pipe. The PVC pipe holster lubricates the soil probe with WD-40 between each soil core and also keeps the soil probe within easy reach. The clever idea came from a client who had spent too much time fiddling with WD-40 spray cans and losing them underneath the pickup seat.

The WD-40 holster is made from 2-inch diameter PVC pipe with a cap glued on the bottom and a threaded fitting on the top with a screw-in plug for storage when not in use. The PVC pipe should be fastened so that the open end faces the soil sampler and the soil probe can be easily placed into the pipe. Fill the PVC pipe with about 3-4 inches of WD-40 in the bottom. With the PVC pipe opening near the hole in the pickup floor, any excess WD-40 drops coming off the soil probe will go down the hole and reduce the mess of spraying WD-40 in the pickup cab.