John Breker

Soil sampling on drown-out, unplanted, and Prevented Planting acres

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Across the northern Great Plains and Canadian Prairies, weather patterns have ranged from too dry to too wet. For the too wet parts, excessive spring and summer rainfall has resulted in extensive stretches of unplanted (Prevented Planting) acres or drown-out acres. As people think about the fall soil sampling season ahead, we are starting to get questions about these unplanted or drown-out fields: When can I start soil sampling? What kind of residual soil nitrate-nitrogen amounts can I expect in the fall?

Extremely wet soil conditions can cause soil nitrogen losses to leaching or denitrification. Warmer soil temperatures and good soil moisture can promote more nitrogen mineralization from soil organic matter. Fallow fields without growing crops (or weeds) can accumulate nitrogen in the soil profile. There are a lot of variables in the equation, and soil testing is the only way to know how much nitrate-N is actually present in the soil profile. Sorry, no points for guessing! The soil nitrate-N level will depend on numerous management and environmental factors, which vary from field to field and zone to zone.

Management Factors

  • Did you apply nitrogen with intent to plant the field? What was the nitrogen fertilizer rate and application timing? Was it applied last fall?
  • Did you do any summer tillage? More tillage promotes nitrogen mineralization.
  • How was your weed control? Did the weeds get large and acquire a lot of nitrogen from the soil profile?
  • Did you plant a cover crop to take up excess water (and nitrogen)?

Environmental Factors

  • Did excessive rainfall cause nitrate leaching on well drained soils?
  • Did excessive rainfall cause denitrification on poorly drained soils?
  • Were summer temperatures warm? Warm temperatures promote nitrogen mineralization.

For immobile soil nutrients (e.g., P, K, Zn), you could start soil sampling anytime, as soon as you can collect good quality soil cores (not too muddy). If these nutrients were applied the previous fall or spring, a soil test will reflect their current availability in soil, following any fixation reactions and nutrient uptake from cover crop or weed growth. For soil nitrate-N, however, the timing will depend on tillage, nitrogen mineralization, and nitrogen uptake from cover crops and weeds.

For “clean” fallow fields (no cover crop or weeds), soil testing may begin in mid-August. It is important to prioritize soil sampling on fallow fields while you can still drive across them. Since these fallow fields have no plant growth to use excess water through fall, the field trafficability might become challenging if excess precipitation continues into fall. To help ensure you can collect good quality soil samples on fallow fields, start soil sampling in August and early September.

For fields with cover crops, soil testing should be delayed until the cover crop is terminated or growth has slowed and nitrogen uptake has stopped. A healthy cover crop can take up a lot of nitrogen through the fall, so you do not want to collect soil samples for nitrate-N too early. In NDSU cover crop projects, fall-planted cover crop mixes can contain 100 to 150 lb/acre N in the plant biomass, which is a sizeable amount of nitrogen that would not be measured as soil nitrate-N.

AGVISE has also performed fallow and cover crop comparison projects; we have seen 35 to 90 lb/acre nitrate-N differences in the 0-24 inch soil profile between fallow and cover crop areas of the same field (Figure 1). To best reflect the amount of residual soil nitrate-N available for next year, it is suggested to wait until cover crop nitrogen uptake has slowed or stopped in October. If more precipitation arrives in fall, the cover crop will continue to use excess soil water and also provide a nice plant residue surface to drive on.

Figure 1. Soil nitrate-N following fallow or cover crop. Cover crop planted in August; soil samples collected in October. AGVISE Laboratories, Northwood, ND. 2020.

We also recommend splitting fields into management zones for soil testing. The unplanted or drown-out parts of the field can very considerably from the rest of the field, which will skew the field-average soil test result and resulting nitrogen fertilizer rate for next year. Often, the unplanted or drown-out parts will have higher soil nitrate-N (no  nitrogen uptake), but sometimes the situation is oddly reversed for no good reason (Figure 2). This data highlights the importance of collecting separate soil samples for the planted and unplanted/drown-out parts of the field.

Figure 2. Soil nitrate-N variability in fields with unplanted or drown-out areas. Paired soil samples in close proximity from the cropped and unplanted/drown-out area in the same field. AGVISE Laboratories, Northwood, ND. 2014.

John Breker

Soil Testing Behind the Combine

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As harvest gets underway, savvy soil samplers are following right behind the combine and starting to collect soil samples. These soil samplers understand the many reasons why taking soil samples right behind the combine gives them the best quality soil samples and data.

In the past, the reasoning to wait until later in the fall to start soil sampling was that there may be additional nitrogen that would be converted to nitrate through the fall as small grain straw and crop residue start to decompose. However, we now know that small grain straw has a high carbon content, and it takes a long time for wheat straw nitrogen to convert to nitrate-N in soil for future crops. Research has shown that soil nitrate-N levels after small grain harvest are quite stable with small changes (up or down) through the fall. Soil sampling right after harvest provides actionable soil nitrate-N data for making fertilizer decisions for next year.

Soil testing behind the combine has several other advantages. If you sample right behind the combine, you beat chisel plows and disk rippers to the field. Taking soil samples before fall tillage allows you to obtain clean and consistent soil cores with your soil probe; this is important for high-quality soil samples. If you sample after tillage, you will be dealing with soil clods that do not feed smoothly into the soil probe. Soil sampling after tillage can also lead to inconsistent sample depths, which will affect soil test levels for P, K, Zn, etc.

Here are some comments by Dr. Dave Franzen, NDSU Extension Soils Specialist (retired) about soil testing right after harvest:

“It is more the rule than the exception that soil sampling begins in mid-September, rather than starting immediately following small grain harvest. However, many producers miss an excellent window for soil testing by waiting too long. The reason for waiting is the hope that additional nitrogen will be made available through mineralization (decomposition of crop residue and organic matter). A review of research has shown that soil nitrate levels change very little, up or down, following small grain harvest.”

Soil sampling right after harvest is recommended and has numerous advantages

  1. Producers are more likely to use the actual soil test results for deciding fall nitrogen fertilizer rates if the soil test results are in their hands before fall fieldwork begins.
  2. Soil sampling before fall tillage provides more consistent 0-6 inch soil cores, which gives the best soil sample quality for phosphorus, potassium, zinc, organic matter, and other non-mobile soil nutrients tested on topsoil.
  3. Soil sampling right after harvest guarantees that fields will be soil sampled on time and not missed due to weather problems that could happen later in the fall.
John Breker

Troubleshooting Problems with Plant Analysis

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A green growing crop is a delightful sight, and it is used by many people as an indicator of crop nutrient status. If you have fields with some yellow areas or slow growth, these symptoms may indicate a nutrient deficiency. Most commonly, yellow-looking plants may be deficient in nitrogen, sulfur, or both. If detected early, there may still be time for rescue treatment.

Plant analysis is not a magic or foolproof tool, but it can provide useful information in diagnosing problems when combined with soil analysis and good field scouting. During the summer, our technical staff receives many questions from agronomists, crop consultants, and farmers on troubleshooting problems in their fields. The following tips and tricks will help you identify potential nutrient deficiencies early.

Troubleshooting nutrient deficiencies based on visual symptoms can be difficult if symptoms look similar (yellow for nitrogen and sulfur deficiency in the early season) or indistinct (slow growth for phosphorus deficiency). Proper troubleshooting requires you to collect paired plant and soil samples from the area with poor plant growth and an adjacent area with good plant growth. A single plant sample from the poor area is seldom enough information to accurately identify the nutrient deficiency. A soil sample from the same area is needed to determine if the soil nutrient supply is truly lacking or if reduced plant nutrient uptake is caused by another factor (e.g., soil saturation, soil compaction, cool temperature, disease).

With paired plant and soil samples from both good and bad areas, you can compare the results and determine if the symptoms are caused by one or more nutrients or non-soil issues. This comparison is particularly important for secondary and micronutrients that may also reduce plant uptake of other nutrients such as nitrogen, which could otherwise misidentify the deficiency and lead to the wrong corrective treatment (a common problem when only one plant sample is collected). Plant and soil samples should be collected within 7-10 days of symptom development to identify the nutrient deficiency and to have enough time for a rescue fertilizer application if possible. Plant samples collected after this window may be suspect as other issues may develop and confound the results.

The picture below shows patchy yellowing in a spring wheat field during tillering stage. Plant and soil samples (good and bad) determined that sulfur was deficient in yellow areas, and the grower still had options for applying sulfur fertilizer to correct the deficiency. This is a great example of paired plant and soil analysis helping the farmer choose the right corrective action, rather than blindly putting more nitrogen on yellow wheat.

Yellow wheat showing potential sulfur deficiency (upper leaves yellowing first), but there might be soil nitrogen losses too. A paired plant and soil sample is the best way to decide the right corrective action.

Collecting plant and soil samples for troubleshooting nutrient deficiencies:

  1. Collect one plant sample in the area with possible nutrient deficiency symptoms (bad) and one plant sample in an adjacent area (~50 feet) into the crop that looks normal (good).  Collect the correct plant part for that plant growth stage (see instructions on plant sample bag or plant sampling guide).
  2. Collect one soil sample (0-6 inch) from each location where you collected the “good” and “bad” plant samples.
  3. Take photographs of individual plants that show distinct leaf symptoms (not landscape photographs) from each location where you collected the “good” and “bad” plant samples. Keep these photographs for your records; these will help in interpretation of plant analysis results.
  4. Submit plant and soil samples for Complete Nutrient Analysis (also called Option F for soil samples).

If you have fields with areas of poor plant growth, now is the time to collect plant and soil samples to determine if a nutrient deficiency is the issue. The troubleshooting procedure outlined above will help you detect nutrient deficiencies early and decide upon the proper corrective action if needed. To learn more about proper plant and soil sample collection and interpreting reports, please see the resources below.

Plant Analysis Guides
Plant Sampling Guide
Interpreting Plant Analysis Reports 

Soil Analysis Guides
Soil Sampling Guide
Interpreting Soil Test Reports

John Breker

Protecting Spring-Applied Nitrogen Fertilizer from Ammonia Volatilization

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As the spring planting season gets underway, agronomists and farmers are asking about the best ways to protect spring-applied nitrogen. How much nitrogen might I lose if I cannot incorporate it? Does vertical tillage incorporate fertilizer enough? We have compiled some resources to help answer those questions.

There are three ways to lose nitrogen fertilizer: ammonia volatilization, denitrification, and nitrate leaching. In excessively wet soils, nitrogen can be lost through nitrate leaching and denitrification. However, for spring-applied nitrogen, ammonia volatilization is the main concern with dry soil conditions and unpredictable precipitation forecasts.

When you apply ammoniacal fertilizers (e.g. anhydrous ammonia, urea, UAN, ammonium sulfate) to the soil surface without sufficient incorporation, some amount of free ammonia (NH3) can escape to the atmosphere. Sufficient incorporation with tillage or precipitation is needed to safely protect that nitrogen investment below the soil surface.

Ammonia volatilization risk depends on soil and environmental factors (Table 1) and the nitrogen fertilizer source (Table 2). Typically, we are most concerned about ammonia volatilization for surface-applied urea or UAN. It is not easy to estimate how much nitrogen might be lost, and sometimes the losses can be substantial. Although you cannot change the soil type or weather forecast, you do have control over the nitrogen source and application method (Table 2) to protect your nitrogen investment.

Table 1. Relative risk factors for ammonia volatilization
Factor High risk Low risk
Soil pH >7 <6
Soil moisture Moist Dry
Soil temperature >70 °F <50 °F
Rainfall, irrigation Little or none, heavy dew >0.3 inch after N application
CEC (cmol/kg) <10 >25
Soil surface residue >50% residue cover (no-till, pasture, turf) Bare
Application method Surface broadcast Incorporated, subsurface band
Havlin, J.L., S.L. Tisdale, W.L. Nelson, and J.D. Beaton. 2014. Soil fertility and fertilizers: An introduction to nutrient management. 8th ed. Pearson, Upper Saddle River, NJ.
Table 2. Estimated ammonia volatilization for different nitrogen sources, application methods, and rainfall scenarios on soil with pH > 7 (high risk).
Fertilizer source Application method Precipitation after fertilizer application
> 0.5 inch rain within 2 days Little or no rain likely within 7 days
% fertilizer nitrogen lost
Urea or UAN Broadcast 0-20 2-40
Dribble 0-15 2-30
Incorporated 0-10 0-10
Ammonium sulfate (AMS) Broadcast 0-40 5-60
Incorporated 0-10 0-30
Ammonium nitrate Broadcast 0-20 5-30
Incorporated 0-10 0-20
Anhydrous ammonia Injected 0-2 0-5
Messinger, J.J. and G.W. Randall. 1991. Estimating nitrogen budgets for soil-crop systems. In: Follett, R.F., D.R. Keeney, and R.M. Cruse, editors, Managing nitrogen for groundwater quality and farm profitability. SSSA, Madison, WI. pp. 82-214.

Practices to reduce ammonia volatilization for urea or UAN, in order of most effective practice

  • Apply urea in subsurface bands at least 3 inches below the soil surface. A shallow urea band (1 or 2 inches deep) acts like a slow-release anhydrous ammonia band, and nobody should ever apply anhydrous ammonia that shallow.
  • If nitrogen will be broadcast with incorporation, make sure the fertilizer is sufficiently incorporated at least 2 inches below the soil surface to ensure good soil coverage. A chisel plow or field cultivator are good incorporation tools. The popularity of high-speed disks (vertical tillage) has led some people to think that it counts as a meaningful incorporation event. In reality, it just moves soil and crop residue around on the soil surface without really incorporating the fertilizer. Take a look after you run across the field and you will see white urea granules remaining on the soil surface. Do you remember the old soil-applied herbicide incorporation videos from the 1970s? Those classic videos provide great examples of what a thorough incorporation job really requires. NDSU Extension has posted them online: https://vimeo.com/216680310/e843149fdd
  • If nitrogen will be broadcast without incorporation, try to time the fertilizer application right before rain (at least 0.5 inches of precipitation). Soils with good crop residue cover (no-till) may require more rain to sufficiently move urea or UAN into the soil.
  • If no rain is forecasted in the near future, consider applying a urease inhibitor on urea or UAN to provide temporary protection until rain arrives. The active ingredient NBPT has been widely researched and shown to reduce nitrogen losses; make sure the active ingredient rate is 1.3 to 1.8 lb NBPT per ton of urea to ensure effective NBPT activity and protection. NBPT begins to break down after 7 to 14 days. In addition, it is important to remember that nitrification inhibitors like nitrapyrin and DCD do not protect against ammonia volatilization.

These practices should also be considered if you will be applying in-season nitrogen fertilizer later in the summer. It is always best to apply nitrogen below the soil surface, such as injected anhydrous ammonia or coulter-injected UAN, to protect nitrogen fertilizer. For surface-applied urea or UAN, you should time the fertilizer application just before a rainfall or consider NBPT to extend the rainfall window.

The higher ammonia loss potential for ammonium sulfate (Table 2) often surprises people (and we get questions about it). On calcareous soils with high pH, the initial reaction products of ammonium sulfate [(NH4)2SO4] and calcium carbonate (CaCO3) can produce free ammonia, which may be lost if ammonium sulfate is lying on the soil surface. This is a similar reaction process to free ammonia formation with diammonium phosphate (DAP, 18-46-0) applied to calcareous soils. This is why AMS and DAP are not suggested as seed-placed fertilizers on calcareous soils because of the ammonia toxicity risk to seedlings. Please note that urease inhibitors like NBPT will not protect ammonium sulfate from ammonia volatilization.

Helpful resources

Nitrogen extenders and additives for field crops (NDSU) https://www.ag.ndsu.edu/publications/crops/nitrogen-extenders-and-additives-for-field-crops

Should you add inhibitors to your sidedress nitrogen application? (Univ. Minnesota) https://blog-crop-news.extension.umn.edu/2020/06/should-you-add-inhibitors-to-your.html

Split the risk with in-season nitrogen (AGVISE Laboratories) https://www.agvise.com/split-the-risk-with-in-season-nitrogen/

John Breker

Switching More Acres to Soybean?

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The spike in nitrogen fertilizer prices over the past month has prompted many growers to think about switching more acres to soybean in 2026. The high nitrogen fertilizer prices are squeezing the potential profitability of any crop requiring nitrogen fertilizer, such as corn, dry bean, canola, or wheat. The symbiotic nitrogen fixing behavior of soybean is an impressive feat of nature that helps reduce nitrogen fertilizer expenses in farm budgets.

If you do plan to plant more soybean acres in 2026, remember that soybean still has its own crop nutrient needs and removal, like phosphorus and potassium, that cannot be ignored for the soybean crop or across the crop rotation. In addition, iron deficiency chlorosis (IDC) is a common problem in soybean fields across the region, and soybean cyst nematode (SCN) can debilitate and cripple soybean yield now and into the future.

Before you plant soybean on any acre, it is important to have current soil test information for IDC and SCN. These two problems are best managed with the right soybean variety, and there is a nice window before spring planting to collect soil samples.

Soybean Fertility (Phosphorus and Potassium)

Soybean does not respond to phosphorus as dramatically as grass crops like corn or wheat do. Nevertheless, medium to high soil test P is required to achieve good soybean yields. Soybean responds to broadcast P placement better than seed-placed P or sideband P. In no-till regions where soybean is often planted with air drills, seed-placed P or sideband P is often the only opportunity to apply phosphorus in the system. You must pay special attention to seed-placed fertilizer safety with soybean.

Soybean removes far more potassium in harvested seed than canola or wheat. Soybean yielding 50 bu/acre removes about 60 lb/acre K2O, while wheat yielding 80 bu/acre removes only 25 lb/acre K2O. Pay close attention to potassium removal across the crop rotation. After soybean is added to the crop rotation, cumulative crop K removal over numerous years increases greatly, and declining soil test K is observed over time.

Do not place potassium fertilizer with soybean seed; delayed seedling emergence and reduced plant population can occur. Any potassium fertilizer should be broadcasted or banded away from seed.

Soybean Iron Deficiency Chlorosis (IDC)

Soybean is very susceptible to iron deficiency chlorosis (IDC). Soybean IDC is not caused by low soil iron but instead by soil conditions that decrease iron availability and uptake by soybean roots. Soybean IDC risk and severity are primarily related to soil carbonate content (calcium carbonate equivalent, CCE) and worsened by salinity (electrical conductivity, EC). These primary risk factors (carbonate and salinity) can be measured with routine soil testing.

Soybean IDC is common in the upper Midwest, northern Great Plains, and Canadian Prairies, where soils frequently have high carbonate and/or salinity. Within a field, IDC symptoms are usually confined to soybean IDC hotspots with high carbonate and salinity; however, symptoms may appear across a field if high carbonate and salinity are present throughout the field. Soybean IDC severity is made worse in cool, wet soils and soils with high residual nitrate-N. Soil pH alone is not a good indicator of soybean IDC risk because some high pH soils lack high carbonate and salinity, which are the two principal risk factors.

Guidelines for managing soybean IDC

  1. Soil test each field, zone, or grid for soil carbonate and salinity. This may require soil sampling prior to soybean (possibly outside of your usual soil sampling rotation), or consult previous soil test records.
  2. Plant soybean in fields with low carbonate and salinity (principal soybean IDC risk factors).
  3. Choose an IDC tolerant soybean variety on fields with moderate to high carbonate and salinity. This is your most practical option to reduce soybean IDC risk. Consult seed dealers, university soybean IDC ratings, and neighbor experiences when searching for IDC tolerant soybean varieties.
  4. Plant soybean in wider rows. Soybean IDC tends to be less severe in wide-row spacings (more plants per row, plants are closer together) than narrow-row spacings or solid-seeded spacings.
  5. Apply chelated iron fertilizer (high-quality FeEDDHA or FeHBED) in-furrow at planting. In-furrow iron fertilizer application may not be enough to help an IDC susceptible variety in high IDC risk soils (see points #2 and #3).
  6. Avoid planting soybean on soils with very high IDC risk.

Soybean Cyst Nematode (SCN)

Soybean cyst nematode (SCN) is the #1 pathogen causing soybean yield loss in the United States. It is a microscopic parasitic worm that lives in soil and attacks the roots of susceptible soybean and dry bean varieties. Soybean cyst nematode is found across the soybean-growing regions of the United States; it first reached Manitoba in 2019.

Soybean cyst nematode is best managed with crop rotation and SCN-resistant soybean varieties. Soil sampling for SCN is your best tool to learn if you have SCN and also if the SCN resistance traits in your soybean varieties are still working. In recent years, AGVISE Laboratories has documented failing SCN control with PI8878 resistance (most common type) and a continuing increase in SCN egg counts across the region. In some places, the SCN egg counts are so high that no soybean crop (resistant or not) should be planted for multiple years. Once you have it, SCN is nearly impossible to eliminate from fields. A current SCN soil sample will help you choose the right SCN-resistant soybean variety and manage SCN populations now and into the future.

John Breker

Phosphorus Fertilizer Forms: Orthophosphate or Polyphosphate (Ortho-P or Poly-P)

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

Each spring, we get questions about what form of phosphate fertilizer is better. The debate over orthophosphate and polyphosphate has raged for decades. Just when we think we have put the issue to bed, it comes up again! These questions originate from the simple fact that orthophosphate (ortho-P) is the form that plant roots are able to uptake. This simple fact, however, is a simplistic understanding of phosphorus fertilizer forms and chemistry in soil. The truth is either form of phosphate fertilizer form (ortho-P or poly-P) will provide the same crop yield response when applied at the same rate of phosphorus. With “new” fertilizer products and additives on the market, it is time to revisit the basics of phosphorus fertilizer materials and their behavior in soil.

All dry phosphate fertilizers are orthophosphate forms; this includes monoammonium phosphate (MAP, 11-52-0), diammonium phosphate (18-46-0), and triple superphosphate (TSP, 0-46-0). Yes, this means that all dry phosphate fertilizers are orthophosphate forms. Liquid phosphate fertilizers are usually some blend of orthophosphate and polyphosphate of varying proportions. There are pure liquid ortho-P fertilizers available, but the liquid poly-P fertilizers will also contain some smaller proportion of ortho-P in solution too. Either way, all phosphorus fertilizer materials, dry or liquid, ortho-P or poly-P, will act the same once applied to soil.

In fertilizer manufacturing, all products start as phosphoric acid derived from rock phosphate. You could use liquid phosphoric acid as a phosphorus fertilizer material itself, but it is corrosive and difficult to handle. The next step is converting phosphoric acid to a more stable product that is easier to handle. If you want to make a high concentration liquid phosphorus fertilizer, you must also remove some water from the phosphoric acidic. The dehydration process (removal of water) produces linked chains of orthophosphate, which are called polyphosphate chains, resulting in a denser and more concentrated liquid phosphate material. The name “polyphosphate” literally means “many phosphate.” The dehydration process is never 100% efficient, so some orthophosphate always remains in solution. In most polyphosphate fertilizers, like 10-34-0, the breakdown is around 75% polyphosphate and 25% orthophosphate. This results in a higher concentration liquid phosphorus fertilizer with more pounds of phosphorus per gallon, meaning that you have to haul less material to the field to achieve the same phosphorus rate.

As a fertilizer applied to soil, does this mean polyphosphate is less available than orthophosphate to plant roots? When polyphosphate is applied to soil, the fertilizer reacts quickly with water in soil and breaks into orthophosphate again. The “many phosphate” becomes normal orthophosphate again, and the rehydration step is very fast. Even at cool soil temperatures (40 deg F), over 40% of polyphosphate converts to orthophosphate within 72 hours. Within one to two weeks, the conversion is complete, leaving all plant-available orthophosphate for crop uptake.

In the end, it all comes back to crop yield, right? In studies across the Midwest, Great Plains, and Canadian Prairies, the performance of orthophosphate and polyphosphate have been equal, as long as you apply the same rate of phosphorus. As stated earlier, all phosphorus fertilizer materials, dry or liquid, ortho-P or poly-P, will act the same once applied to soil.

BONUS A unique property of ammonium polyphosphate (APP, 10-34-0) is the ability to chelate or sequester metal cations, such as micronutrients like zinc, in the polyphosphate molecule. This allows polyphosphate solutions to maintain a higher concentration of micronutrients in solution than pure orthophosphate. Ammonium polyphosphate can maintain 2% Zn in solution, compared to only 0.05% Zn with pure orthophosphate.

John Breker

Fall-applied Nitrogen Fertilizer: A Couple Simple Rules

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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

AGVISE Demonstration Project: Lowering Soil pH with Elemental Sulfur

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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

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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

Jodi Boe

Lime Works: The Results Are In

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This article was originally published in the AGVISE Laboratories Winter 2023 Newsletter.

In the fall of 2022, I hired a custom applicator to haul and spread lime across 238 acres of my family’s farm in western North Dakota. The reason? To increase soil pH on five fields with very low soil pH. One field even had a soil pH of 4.7, so these were good candidate fields for a practical case study for liming on a real farm operation.

I wrote more about the soil sampling process and lime application in the AGVISE Winter 2022 newsletter (https://www.agvise.com/wp-content/uploads/2022/11/AGVISE-Newsletter-2022-Winter.pdf). Each field received approximately 2 ton/acre sugar beet lime (1.4 ton ENP/acre) from Sidney Sugar in Sidney, MT, and the lime was disced to 3 inches for incorporation. After one year, the soil pH had already increased by 0.36 pH-units in the 0-6 inch soil depth. The 2023 growing season was relatively wet in southwest North Dakota, and the additional soil water certainly helped the lime react and neutralize soil acidity quickly. The incorporation with a disc also helped distribute the lime more evenly and deeply, allowing the lime to react faster. One negative side effect of tillage was a flush of annual weeds, particularly green and yellow foxtail. This was the first tillage event on these fields in 12 years, so I expect the annual weed community to diminish as we return to no-till after the one-time tillage pass.

Figure 1. Zone soil pH map of a field receiving 2 ton/acre sugar beet lime in fall 2022. Each zone increased roughly 0.36 pH-units from 2022 to 2023. (Maps created in ADMS 32, GK Technology, Inc.)

Lime also works without incorporation, just at a slower pace. In 2021, we established a no-till lime trial to investigate lime rates without incorporation. Lime was applied in May 2021, and the fall 2023 soil pH results are shown in Figure 2. The highest lime rate at 2.5 ton ENP/acre increased soil pH in the upper 0-3 inch soil depth by 0.71 pH-units over 2.5 years. So far, no effect on soil pH in the lower 3-6 inch soil depth has been observed. In most no-till systems, the most acidic part of the soil profile is located at the soil surface, and a lime application correcting soil pH in the upper 0-3 inch soil depth is still effective. This is where seedlings and roots are most vulnerable to soil acidity, so correcting soil pH at the soil surface is critical and can be accomplished with a surface application of lime in no-till systems.

Surface Soil pH (0-3 inch) in No-till Lime Trial, October 2023

Figure 2. Soil pH following surface application of lime after 2.5 years in a no-till cropping system in southwestern North Dakota.