Banding Phosphorus and Potassium: Stretch your fertilizer dollars further

This article originally appeared in the AGVISE Laboratories Winter 2022 Newsletter

Broadcast or band? For phosphorus and potassium, these are big fertilizer questions. In recent months, high fertilizer prices have prompted farmers and agronomists to consider other strategies to reduce fertilizer costs without jeopardizing crop yield. Among the most common and effective options is placing fertilizer in a tight band below the soil surface, also known as a subsurface band.

Subsurface banding helps improve fertilizer recovery and efficiency. It ensures that fertilizer is placed in the plant root zone, facilitating direct uptake of crop nutrients. It also minimizes potential fixation reactions (aka tie-up) that reduce soil nutrient availability, allowing more phosphorus or potassium to remain available in soil for plant uptake. You ultimately get more bang for your buck on each pound of fertilizer applied. In addition, placing fertilizer below the soil surface protects fertilizer from

Idealized crop response to phosphorus as affected by fertilizer placement and soil test level (figure from J. Prod. Agric. 1:70-79).

soil erosion and runoff losses via wind and water. This is important for fall-applied phosphorus and potassium because spring snowmelt runoff and wind erosion can move fertilizer lying on the soil surface from neighbor to neighbor and watersheds beyond.

When we discuss banding phosphorus and potassium, it also comes along with the question, “How far can I cut fertilizer rates?” It is important to recognize that the improved efficiency of banding over broadcast is a function of soil test levels (figure) and proximity to the seed row. If you have high soil test levels (>15 ppm Olsen P), then the expected crop yield response to fertilizer, whether broadcast or banded, is lower. Banding fertilizer still helps with the fertilizer recovery, but the expected crop yield increase is often similar to broadcast. However, if you have low soil test levels, then the expected crop yield response is much greater with banding.

Where does seed row proximity fit in? The greatest efficiency comes with in-furrow or near-seed placement (e.g. 2×2 band), allowing effective fertilizer rates of one-half to two-thirds their broadcast equivalent. The near-seed placement also provides the starter effect, which enhances early plant growth and development in cool, wet soils of the upper Midwest and northern Great Plains. Of course, you must watch seed safety with any seed-placed fertilizer in the furrow.

For deep-band or mid-row band placement, the benefits over broadcast begin to disappear. These are still great placement options for anhydrous ammonia or urea, but the greater distance between the seed row and fertilizer band does not provide the same efficiency for immobile soil nutrients like phosphorus and potassium. This will surprise some people hoping that strip-till with deep-banded phosphorus and potassium or a one-pass air seeder with mid-row banders might be their answer to reducing fertilizer costs. For these “far-from-seed” banding options, reduced fertilizer rates are not suggested, and some in-furrow or near-seed banded fertilizer should still be applied for the current crop.

 

Fall-applied Nitrogen Fertilizer: A Couple Simple Rules

The beginning to mid-October is when soil temperatures across the northern Great Plains and Canadian Prairies typically drop below 50 °F (10 °C). This is the soil temperature threshold that we wait to reach before applying fall-applied nitrogen fertilizer. It is important to wait until soil temperatures are cold enough (<50 °F) to help 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. The colder soil temperatures slow microbial activity, thus keeping more nitrogen in the safer ammonium-nitrogen form. This applies to any ammoniacal nitrogen fertilizer source, which includes anhydrous ammonia, urea, and ammonium sulfate.

Map courtesy of the North Dakota Agricultural Weather Network (NDAWN).
You can find an updated average bare soil temperature map here

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 prevent ammonia volatilization, another potential nitrogen loss mechanism. Fall precipitation (rain or snow) is too sporadic and unreliable to be considered an effective incorporation “strategy” for fall-applied nitrogen. 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 enough soil coverage to prevent ammonia volatilization.

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 delay nitrification, offering additional insurance to keep nitrogen in the safer ammonium-nitrogen form for longer. However, please note that its efficacy decreases with warmer soil temperatures, so it is no replacement for cool soil temperatures (<50 °F).

In conclusion, fall-applied nitrogen is a great 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 fall will still be there next spring.

Updated Residual Soil Nitrate Trends (Variability is high this year)

The 2022 growing season may seem like a long way off, but spring will be here before we know it. In fact, many growers are already making (or have made) crop choices and seed variety decisions for 2022. One factor that must be considered when making crop and variety selections for 2022 is residual soil nitrate-nitrogen following the 2021 growing season. For many in the northern Great Plains and Canadian Prairies, the 2021 growing season was hot and dry, which resulted in high residual soil nitrate levels following many crops. An update on average residual nitrate levels after wheat, broken down by geography, is below (Table 1). Residual soil nitrate-nitrogen following other crops, including soybean, are also higher than average (Table 2). This highlights the importance of soil sampling, even after crops we do not typically think of leaving high residual soil nitrate behind.

The data in the tables represents a snapshot of the samples we have tested so far this fall. While the average residual soil nitrate-nitrogen for an area may be interesting to talk about, it is not a replacement for actual soil test results from you or your growers’ fields. The data shows that over 30% of the wheat fields in many areas (see the right-hand column of the table) test over 100 lb/acre soil nitrate (0-24 inch depth). Droughts like 1988 and 2021 are very uncommon and leave us in situations that we are not used to dealing with. Using an average soil nitrate level from a region to decide an N rate on an individual field would be like deciding to apply an insecticide on every acre of the farm without even looking at each field to see if the insect is present. You need actual soil test data on each field to make informed decisions.

Table 1. Residual nitrate trends as of Sept. 17, 2021 from more than 20,000 soil samples taken after wheat. Regions with less than 100 soil samples are not included in the table.

Table 2. Residual nitrate trends as of Sept. 17, 2021 for crops other than wheat. Regions with less than 100 soil samples for each respective crop are not included in the table.

High Fertilizer Prices

According to the September 15, 2021 DTN fertilizer price survey, retail fertilizer prices continue to rise. The average price per pound of nitrogen by fertilizer product is $0.61/lb N for urea, $0.46 lb/N for anhydrous ammonia, and $0.66/lb N for UAN-28. This represents a 55%, 73%, and 71% increase in price compared to prices for the same fertilizers this time last year. Long story short, fertilizer is expensive. High residual soil nitrate following wheat may help reduce input costs in 2022, as long as you know what the residual soil nitrate in your fields is and take advantage of it by growing a crop that requires nitrogen fertilizer. If you have a soil nitrate test of 80 lb/acre (0-24 inch) after wheat, that is about 50 lb more than normal carry over. The extra 50 lb/acre soil nitrate is worth $30.00/acre (based on the current urea price).

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

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 window is generally between growth stages V4 and V8, before rapid plant nitrogen uptake occurs. Split-applied nitrogen has 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 with high in-season nitrogen losses through nitrate leaching or denitrification.

The target timing for PSNT sampling is when corn is 6 to 12″ tall. Twelve-inch corn is often V4 or V5 (like in the picture 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 stages. 

Whether your nitrogen management plan includes a planned sidedress nitrogen application or not, the Pre-Sidedress Soil Nitrate Test (PSNT) is one tool to help make decisions about in-season nitrogen. You may also hear this test 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 Pre-sidedress Soil Nitrate Test (PSNT), taken when corn is 6 to 12 inches tall, can 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. 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 recommend 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 program by choosing the “Corn Sidedress N” crop choice and submitting a 0-12 inch soil sample for nitrate analysis. AGVISOR will generate sidedress nitrogen fertilizer guidelines, using the 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 will be 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-blended 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 blend the entire soil sample in the laboratory before analysis. AGVISE provides 24-hour turnaround on PSNT soil samples. The soil samples are analyzed and reported the next business day after arrival. Soil test results are posted on the online AGVISOR program for quick and easy access. With AGVISE, you get not only great service but also the highest quality data with four decades of soil testing experience.

Pre-Sidedress Soil Nitrate Test (PSNT) resources

Please call our technical support staff if you have any questions on PSNT and interpreting the soil test results for sidedress nitrogen application.

Protect Nitrogen Fertilizer from Ammonia Volatilization

Recent rain and snow have brought much-needed precipitation to the northern Great Plains and upper Midwest regions. Some degree of drought conditions stretch from Alberta to Iowa, and agronomists and farmers are wondering the best ways to protect spring-applied nitrogen as the planting season continues. 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 fertilizer nitrogen: ammonia volatilization, denitrification, and nitrate leaching. In excessively wet soils, denitrification and nitrate leaching are a concern. However, for spring-applied nitrogen, ammonia volatilization is the main concern with dry soil conditions and unpredictable rainfall 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. With dry soil conditions, this is important to remember because we must balance the need to protect nitrogen fertilizer while conserving soil water for seed germination and emergence.

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.

Practices to reduce ammonia volatilization, in order of most effective: 

  • Apply urea in subsurface bands at least 3 inches below the soil surface. A shallow urea band (1 or 2 inches) 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 is usually needed. 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 any fertilizer. Take a look after you run across the field and you will see white urea granules everywhere. There are soil-applied herbicide incorporation videos from the 1970s that show what a thorough incorporation job really requires.
  • If nitrogen will be broadcast without incorporation, try to time the fertilizer application right before rain (at least 0.3 inch of precipitation). Soils with good crop residue cover (no-till) may require more rain to sufficiently move urea or UAN into the soil surface.
  • 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 university research-proven urease inhibitor is NBPT, available in products like Agrotain (Koch) and its generic cousins. For generic products, 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 breakdown 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 to corn or wheat 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 will want to time the fertilizer application just before a rainfall or consider NBPT to extend the rainfall window.

Resources on ammonia volatilization and urease inhibitors

Nitrogen extenders and additives for field crops, NDSU

How long can NBPT-treated urea remain on the soil surface without loss?, NDSU

Should you add inhibitors to your sidedress nitrogen application?, University of Minnesota

Split the risk with in-season nitrogen, AGVISE

Starter Fertilizer: Choosing the Right Rate

Starter fertilizer placed with or near the seed is essential for vigorous early season growth in grass crops such as corn and wheat. We plant these crops early because we know vigorous early season growth is important to achieving high crop yields. Early planting also means cold soils, and starter fertilizer is necessary to get the crop going with a good start. Each spring, we receive many questions about starter fertilizer placement and seed-safe fertilizer rates. These questions come from farmers who want to plant as many acres per day as possible, take advantage of more efficient banded phosphorus placement, and of course reduce fertilizer costs.

The two most common questions we get are “What is highest rate of starter fertilizer I can apply with the seed?” and “What is the lowest rate of starter fertilizer I can apply with the seed and still get a starter effect?” South Dakota State University (SDSU) made a downloadable spreadsheet that calculates the maximum seed-safe fertilizer rate (Figure 1). The spreadsheet will ask for the crop choice, fertilizer product, seed opener width, row spacing, tolerable stand loss, soil texture, and soil water content. The spreadsheet calculations are based on SDSU greenhouse and field studies.

Seed Safety Calculator from SDSU for Starter Fertilizer Article

Figure 1. Fertilizer Seed Decision Aid from South Dakota State University. Download the spreadsheet here

Research has shown, that to achieve the full starter effect, a fertilizer granule or droplet must be within 1.5 to 2.0 inches of each seed. If the fertilizer granule or droplet is more than 1.5 to 2.0 inches away from the seed, the starter effect is lost. To illustrate the role of starter fertilizer rates and seed placement, AGVISE put together displays showing the distance between fertilizer granules or droplets at various rates and row spacings. For example, take a look at wheat planted in 7-inch rows with 30 lb/acre P2O5 (57 lb/acre 11-52-0) and corn planted in 30-inch rows with 30 lb/acre P2O5 (7.5 gal/acre 10-34-0). You need to maintain a sufficient starter fertilizer rate to keep fertilizer granules or droplets with 1.5 to 2.0 inches of each seed.

Starter fertilizer demonstration example for starter fertilizer article

Figure 2. Two examples from the AGVISE Starter Fertilizer Display series. Find more crops and fertilizer rates here.

In the northern Great Plains and Canadian Prairies, most fertilizer is applied at planting and often as seed-placed fertilizer. This creates a challenge to prevent soil nutrient mining when balancing seed safety and crop nutrient removal with higher crop yield potential. Soil nutrient mining occurs when you apply less fertilizer than crop nutrient removal, resulting in soil test P and K decline over time. Some broadleaf crops, like canola and soybean, are very sensitive to seed-placed fertilizer, allowing only low seed-placed fertilizer rates. In contrast, most cereal crops can tolerate higher seed-placed fertilizer rates. To maintain soil nutrient levels across the crop rotation, you need to apply more phosphorus fertilizer in crops that allow greater seed safety. You can apply more phosphorus fertilizer with crops like corn or wheat, which allows you build soil test P in those years, while you mine soil test P in canola or soybean years. If you cannot the maintain crop nutrient removal balance with seed-placed fertilizer, then you need to consider applying additional phosphorus in mid-row bands or broadcast phosphorus at some point in the crop rotation.

Table 1. Seed-safe fertilizer rates may not meet crop removal. In the example, the seed-safe limit is based on 1-inch disk or knife opener and 7.5-inch row spacing for air-seeded crops and 30-inch row spacing for corn. Phosphorus (P) balance: Seed-safe limit (lb/acre P2O5) minus crop P removal (lb/acre P2O5). A negative P balance indicates the seed-safe limit does not meet crop removal, which may decrease soil test P.

Starter fertilizer is an important part of any crop nutrition plan. Here are more resources to help you make the best decisions on starter fertilizer materials, placement, and rates.

Fertilizer Application with Small Grain Seed at Planting, NDSU

Safe Rates of Fertilizer Applied with the Seed, Saskatchewan Agriculture

Using banded fertilizer for corn production, University of Minnesota

Corn response to phosphorus starter fertilizer in North Dakota, NDSU

Wheat, barley and canola response to phosphate fertilizer, Alberta Agriculture

Starter Fertilizer Display: How low can YOU go?

When profits are squeezed, more farmers are asking about optimal starter fertilizer rates and how low starter fertilizer rates can be. These questions are the result of wanting to keep fertilizer costs down, to plant as many acres per day as possible, and to take advantage of more efficient, lower rates of banded phosphorus fertilizer compared to higher rates of broadcast phosphorus fertilizer.

To illustrate the role of starter fertilizer rates and seed placement, we put together displays showing the distance between fertilizer granules or droplets at various rates and row spacings. You can see several pictures with canola, corn, soybean, sugar beet, and wheat. We greatly thank John Heard with Manitoba Agriculture for helping with the displays.

The displays show the normal seed spacing for several crops with different dry or liquid fertilizer rates alongside the seed. These displays help visualize the distance between the seed and fertilizer at several rates. University research shows that to achieve the full starter effect, a fertilizer granule or droplet must be within 1.5-2.0 inches of each seed. If the fertilizer granule or droplet is more than 1.5-2.0 inches away from the seed, the starter effect is lost. Some people wonder about these displays, but you can prove it to yourself pretty easily. Just run the planter partially down on a hard surface at normal planting speed. You will see what you imagine as a constant stream of liquid fertilizer, ends up being individual droplets at normal speed, especially with narrow row spacings and lower fertilizer rates.

These displays help illustrate the minimum starter fertilizer rate to maintain fertilizer placement within 1.5-2.0 inches of each seed for the full starter effect. In addition to an adequate starter fertilizer rate, additional phosphorus and potassium should be applied to prevent nutrient mining, causing soil test levels to decline in years when minimum fertilizer rates are applied.

Split the Risk with In-season Nitrogen

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.

Phosphorus and the 4Rs: The progress we have made

The year 2019 marked the 350th anniversary of discovering phosphorus, an element required for all life on Earth and an essential plant nutrient in crop production. Over the years, we have fallen in and out of love with phosphorus as a necessary crop input and an unwanted water pollutant. Through improved knowledge and technologies, we have made great progress in phosphorus management in crop production. Let’s take a look at our accomplishments!

Right Rate

Phosphorus fertilizer need and amount is determined through soil testing, based on regionally calibrated soil test levels for each crop. Soils with low soil test phosphorus require more fertilizer to optimize crop production, whereas soils with excess soil test phosphorus may only require a starter rate. Across the upper Midwest and northern Great Plains, soil testing shows that our crops generally need MORE phosphorus to optimize crop yield (Figure 1), particularly as crop yield and crop phosphorus removal in grain has increased. Since plant-available phosphorus varies across any field, precision soil sampling (grid or zone) allows us to vary fertilizer rates to better meet crop phosphorus requirements in different parts of the field.

For phosphorus and the 4Rs article

Figure 1. Soil samples with soil test phosphorus below 15 ppm critical level (Olsen P) across the upper Midwest and northern Great Plains in 2019.

Right Source

Nearly all phosphorus fertilizer materials sold in the upper Midwest and northern Great Plains are some ammoniated phosphate source, which has better plant availability in calcareous soils. Monoammonium phosphate (MAP, 11-52-0) is the most common dry source and convenient as a broadcast or seed-placed fertilizer. Some new phosphate products also include sulfur and micronutrients in the fertilizer granule, helping improve nutrient distribution and handling. The most common fluid source is ammonium polyphosphate (APP, 10-34-0), which usually contains about 75% polyphosphate and 25% orthophosphate that is available for immediate plant uptake. Liquid polyphosphate has the impressive ability to carry 2% zinc in solution, whereas pure orthophosphate can only carry 0.05% zinc. Such fertilizer product synergies help optimize phosphorus and micronutrient use efficiency.

Right Time

Soils of the northern Great Plains are often cold in spring, and early season plant phosphorus uptake can be limited to new seedlings and their small root systems. We apply phosphorus before or at planting to ensure adequate plant-available phosphorus to young plants and foster strong plant development. In-season phosphorus is rarely effective as a preventive or corrective strategy.

Right Place

Proper phosphorus placement depends on your system and goals. Broadcasting phosphorus fertilizer followed by incorporation allows quick application and uniform distribution of high phosphorus rates. This strategy works well if you are building soil test phosphorus in conventional till systems. In no-till systems, broadcast phosphorus without incorporation is not ideal because soluble phosphorus left on the surface can move with runoff to water bodies.

In no-till systems, subsurface banded phosphorus is more popular because phosphorus is placed below the soil surface, thus less vulnerable to runoff losses. In general, banded phosphorus is more efficient than broadcast phosphorus. In the concentrated fertilizer band, less soil reacts with the fertilizer granules, thus reducing phosphorus fixation, allowing improved plant phosphorus uptake. Some planting equipment configurations have the ability to place fertilizer near or with seed, which further optimizes fertilizer placement and timing for young plants.

For more information on 4R phosphorus management, please read this excellent open-access review article: Grant, C.A., and D.N. Flaten. 2019. J. Environ. Qual. 48(5):1356–1369.

Caution: Ammonium Sulfate with Seed

Seed-placed fertilizer is a common practice to increase seedling vigor and optimize fertilizer placement and crop response. This is a popular strategy to apply phosphorus for canola, corn, and wheat. However, the seed-placed fertilizer rate cannot exceed seed safety limits, otherwise seedling germination and plant population may be reduced. Sulfur is very important in canola growth and development, so farmers often try placing ammonium sulfate (AMS) with canola seed as well! This can create big problems.

A team of agronomists and soil scientists at the University of Manitoba conducted greenhouse and field studies, examining the effect of seed-placed ammonium sulfate on canola plant population and seed yield. The plant population loss was much greater on soils with pH > 7.5 (Figure 1). The high pH soils contained calcium carbonate (CaCO3), which reacts with ammonium sulfate to create calcium sulfate (gypsum) and ammonium carbonate. The higher reaction pH of ammonium carbonate produces free ammonia (NH3). Free ammonia (NH3) in soil is toxic to living organisms and kills germinating seeds. Acute ammonia toxicity is a major concern with fertilizer materials that liberate free ammonia (NH3) in soil, such as anhydrous ammonia (82-0-0) or urea (46-0-0), ultimately reducing plant population if you are not careful with fertilizer rate and placement.

For Caution: Ammonium Sulfate with Seed post

Figure 1. Ammonium sulfate (AMS, 21-0-0-24S) included with seed-placed monoammonium phosphate (MAP, 10-52-0) reduced canola plant population. Soil carbonate content is 21% CCE and 0.5% CCE in knoll soil and hollow soil, respectively. Brandon, Manitoba.

Across the landscape, soil pH and carbonate content will vary. The well-drained lower landscape positions (swales, hollows) often have acidic to neutral pH and little carbonate. The upper landscape positions (knobs, knolls), suffering decades of soil erosion, often have high pH and ample carbonate (Figure 1). The risk of plant population loss is greater on eroded knobs where adding ammonium sulfate can create ammonia toxicity concern.

Considerable yield loss will occur if canola plant population is less than 70 plants per square meter. Even with low fertilizer rates, the interaction of seed-placed ammonium sulfate and phosphorus can greatly reduce canola plant population. In Manitoba, 25% plant population loss was observed with only 8 lb/acre S and 18 lb/acre P2O5 (Figure 2).For Caution: Ammonium Sulfate with Seed article

Figure 2. Ammonium sulfate (AMS, 21-0-0-24S) included with seed-placed monoammonium phosphate (MAP, 10-52-0) reduced canola plant population. Carman, Manitoba, 2011.

Sulfur is vital for successful canola production, but it must be applied safely. There are new air drill configurations with innovative seed and fertilizer placement options. Seed safety is paramount with seed-placed fertilizer. Ammonium sulfate should be broadcasted or banded away from seed (mid-row). Keeping ammonium sulfate away from seed will also allow you to maximize seed-placed phosphorus rates and efficiency without jeopardizing seed safety.

Placing ammonium sulfate with seed should be an emergency option only. Canola plant population loss should be expected, even at low ammonium sulfate rates, on soils with pH greater than 7.5 and calcium carbonate.