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.

John Breker

Soil Nitrogen Trends – Fall 2023: Some Up, Some Down

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The 2023 drought was an all-too-soon reminder of the widespread 2021 drought. It covered much of the upper Midwest, Great Plains, and Canadian Prairies. From previous experience with droughts, we expected that residual soil nitrate-N following crops would be higher than normal, caused by the drought and reduced crop yields. The first wheat fields that were soil tested in August and September confirmed our expectation that residual soil nitrate-N was already trending higher than normal. Yet, some regions were spared the drought and received above-average rainfall, and achieved record-setting crop yields. For these regions, the amount of residual soil nitrate-N after high-yielding crops was near or below average. 

The 2023 AGVISE soil test summary data highlights the great variability following the drought. The median amount of soil nitrate-nitrogen across the region was higher than the long-term average following wheat. Over 28% of wheat fields had more than 60 lb/acre nitrate-N (0-24 inch) remaining. Yet, another 17% of wheat fields had less than 20 lb/acre nitrate-N remaining, suggesting either lost crop yield or protein due to insufficient nitrogen nutrition. For any given farm, the great variability in residual soil nitrate-N across all acres makes choosing one single nitrogen fertilizer rate impossible for next year, and soil testing is the only way to decide that right rate for each field.

Through zone soil sampling, we are also able to identify that residual soil nitrate-nitrogen can vary considerably within the same field. This makes sense because we know that some areas of the field produced a fair or good yield, leaving behind less soil nitrate, while other areas produced very poorly and left behind much more soil nitrate. These differences across the landscape are driven by soil texture, soil organic matter, and stored soil water as well as specific problems like soil salinity or low soil pH (aluminum toxicity). Although the regional residual soil nitrate-nitrogen trends were higher overall, it is truly through zone soil sampling that we can begin to make sense of the field variability that drives crop productivity and determine the right fertilizer rate for next year.

For fields that have not been soil tested yet, there is still time to collect soil samples in winter. Nobody wants to experience another drought, but this kind of weather reminds us how important soil nitrate testing is every year for producers in the Great Plains and Canadian Prairies. Each year, AGVISE summarizes soil test data for soil nutrients and properties in our major trade regions of the United States and Canada. For more soil test summary data and other crops, please view our soil test summaries online: https://www.agvise.com/resources/soil-test-summaries/

Cindy Evenson

Choosing the Right Phosphorus Method

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This article originally appeared in the AGVISE Laboratories Spring 2023 Newsletter under President’s Corner

The phosphorus soil test debate never ends. Should I use the Olsen test, or maybe Bray-1 would be better? What about the Mehlich-3 method, and should that extract be analyzed on an ICP or with a colorimetric method? Perhaps, Bray-2 or the Haney extractable P is something to consider? This whole phosphorus test dilemma can be quite confusing; however, the answer is quite simple. Use the soil phosphorus test that is calibrated for your region!

In the upper Midwest, the Olsen test is the most reliable method to determine phosphorus availability and has the most correlation and calibration data with field trials. Many hours have been spent by university researchers putting out field trials to determine phosphorus fertilizer rates for various crops. The researchers have evaluated various phosphorus methods, and the two most common methods are the Bray-1 and Olsen extractants. The Bray-1 method is the older method, developed in Illinois. It works well on soils with pH below 7.3. Once the soil pH is above 7.3, the extractant may fail. If the test fails, it will produce a result near zero.

The Olsen method is required on calcareous soils (pH > 7.3), but it also works well on acidic soils. There is a common misconception that the Olsen method is only suitable on calcareous soils. In fact, the Olsen method is widely used across the world because of its versatility on acidic and calcareous soils. It is a perfect fit for our region because it works so well across a wide soil pH range and on diverse soil types. In the AGVISE Newsletter Spring 2017 issue, retired AGVISE President Robert Deutsch compiled soil test data for the Bray-1 and Olsen methods with over 25,000 soil samples. The graphs highlight how robust the Olsen phosphorus method is, working on acidic and calcareous soils alike.

The Mehlich-3 method has gained popularity in the southeast United States and the central Midwest. In these regions, the soils are more weathered and often do not have problems with high calcium carbonate content. At the University of Minnesota, Dr. Dan Kaiser has worked on Mehlich-3 method correlation on Minnesota soils for quite a few years. For some soils, the Mehlich-3 method performed as expected, while some others had Mehlich-3 results 8 to 10 times higher than expected. For these reasons, the Mehlich-3 method has not been approved for use in the upper Midwest or northern Great Plains.

As of this time the only phosphorus soil tests recommended for soils in the upper Midwest are the Olsen and Bray-1 extracts. If someone mentions using any other phosphorus soil test, it has not been tested or correlated to the soils in this region.

John Breker

Are Soybean Iron Deficiency Chlorosis (IDC) Ratings Getting Worse?

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This article originally appeared in the AGVISE Laboratories Spring 2023 Newsletter

For the past three years, we have seen severe and widespread soybean iron deficiency chlorosis (IDC) symptoms across the region. In fact, some seasoned agronomists have commented that 2022 was the worst soybean IDC year that they had experienced in decades. Soybean IDC is a serious risk on soils with high calcium carbonate or salinity, which interfere with iron uptake and utilization in soybean. With all that we have learned about soybean IDC risk and management over the past 30 years, we have to ask, “What is going on? Why is soybean IDC continuing to get worse?”

The NDSU soybean IDC trial data suggests it might be the soybean varieties. Each year, seed companies submit soybean varieties to NDSU for independent evaluation of soybean IDC ratings (https://www.ag.ndsu.edu/varietytrials/). The NDSU trial sites impose high soybean IDC risk, where the best and worst soybean varieties are thoroughly tested alike for soybean IDC tolerance. In recent years, the problem is that the year-after-year average soybean IDC rating continues to get worse (see figure). In 2022, the average soybean variety scored 3.5 on the NDSU scale (1-good, 5-bad). Adverse soil and weather conditions may explain part of the worsening problem in the NDSU trials, but it is apparent that few soybean varieties can handle severe soybean IDC on their own. In defense of soybean breeders, there are a lot of different breeding objectives on their plates right now, including herbicide tolerance packages, disease and insect pests, and seed yield, of course!

This means we need to revisit and use all of our options in the soybean IDC toolbox. We have known about these effective management tools for over 20 years, and we are going to need to use all of them until soybean variety IDC tolerance can get to where we need it.

Steps to better soybean IDC management

  • Soil test each field, zone, or grid for carbonate and salinity to evaluate soybean IDC risk potential.
  • Plant soybean in fields with low soybean IDC risk. Choose a tolerant soybean variety, if you can. Some high IDC-risk fields may not be suitable for soybean.
  • Use a chelated iron fertilizer (high-quality EDDHA or HBED chelate) with seed at planting. Liquid and dry products are now available.
  • Plant soybean in wider rows. Soybean IDC tends to be less severe in wider rows.
Jodi Boe

Two Graphics You Should Know Before the 2023 Growing Season

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This article originally appeared in the AGVISE Laboratories Spring 2023 Newsletter

The goal of AGVISE Newsletters is to inform you and your customers of important soil fertility information relevant to our area. Often, visuals or graphs are much more powerful at communicating a message than words. With that in mind, I want to share two figures I think you should know about going into the 2023 growing season with a short synopsis and where you can find more information on the topic.

Adapted from the “Biostimulants” episode of the University of Minnesota Extension Nutrient Management Podcast https://nutrientmanagement.transistor.fm/episodes/biostimulants-52305fc9-6c01-4907-8507-2bcf4c708a08

Right now, there are many biological products and fertilizer additives on the market. In particular, asymbiotic nitrogen-fixing products have gained a lot of attention, but many have little or no university research evaluating them. Any grower wanting to try new products should test them on a small acreage first, before adopting them across the whole farm. To the left is a diagram of how such an on-farm trial would look. The key factors of a meaningful on-farm trial include a control treatment (the standard practice), the standard practice plus the product, and randomized replication (at least three replicates, randomized so that one treatment is not always on the east or west side of the trial area).

If the standard nitrogen rate will be reduced when the product is used, a treatment should also be included that compares the same reduced nitrogen rate without the product (this three-treatment setup is what is pictured). If the standard nitrogen rate is higher than the crop N requirement, maybe if you do not have a current soil nitrate-N test or just general overapplication, a reduced nitrogen rate plus the product that produces the same crop yield as the standard practice does not mean that the product is producing additional nitrogen for the crop; it may just mean that the grower can cut back their standard nitrogen rate.

Slide from Dr. John Jones’ 2023 AGVISE Seminar presentation, Phosphorus and Potassium: A Fresh Look with Fresh Data https://www.agvise.com/resources/seminars-and-events/

With fertilizer prices remaining high, it is tempting to cut back phosphorus and potassium inputs to save money. As tempting as this is, do not cut back farther than the fertilizer rates needed to meet the critical soil test level, as optimum soil-test P and K levels are required to achieve the highest response from nitrogen fertilizer. While working to update the Wisconsin phosphorus and potassium fertilizer guidelines, Dr. John Jones at the University of Wisconsin has put together some excellent data illustrating the reality of Liebig’s Law of the Minimum: when P and K fertility needs are unmet, the return from nitrogen fertilizer investment will be reduced compared to when P and K are at optimum levels. This means pouring on more nitrogen will not increase crop yield unless you are doing a good job of managing P and K too. Although not shown here, Dr. Jones also has data showing that corn and soybean yield response to P is reduced when K fertility needs are unmet.

John Breker

High Fertilizer Prices? Using Crop Removal P & K Rates is an Expensive Choice

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This article originally appeared in the AGVISE Laboratories Spring 2023 Newsletter

If you thought high fertilizer prices would resolve after one or two years, it is looking like those prices are becoming the new norm. At such prices, every fertilizer dollar you spend must be spent to guarantee the best bang for each buck. This means soil testing makes more dollars and sense than ever.

Phosphorus and potassium are best managed with current soil test information to maximize crop yield potential and profitability. Yet, some people continue to apply phosphorus and potassium at crop removal (CR) rates as a way to maintain the soil fertility status quo. This is a major oversight because CR-based rates maintain soil fertility in a way that overapplies fertilizer to parts of the field with high soil test P or K that do not need more fertilizer, yet underapplies fertilizer to parts with low soil test P or K and ultimately sacrifices crop yield. This is particularly troublesome if the factor that limited crop yield was one of those nutrients! As a result, the reduced crop yield leads to a lower CR-based fertilizer rate that fails to fix the soil fertility issue, and you stay in a low soil fertility rut. For example, if soil test P is very low and limits crop yield, a crop removal-based P rate will undershoot the actual crop P requirement, resulting in reduced crop yield and continued nutrient mining year after year. A soil test-based P rate will show you exactly where more fertilizer is required to maximize crop yield and where you can reduce fertilizer rates to maximize profitability.

Another serious reason to avoid CR-based rates is the risk of off-site nutrient losses, especially phosphorus. When CR-based rates are applied on soils with high or very high soil test P, this increases the risk for environmental P loss to waterways that can degrade water quality and result in regulatory oversight. Precision soil sampling (grid or zone) and soil test-based fertilizer rates is the best way to maximize crop yield, profitability, and protect the environment.