Soil Organic Matter and Cover Crop-based Nitrogen Recommendations for Corn

Introduction

Nitrogen (N) recommendations for corn have always been challenging to develop because of the dynamic nature of the many different N sources that contribute to meeting crop needs (e.g., manure, fertilizer, crop residues, and soil organic matter). Yield goal-based N recommendations, which use the expected crop yield multiplied by an N requirement coefficient (i.e., 1 lb N/bu corn yield), have served well for many years as a starting point for making corn N fertilizer recommendations. However, there are many methods available to improve upon these recommendations, such as using the pre-sidedress soil nitrate test, chlorophyll meter test, late-season stalk nitrate test, computer models, and learning from experimentation with different N rates in farmer-conducted trials.

In this article, we describe a new method of making N recommendations for corn that explicitly accounts for N supply from soil organic matter and cover crop residues. This method has been under development at Penn State for nearly a decade and, while still experimental in nature, has firm scientific underpinnings. This article explains the principles of the system, the required inputs, and suggested sampling methods, and an appendix with the formulas used for the calculations. We have also developed an online graphical interface and a spreadsheet template for completing the calculations.

From Yield Goal- to Delta Yield-Based N Fertilizer Recommendations

This new N recommendation method is based on a different concept of calculating crop N fertilizer requirements than the yield goal-based system. Instead, the recommendations are calculated based on a metric called delta yield, which is the difference between the potential corn yield at a site given no N limitations and the corn yield that could be produced with no fertilizer added (Figure 1). The corn yield without fertilizer added reflects the amount of N supplied by soil organic matter and cover crop residues in cases where cover crops are used. Delta yield is a better predictor of economically optimal N rates for corn production than the yield goal alone because it accounts for the N supplied by soil and determines the fertilizer requirement based not on the entire yield goal but the additional yield beyond what the soil is capable of producing without fertilizer.

We have used a relatively large database of corn yield response trials to calibrate an equation to predict the agronomically optimum N rate (AONR) for corn (N needed to maximize yield) based on delta yield in Pennsylvania (see equations in Appendix). This calibration can also be used to determine the economically optimum N rate (EONR) for corn, which is the rate that maximizes the return on investment in N fertilizer purchases. Because corn yields usually increase with diminishing returns (Figure 1), often the last few units of N needed to maximize yield do not increase the yield enough to pay for the cost of the additional N. The EONR is a dynamic component of the N fertilizer recommendation because it depends on the relative costs of N fertilizer and the value of corn, known as the fertilizer:crop price ratio (PR), which can change based on fluctuating markets.

An important component of calculating N recommendations using delta yield is being able to predict the corn yield that a soil can produce without N fertilizer added. Predicting this unfertilized corn yield is where our new N recommendation system is able to account for N supply from soil organic matter and cover crop residues. Using hundreds of measurements of unfertilized corn yield produced in different research stations and on-farm experiments across Pennsylvania, encompassing a wide range of soil types, cover crop types, and growing season weather conditions, we have calibrated a series of equations that predict unfertilized corn yield based on site-specific soil and cover crop measurements. Table 1 provides a summary of the required soil and cover crop inputs, as well as suggested methods of measurement.

After the unfertilized corn yield is predicted based on the inputs in Table 1, it can be compared to the expected yield potential for a site based on previous experience to calculate delta yield, which is then used to predict AONR and EONR based on a user-defined fertilizer:crop PR. The following sections describe the soil and cover crop factors that affect N supply to corn, the soil and cover crop inputs and sampling methods required to generate N recommendations, how to calculate the N recommendations, and how to meet the N recommendations with different fertilizer and manure sources.

Factors Affecting N Supply to Corn

Soil Texture

Soil texture is an important property of soil that regulates the decomposition of soil organic matter, which is made up of carbon, nitrogen, and many other elements. Greater mineral surface area, which occurs with a greater clay content of the soil, has a stabilizing effect on soil organic matter through the adsorption of organic matter molecules onto clay surfaces. In soils with greater sand content, organic matter is less stabilized and more likely to be decomposed by microbes, resulting in carbon dioxide respiration into the atmosphere. This process is tied to nitrogen mineralization (the conversion of organic nitrogen into ammonium) because when microbes respire carbon, the N contained in that organic matter is released into the soil as ammonium. Greater microbial respiration, which is a result of faster organic matter decomposition and more cycles of microbial attack, will promote more N mineralization. In our new recommendation system, this process is captured by a term called humification efficiency. It roughly translates into the proportion of decomposed carbon (C) that remains in microbial biomass or stabilized organic matter within the period of a corn growing season, as opposed to being respired. Humification efficiency is, therefore, inversely related to N mineralization. During the calibration of our equations to predict unfertilized corn yield, we have found that humification efficiency can be predicted by soil % sand and % clay. Because the humification efficiency controls N mineralization from soil organic matter and cover crop residues, calculating this value is the first step in the process of predicting unfertilized corn yield.

Soil Carbon and Nitrogen

Soil organic matter is composed of many elements but is roughly half C, and in agricultural soils, every 10 parts of C contain 1 part of N, on average. In a topsoil that weighs 2,000,000 lbs/ac, a soil with 2% organic matter will roughly contain 20,000 lbs/ac of C and 2,000 lbs/ac of N in organic matter. Soil organic matter is the largest pool of N in the soil but is largely not available for crop uptake until it becomes mineralized into ammonium by microbes in the soil. This mineralization process occurs during decomposition, as described earlier, and is controlled by temperature, moisture, microbial physiology, and the stabilization of organic matter by mineral surfaces. All else held equal, the more organic matter in the soil, which we can measure as % C, and the more N per unit of C, which we can measure as soil C:N ratio (%C/%N), the greater N mineralization will be. Soil C:N ratio is an important regulator of N mineralization because N will be released from organic matter as the C is decomposed by microbes in proportion to the relative concentration of C and N in the soil, where a low C:N ratio represents a high concentration of N relative to C. We have calibrated an equation based on over a hundred measurements of unfertilized corn yield to predict the contributions of soil organic matter mineralization to the yield of unfertilized corn (see Appendix). The equation for soil organic matter contributions to unfertilized corn yield is calculated using the humification efficiency previously described and the soil %C and soil C:N ratio.

Cover Crop Nitrogen Content and C:N ratio

Cover crops are used to meet a wide variety of goals, some of which are related to N management. Grass and brassica cover crops can be effective N scavengers to prevent wintertime N leaching losses and can recycle this N into organic matter. Legumes can fix N from the atmosphere through a symbiotic association with rhizobial bacteria living in root nodules and will cycle this N back into the soil upon termination and decomposition. The process of N mineralization from cover crop residues is microbially driven and has very similar controls to that of soil organic matter decomposition and N mineralization described earlier. All else held equal, the greater the N content of the cover crop residue and the more N per unit of C in the cover crop residue (i.e., a low C:N ratio), the more N will be mineralized. Different than soil organic matter, however, which usually has enough N relative to C that decomposition results in ammonium release into the soil (N mineralization), some cover crop residues cause N immobilization. Nitrogen immobilization is a tie-up of nitrate or ammonium N in microbial biomass that occurs when high C:N crop residues (i.e., low N per unit C) are decomposing. As microbes decompose the low N residues and build their biomass with C, the microbes cannot derive enough N from the residues to meet their physiological needs for growth, so the microbes scavenge ammonium or nitrate N from the soil. This process reduces the inorganic N available in the soil to meet the needs of crop growth and may increase the amount of supplemental N additions (i.e., fertilizer or manure) needed for the crop.

The C:N ratio tipping point between N mineralization and immobilization, which we refer to as the critical C:N ratio for immobilization, is often between 20:1 and 30:1. It is controlled in part by microbial physiology, community composition, and the turnover of microbial biomass through multiple generations of microbes. These aspects control the microbial carbon use efficiency (the proportion of decomposed C retained in biomass as opposed to respired) at the organismal and community levels. In our equations, microbial carbon use efficiency is one component of the humification efficiency factor, which is calculated based on % sand and % clay. Therefore, in our equations, the critical C:N ratio for immobilization is controlled by soil texture. Increasing the % clay relative to % sand increases the humification efficiency, meaning more C and N is retained in microbial biomass, leading to a lower cover crop C:N ratio at which N immobilization will occur.

We calibrated an equation to predict the effect of cover crop residue N mineralization or immobilization on unfertilized corn yield using hundreds of experimental observations where the yield of unfertilized corn grown after a cover crop was compared to corn grown after no cover crop (see Appendix). This equation incorporates the humification efficiency factor and the cover crop N content (lbs N/ac) and C:N ratio at termination. It also translates the effect of N mineralization or immobilization into a corn yield credit using a calibrated coefficient. The calibrated coefficients are different for winter-killed cover crops and winter-hardy cover crops that are either mineralizing or immobilizing, which accounts for N losses that affect the efficiency with which N mineralization or immobilization translates into corn yield response.

Required Soil and Cover Crop Inputs and Sampling Methods

Several measurements of soil and cover crop residues are needed to make N recommendations using this new method. Here, we describe the types of measurements needed and recommendations on how to conduct the sampling and obtain the analysis.

Soil texture, carbon, and nitrogen

Soil measurements of % sand, % clay, % C and % N can be made from a composite soil sample, such as those traditionally collected for routine fertility analyses. Because these measurements of a soil are relatively stable, the sample can be collected at any time of year. Although soil % C and % N can change over time due to management, they are relatively slow-changing properties, so it is probably sufficient to collect soil samples for this measurement every three years, similar to the recommended frequency for regular fertility sampling. Therefore, the same soil sample used for regular fertility analysis could be used for this N recommendation system. One important point about soil sampling, however, is that the equations are calibrated based on a 0-to-8-inch sampling depth, which may be different than your traditional fertility sampling depth (many routinely sample only to 6-inch depth). It is especially important to follow the 8-inch depth sampling guidelines in no-till fields, where soil organic matter can be highly stratified with the highest organic matter concentrations at the soil surface. In these conditions, taking a soil sample to 8-inch depth will result in a lower % C and % N than a 6-inch depth sample, because the 8-inch depth mixes in an additional 2 inches of soil with lower C and N concentrations.

The most accurate way to measure % C and % N in a soil sample is to use an elemental combustion analyzer. In this method, the soil sample is heated to a very high temperature, and the carbon dioxide and nitrogen gases released during combustion are measured by the analyzer. This method currently costs $30 at the Penn State Agricultural Analytical Services Lab. Other soil testing labs most likely offer elemental combustion analysis for carbon and nitrogen as well. After receiving the results of this analysis, calculate the soil C:N ratio as % C divided by % N. Soil %C and C:N ratio have one of the largest influences on the N recommendations developed by the tool, so we suggest placing the highest priority on obtaining a reliable estimate of this soil property.

While the most accurate method to determine % C and % N and soil C:N in a soil sample is by elemental combustion analysis, there are also crude rules of thumb to convert a measurement of organic matter into these variables. Using these rules of thumb can reduce the accuracy of the prediction but makes the recommendation tool accessible if you cannot obtain % C and % N by the recommended method. To convert soil organic matter measured using the loss on ignition method into % C, multiply the organic matter % by 0.44 if the soil test reports %LOI (loss on ignition) or multiply by 0.59 if the soil test reports % OM (organic matter). Agricultural soils often have a soil C:N ratio near 10:1, although variation does occur, so you can use a default value of 10:1 in the absence of a measured soil C:N ratio.

Sand and clay percentages can also be measured from the same composite soil sample collected from 0 to 8-inch depth. The most accurate measurement of soil texture is to conduct a particle size test using the hydrometer, which is offered by the Penn State Agricultural Analytical Services Lab for $25. Because soil texture does not change due to management (unless significant fill soil is introduced or deep tillage brings up clay-rich subsoil), soil texture only needs to be measured once in a lifetime, not every three years. This makes the soil texture analysis a one-time investment in being able to use this N recommendation method. Although soil texture is an important control on N mineralization, it has a relatively small influence on the overall N recommendations relative to other factors, such as soil %C and C:N ratio. Therefore, it may be sufficient to estimate the % sand and % clay content of a soil from NRCS soil survey maps. We recommend an online tool called SoilWeb that has a relatively easy-to-use interface for browsing soil survey maps and obtaining soil texture information from soil map units. More information about how to use this service to determine soil texture is described here.

Cover crop nitrogen content and C:N ratio

Cover crop information that is needed to make N recommendations using this tool includes the biomass N content (lbs N/ac) and C:N ratio of winter-hardy cover crop species in spring at their maximum growth prior to termination. If winter-killed species are used, either as monocultures or as a component of mixtures, the biomass N content of just the winter-killed species at peak growth in the fall, prior to winterkill, can be used. The N contribution from winterkilled species is much less than that of winterhardy species, especially when the winterkilled species are just a small component of a mixture. If this is the case, it is possible to omit the winter-killed species from the calculation.

There are multiple ways to estimate cover crop N content and C:N ratio. The most accurate, but also perhaps the most time-consuming and expensive, method is to sample the cover crop biomass by clipping the tissue from the soil from a quadrat of a known area. The biomass can be dried, weighed, and then submitted to a lab for analysis of % C and % N. An alternative method of estimating cover crop biomass N content that we developed is to use a handheld Greenseeker sensor to measure the cover crop NDVI (normalized difference vegetation index). We have developed a calibration of the NDVI value from a handheld Greenseeker sensor to predict the cover crop biomass N content for different species and mixtures in the fall or spring. Many of the Penn State Extension Agronomy Team Educators have handheld Greenseeker sensors that they can loan to interested farmers. More information about how to use a handheld Greenseeker to measure cover crop biomass N content is available here. Finally, Table 2 provides average values and typical ranges for biomass N content of several different common cover crop species and mixtures, which could be used for planning purposes, scenario development, or to estimate an N recommendation in the absence of better data.

Cover crop C:N ratio is the other cover crop measurement needed to develop N recommendations using this tool. Cover crop C:N ratio can be measured by a laboratory analysis of % C and % N. This could be conducted on a “grab sample” of representative cover crop tissue cut at the soil surface from a few spots in the field, rather than from a quadrat of known area. This property of the cover crop could also be measured from a lookup table of typical cover crop C:N ratio values based on species and growth stage, provided in Table 2. The cover crop C:N ratio has a relatively small impact on the final N recommendation, so it should be sufficient to estimate this property of the cover crop. Additionally, an uncertainty analysis could be conducted using the tool, testing different potential values within a typical range of the species and growth stage, and the user could choose a more conservative value (higher C:N ratio) to reduce the risk of an N shortage.

^a Early maturity is considered the tillering growth stage; medium maturity is the jointing to boot stage; late maturity is the heading stage and later.

^b Adjust the average C:N for fields with manure application: Reduce C:N by 5 for fields with manure applied annually and/or reduce C:N by 5 more for fields with manure applied to growing cover crops (fall or early spring).

^c Adjust to the higher end of this range for mixtures with mature grasses.

Compiled by K. Arrington and M.B. Gavin from a database of cover crop samples collected from the Penn State Research Station (Rock Springs, PA) and commercial farms across Pennsylvania (2011 – 2021; > 1200 samples).

Calculating and Fulfilling N Recommendations

We have developed both an online graphical interface and an Excel spreadsheet to facilitate the calculations of N recommendations based on the inputs described above. Furthermore, the appendix of this article includes the full series of equations used to calculate the recommendations for those who may be interested in doing the calculations by hand or just knowing what happens inside the interface or spreadsheet. Using either the graphical interface or the Excel spreadsheet, enter the inputs for a realistic corn yield goal based on the average of yields obtained from multiple years of production on a given field and the soil and cover crop measurements described above. There is also a spot to add the cost of N fertilizer and the price of corn, which adjusts the EONR based on the fertilizer:crop price ratio. The online interface or spreadsheet will handle all the calculations and report the recommended supplemental N fertilizer requirement needed to meet the yield goal.

Fulfilling N Recommendations with Manure and Fertilizer N Sources

The recommended N fertilizer calculated by this new method represents the total quantity of available N that should be supplied to the corn during the growing season through a combination of manure applied in the fall or spring, fertilizer applied at planting, and fertilizer applied as a sidedress. Deciding on how to achieve this target rate of available N through these various N sources is part of the "art of agronomy" and is likely to be different for each farm operation. Furthermore, limited research has been conducted on how manure applications made in the fall or spring on living cover crops should be credited because some of the N contained in that manure will be scavenged and recycled by the cover crop, where it is accounted for in the cover crop N mineralization equation. Our current best estimates of how to account for manure N and strategically apply N fertilizer when using this new recommendation system are described below.

To account for N available from manure, we suggest using the manure N availability factors in the Penn State Agronomy Guide. However, some adjustments may need to be made in how you use these factors in combination with the N recommendation method described here. Table 3 describes the manure N availability factors to use for various combinations of manure and cover crop management scenarios.

After the manure N availability is calculated and subtracted from the total N recommendation made by this new tool, the remainder of the N requirement should be met through fertilizer applications. If the remaining N balance is a small amount, for instance, less than 30 lbs N/ac, it can be applied entirely at planting, as it can be difficult to sidedress low rates of N. For remaining N balances between 30 and 80 lbs N/ac, fertilizer N can be split approximately half and half between at-planting and sidedress applications. For remaining N balances greater than 80 lbs N/ac, fertilizer applications can be split one-third at planting and two-thirds at sidedress.

If there is uncertainty about the amount of N required, additional soil testing prior to sidedressing can be conducted using the pre-sidedress soil nitrate test (PSNT), which has been recently recalibrated to predict sidedress N requirements in no-till crop production when monoculture grass or legume cover crops or no cover crops are used. The new PSNT calibration indicates it can accurately predict N requirements at sites with a long-term manure history (two or more years of manure applications in the last five years). For valid results from the PSNT, no more than 50 lbs/ac of N fertilizer should be applied at planting (manure applications are okay). Results of the PSNT sampling can be used to calculate the sidedress N requirement using the formula provided in the fact sheet.

Role of Weather

Weather plays an important role in affecting N requirements for corn, both through the effect on the overall corn yield potential, which sets the N demand by the crop, and the effect on soil N contributions from mineralization. The equations for developing N recommendations presented here have been calibrated to account for the average weather conditions in Pennsylvania and should therefore not be used to make N recommendations in other states. One way that the equations have been calibrated for the average weather conditions is in the coefficients of the cover crop and soil organic matter N mineralization equations, which factor in the extent to which soil organic matter and cover crop residues will decompose during a typical corn growing season. When calibrating the equations to predict unfertilized corn yield, we also accounted for the rainfall that occurred during June, July, and August, which are the months that corn yield is most affected by the weather. Greater than average (but not excessive) rainfall increased yields, while less than average rainfall decreased yields. By accounting for the effect of rainfall deviations from the average, our equations are tuned to predict the unfertilized corn yield in a year with average rainfall in June, July, and August. Because N fertilizer applications usually need to be made before the precipitation in June, July, and August is known, the tool does not have an input for rainfall adjustments. Rather, we used the rainfall data during the calibration process to adjust the unfertilized yield prediction to reflect average precipitation levels. Therefore, when selecting the realistic corn yield goal for a site, it is important to use a true average of the realized yield in a field over a series of years that represents the typical range of weather conditions. In this way, the tool provides an N recommendation based on the average growing season weather in Pennsylvania. Choosing a realistic corn yield goal that is higher than the true average for a site (such as choosing the highest yield ever achieved) may increase the N recommendation above what is economically optimal.

Future Work

It is important to recognize that although this new approach to making N recommendations is based on a large body of research conducted on many different commercial farm and university research station fields over many growing seasons, the recommendation method as a whole is still highly experimental. Future years of research will focus on holistically testing this approach in a production setting to better understand its performance, identify weaknesses, resolve uncertainties, and develop scalable workflows. Areas of research and development that we are actively pursuing to improve the usability and accuracy of this new N recommendation method include:

• Determining whether satellite imagery can be used to estimate cover crop biomass N content.
• Developing methods to estimate cover crop C:N ratio without tissue sampling.
• Determining if additional soil analyses of labile C and N fractions improve the accuracy of the N recommendations.
• Improving the underlying equations by accounting for microbial physiology and soil texture controls on N mineralization.
• Understanding how to credit manure applications towards meeting the N fertilizer recommendations.
• Determining how to account for N availability when cover crops are harvested for forage.

Conclusion

By now, you can appreciate the complex set of factors that interact to regulate N supply from soil organic matter and cover crop residues. Given this complexity, it is no wonder that for so long, N recommendations for corn have not included adjustments for soil organic matter and cover crops based on site-specific measurements. While the new process for making N recommendations described in this fact sheet is a noteworthy milestone in the trajectory of corn N management, it does not mark the end of a journey but rather the beginning of a new chapter in our pursuit to advance the efficiency and economy of agricultural production. We will surely continue to learn and improve as we continue this pursuit, and we welcome and appreciate your company on this journey.

Appendix

Equations to calculate soil organic matter and cover crop-based N recommendations for corn

Step 1: Calculate the humification efficiency (ε)

The first calculation is the humification efficiency, based on % sand and % clay.

ε = 0.014 + 0.011 x %Clay + 0.0053 x %Sand
Equation 1

Where %Clay and %Sand are the percentages of each soil texture size class, in values of 0 to 100.

Step 2: Calculate the soil organic matter credit

The next calculation is the soil organic matter yield credit, based on soil % C, soil C:N ratio, and humification efficiency.

Where ΔY_SOM is the corn yield credit from soil organic matter, %SoilC is the total soil carbon concentration in a 0-8" sample depth, ε is the humification efficiency (calculated in Equation 1) and (C:N)_s is the soil C:N ratio (%C/%N).

Step 3: Calculate winter-hardy cover crop credit

Next, the cover crop yield credit is calculated based on the biomass N content, cover crop C:N ratio, and humification efficiency. First, determine if cover crop biomass will be mineralizing or immobilizing. If the cover crop C:N ratio is less than 10/ε, then biomass will be mineralizing; otherwise, cover crop biomass will be immobilizing. Then use Equation 3 as follows:

Equation 3

Where ΔY_whcc is the corn yield credit from winter-hardy cover crop residues, α = 0.55 when cover crops are N mineralizing and α = 1.8 when N immobilizing, N_whcc is the winter-hardy cover crop biomass N content in lbs/ac N at the time of termination in spring, ε is the humification efficiency (calculated in Equation 1) and (C:N)_whcc is the winter-hardy cover crop C:N ratio.

If the calculated winter-hardy cover crop N credit is less than -92, use a value of -92, which represents the greatest extent of N immobilization that can occur from cover crops.

Step 4: Calculate winter-killed cover crop credit

If the cover crop stand has a winter-killed species, then calculate the winter-killed cover crop credit as follows.

Where ΔYwkcc is the corn yield credit from winter-killed cover crops, and Nwkcc is the winter-killed cover crop biomass N content in lbs/ac N at the time of peak fall growth prior to winterkilling.

Step 5: Calculate unfertilized corn yield prediction (Y₀)

Next, the soil organic matter and cover crop yield credits are added together to get an overall corn yield credit, which is then adjusted with a quadratic curvature to account for the diminishing returns of N supply to corn yield. This results in the final unfertilized corn yield prediction.

If (ΔY_SOM + ΔY_whcc + ΔY_wkcc) > 391 then Y₀ = 196, else

Y₀ = (ΔY_SOM+ ΔY_whcc + ΔY_wkcc)−0.00128 x (ΔY_SOM+ ΔY_whcc + ΔY_wkcc)²
Equation 5

Where Y₀ is the predicted unfertilized corn yield in bu/ac, ΔY_SOM is the soil organic matter credit (calculated with Equation 2), ΔY_whcc is the winter-hardy cover crop credit (calculated with Equation 3), and ΔY_wkcc is the winter-killed cover crop credit (calculated with Equation 4).

Step 6: Calculate delta yield (dY)

After the predicted unfertilized yield has been calculated, the delta yield can be determined. Delta yield is simply the difference between the realistic corn yield goal and the unfertilized corn yield prediction. If the unfertilized corn yield prediction is equal to or greater than the corn yield goal, it indicates there is enough soil N supply to meet the needs of the corn at the expected yield goal, and no supplemental N is required.

dY = Y_G – Y₀
Equation 6

Where dY is the delta yield in bu/ac, Y_G is the realistic corn yield goal (average yield for the site) from past experience in bu/ac, and Y₀ is the predicted unfertilized corn yield in bu/ac (calculated with Equation 5). If dY is negative, sufficient N will be supplied from soil and cover crop residues, and no additional N fertilizer is recommended.

Step 7: Calculate agronomic optimum N rate (AONR)

When the delta yield is positive, the amount of N needed to achieve the yield goal is calculated based on the calibration between delta yield and AONR.

AONR = 124 + 0.61 x dY
Equation 7

Where AONR is the N rate needed to reach the realistic corn yield goal in lbs/ac N and dY is the delta yield (calculated with Equation 6).

Step 8: Calculate fertilizer:crop price ratio and EONR

The AONR calculated in step 7 is not the economically optimum N rate, and our calibration indicates that the AONR can be relatively high even with a small delta yield. It is important to take the final step of calculating the EONR based on the fertilizer:crop price ratio. When the delta yield is small, the EONR may be negative or zero because adding even the first unit of N fertilizer does not result in a yield gain that will return more income than the cost of the fertilizer. If the calculated EONR is negative or zero, no supplemental N fertilizer should be used.

Where PR is the fertilizer:crop price ratio, $/lb N is the fertilizer cost of one pound of N and $/bu corn is the market price of corn grain.

Where EONR is the economically optimum rate of nitrogen to apply (the cost of the last pound of N applied equals the value of the corn yield increase from that N application) in lbs/ac N, AONR is the agronomic optimum N rate (calculated with Equation 7), PR is the fertilizer:crop price ratio (calculated with Equation 8), and dY is the delta yield (calculated with Equation 6).

If the calculated EONR is negative, then corn is not likely to have an economically positive response to N fertilizer, and no N is recommended.