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  FO-06125-GO     1993 To Order   
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Best Management Practices for Nitrogen Use Statewide in Minnesota


G. W. Randall and M. A. Schmitt



These nitrogen BMPs were assembled by the University of Minnesota and the Minnesota Department of Agriculture (MDA) as part of the Nitrogen Fertilizer Management Plan developed by the Nitrogen Fertilizer Task Force coordinated by the MDA.

Summary

Nitrogen (N) is an essential plant nutrient that contributes greatly to the agricultural economy of Minnesota crop producers. Unfortunately, the nitrate form of nitrogen can leach into surface and groundwaters if nitrogen management is not practiced carefully. The public will not tolerate contamination of water resources by agricultural production systems; laws regulating use of nitrogen fertilizers could be adopted if contamination from agricultural activities is not minimized.

The research-based Best Management Practices (BMPs) outlined in this bulletin are economically and environmentally sound. It is strongly suggested that farmers adopt them voluntarily. By following these recommendations, agriculture can avoid the threat of fertilizer regulations and enjoy a more profitable and better community.

These nitrogen BMPs were assembled by the University of Minnesota and the Minnesota Department of Agriculture (MDA) as part of the Nitrogen Fertilizer Management Plan developed by the Nitrogen Fertilizer Task Force coordinated by the MDA.


Introduction

Nitrogen (N) is an essential plant nutrient that is applied to Minnesota crops in greater quantity than any other fertilizer. In addition, vast quantities of nitrogen are contained in the ecosystem, including soil organic matter. Biological processes that convert nitrogen to its mobile form, nitrate (NO3), occur continuously in the soil system. (For greater detail see Understanding Nitrogen in Soils FO-3770.) Unfortunately, nitrates can be leached from the rooting part of the soil. To minimize losses to surface- and groundwater, management guidelines have been developed.

In response to the Comprehensive Groundwater Protection Act of 1989, a Nitrogen Fertilizer Management Plan was developed with the purpose of managing nitrogen inputs to crop production so as to prevent degradation of Minnesota water resources while maintaining farm profitability. The central tool for achievement of this goal is the adoption of Best Management Practices (BMPs) for nitrogen. The primary focus of the BMPs is nitrogen fertilizer; however, consideration of other nitrogen sources and agronomic practices is necessary for effective and practical total nitrogen management.

BMPs for nitrogen are broadly defined as "economically sound, voluntary practices that are capable of minimizing nutrient contamination of surface- and groundwater." The BMPs recommended here are based upon research, particularly at the University of Minnesota and other land-grant universities, and upon practical considerations. This ensures that the BMPs are technically sound and likely to be easily adopted by growers.

BMPs are based, in part, upon the concept of total nitrogen management, which accounts for all forms of on-farm nitrogen in the development of crop management plans. In addition to the statewide BMPs shown in this bulletin, a set of BMPs has been prepared for special situations and specific regions of the state (Figure 1). A listing of the bulletins containing these BMPs can be found on the last page of this bulletin.


Figure 1. The five regions for which BMPs are formulated


Figure 1 KEY

Statewide Nitrogen BMPs

Statewide BMPs are applicable to all crops, farming practices, and areas of the state. They are based upon the concept that accurate determination of crop nitrogen needs is essential for profitable and environmentally sound nitrogen management decisions.

Statewide nitrogen BMPs include:

• Develop realistic yield goals.

Unrealistically high yield goals can cause farmers to apply too much nitrogen, resulting in reduced profit and a higher potential for nitrogen loss to groundwater.

Accurate farm records should be used to calculate average yield for each field. The yield goal, specific for each field, should be based on the most recent five-year average, excluding the worst year. Using the most recent five-year crop yields makes sure that technological advances and current management practices for specific fields or crops are taken into account in determining yield potential. Excluding the worst year helps keep catastrophic events from artificially lowering yield potential. If crop rotation practices reduce the number of years that can be included in the most recent five-year average, yield goals can be based on farm-specific information from the previous five years for the specific crop to be fertilized.

Although maximum yield potential may occasionally be limited using this approach, long-term economic analysis indicates that using past average yields as a guide to crop nitrogen needs maximizes profitability while at the same time significantly reduces the potential for nitrate contamination. An average yield approach provides a sound basis for field-specific nitrogen recommendations that are environmentally sensitive and agronomically sound.

• Develop and use a comprehensive record-keeping system to record field-specific information.

Accurate field records should be kept by farmers for use in their crop management decisions because such records are essential to developing realistic yield goals and attaining maximum profitability. This field and farm-specific information should be used to evaluate past experience and to plan future nitrogen management programs. At a minimum, farmers are encouraged to accurately and systematically keep information on crop yields, nitrogen fertilizer and manure applications, and soil test results. The information can be used to monitor and adjust nitrogen management more precisely to maximize profit and protect the environment.

• Adjust nitrogen rate according to soil organic matter content, previous crop, and manure application.

Soil organic matter releases nitrogen that is available to crops. Nitrogen recommendations in Minnesota should be adjusted for soil organic matter content if a soil nitrogen test is not used.

Legumes in a crop system can supply substantial amounts of nitrogen to subsequent crops. For example, first-year credits for nitrogen can range from 40 pounds of nitrogen per acre (lb N/A) for soybeans to 150 lb N/A for alfalfa. Similarly, manure application should significantly lower fertilizer nitrogen application rates.

Research results from a 12-year study conducted at Waseca clearly show a 40-lb nitrogen credit when corn followed either soybeans or wheat (Table 1). Optimum nitrogen rates for continuous corn, corn after soybeans, and corn after wheat were 160, 120, and 120 lb N/A, respectively.

Table 1. Effect of previous crop on corn response to nitrogen from 1975-86 at Waseca. (From Randall et. al).
Rate (lb N/A) Previous Crop (bu/A)
Corn Soybeans Wheat
0 75 109 104
40 100 134 130
80 115 146 147
120 125 153 151
160 133 158 154
200 136 158 156

Failing to account for these sources of nitrogen in determining the correct application rate creates a surplus of nitrogen, a surplus that can potentially leach to groundwater. (For greater detail on nitrogen rates as influenced by soil organic matter and previous crop see Fertilizing Corn in Minnesota FO-3790.)

• Use a soil nitrate test when appropriate.

Use of a deep soil test to measure residual soil nitrate in the root zone can save money by reducing nitrogen recommendations in regions of Minnesota where average annual precipitation is approximately 25 inches or less. In western Minnesota, the soil nitrate test is a key management tool for corn, small grain, and sugar beet producers. Soil should be collected prior to planting from a 0–24 inch depth for corn and small grains and from a 0–48 inch depth for sugar beets. (For greater detail see Fertilizing Corn in Minnesota FO-3790; and Fertilizing Wheat in Minnesota FO-3772.)

Recent research in south-central, southeastern, and east-central Minnesota also indicates that a spring soil nitrate test can be helpful in determining if fertilizer nitrogen is needed on medium- and fine-textured soils. Soil samples should be collected before planting from the 0–24 inch depth and analyzed for nitrate-nitrogen (NO3-N). Our current interpretation indicates that if the analysis shows a carryover of 175 lb N/A or more, no fertilizer nitrogen is recommended. If the test indicates less than 175 lb N/A, nitrogen recommendations should be based on realistic yield goal, soil organic matter content, and previous crop. Changes in the nitrate soil test interpretation and more precise recommendations will come in the near future as our data base grows.

This soil nitrate test has the greatest utility when high amounts of residual NO3-N are a distinct probability. These conditions may be expected to exist where:

— manure has been applied repeatedly to a field,

— the yield of last year's crop was much less than expected because of drought, hail, insect damage, etc., and

— continuous corn has received liberal quantities of fertilizer nitrogen.

This test should not be used for first-year corn following alfalfa or where manure has been applied within the previous nine months, because the nitrogen credit will be underestimated.

• Use prudent manure management to optimize nitrogen credit.

Manure management programs should be planned to optimize nutrient use. The amount of nitrogen supplied by manure will vary with the source of manure (type of livestock), handling and storage, application rate, and method of application.

Test manure for nutrient content.

The nutrient content of manure is affected by animal species, type of manure-handling system, livestock housing and bedding system, diet, temperature, and outside contamination, etc.—all of which are specific to the particular livestock operation. The best method to determine nutrient supply from manure is to analytically test the manure for its nutrient content. It is essential that a representative sample of the manure be taken. Inaccurate readings of nutrient content due to a poorly taken sample (not well mixed, insufficient subsamples, poorly agitated) can result in large differences between what is estimated and the actual amounts of nutrients applied. Relying on published average nutrient values for manure can be very misleading; research results for manure analyses in Minnesota vary from these average values by a range from 25 to 300 percent.

Calibrate manure application equipment.

Correct application of manure to cropland is critical to maximize the value of manure to crop producers. For farmers to be able to give credit for nutrients in manure and not create environmental problems, they must be confident of the accuracy of their application rate.

Apply manure uniformly throughout a field.

For manure to be used efficiently as a nutrient source for crop production, the spread pattern must be uniform throughout the field. If this is not done, concentrated zones and deficient zones will exist. These nutrient excesses and shortages within the field create the potential for environmental contamination and poor economic return. It is also important that liquid or slurry manure be well agitated before transport to the field. This helps ensure that nutrient availability from all portions of the storage facility is equal, thus ensuring uniformity between the first loads and last loads of manure hauled to the field.

Injection of manure is preferable, especially on strongly sloping soils.

Injection methods should place liquid manure below the soil surface, thus eliminating surface runoff on sloping soils and volatilization of ammonia from the manure on all soils. New systems such as sweep injectors spread the manure in horizontal patterns under the soil. These systems are not as susceptible to channel erosion as the older knife systems, with which runoff water can flow down-slope in the knife tracks and cause erosion and nutrient loss. Injecting manure is especially important in preventing contamination of surface-water supplies.

Avoid applying manure to sloping, frozen soils.

Application of manure to sloping, frozen soils is discouraged because of the high potential for runoff with snowmelt in the spring. In addition, overwinter volatilization of ammonia from the manure substantially reduces the amount of manure-nitrogen available for the crop.

Incorporate broadcast applications whenever possible.

Broadcasting is a common way to apply solid, slurry, and liquid manures. Because of the high potential for ammonia loss, it is recommended that manure be incorporated within 12 hours to optimize its nitrogen value.

Incorporation is also important for aesthetic and odor reasons. Rapid incorporation of manure may defuse some of the tensions that can occur in a community where manure is broadcast close to nearby neighbors or housing developments. In addition, local ordinances adopted to restrict livestock production systems may be less rigid if broadcast manure is always incorporated quickly.

(For greater detail on manure management consult the appropriate bulletins listed at the end of this bulletin.)

• Credit second-year nitrogen contributions from alfalfa and manure.

Alfalfa and manure can supply nitrogen for more than one crop year. Recent University of Minnesota research indicates that a credit of 75 lb N/A can be adopted for second-year corn following a good stand of alfalfa on medium- and fine-textured soils. A 50-lb nitrogen credit for second-year corn is given when a fair stand (two to three plants/foot) of alfalfa has been plowed down.

The organic nitrogen in manure is transformed to plant-available nitrogen over a period of several years. Between 33 and 50 percent of the organic nitrogen remaining will be converted to plant-available nitrogen each year after the manure is applied. The value of manure analyses and an accurate and uniform application rate can again be seen as they provide information and confidence when assigning a nitrogen credit to previous manure applications.

• Do not apply nitrogen above recommended rates.

Nitrogen application rates higher than current University of Minnesota and other land-grant university recommendations have been shown to significantly increase nitrate leaching losses and subsequent contamination of groundwater. A high degree of confidence can be placed in University of Minnesota and neighboring land-grant university recommendations because they are based on long-term, statistically-based field research studies. Environmental impacts will be reduced if recommendations are followed.

Application of the proper rate of nitrogen has a larger influence on nitrate losses to our water resources than any other nutrient or crop management factor. The concept illustrated in Figure 2 demonstrates the significant impact that nitrogen rate has on crop yield. At nitrogen rates above the optimum level, yield is no longer increased but nitrate losses increase substantially.


Figure 2. Conceptual impact of nitrogen (N) rate on crop yield and nitrate loss from a corn production system.
Figure 2

Nitrate losses can be minimized by using the proper nitrogen rate combined with the other BMPs given in this bulletin and in the region-specific bulletins. Note that some nitrate loss occurs from a row crop even without the addition of fertilizer nitrogen. These losses occur naturally and are primarily related to the mineralization (breakdown) of soil organic matter, which releases nitrate to the soil system.

Research results obtained from continuous corn grown on a Port Byron silt loam in Olmsted County clearly show that increasing fertilizer nitrogen rates does not increase corn yield and does increase the NO3-N leached from the soil (Table 2). The five-year yield averages show a highly profitable yield response to the 150-lb N/A rate and no additional yield increase to the 225-lb N/A rate. At the same time the NO3-N concentration in the soil water at the 7.5-foot depth one year after discontinuing these treatments was substantially higher with the 225-lb rate compared to the 150-lb rate. Very low NO3-N concentrations were found at the nitrogen rates that gave less than optimum yields.

Table 2. Average corn grain yield and NO3-N concentration in soil water at 7.5 feet in Nov. 1992 as influenced by nitrogen rates from 1987-91 for corn in Olmsted Co. (From Randall et al.).
1987-91 rate
(lb N/A per yr)
1987-91 Avg.
Grain Yield (bu/A)
NO3-N Concentration
in soil water at 7.5 feet (ppm)
0 82 2
75 141 4
150 168 17
225 164 32

Somewhat similar results were found in tile drainage water leaving continuous corn plots at the Southern Experiment Station at Waseca (Table 3). Nitrogen was applied annually from 1977 to 1982 to the same plots on a Webster clay loam. Corn grain yields were optimized at the 180-lb N/A rate. Applying 240 lb N/A did not increase yields additionally, yet NO3-N concentrations were increased by 7 parts per million (ppm) with the higher rate. Applying the nitrogen in the spring (a BMP) compared to the fall decreased NO3-N concentrations in a range from 2 to 4 ppm regardless of nitrogen rate while increasing yields 5 to 15 percent. This illustrates the mutual benefit of combining other BMPs, such as time of application, with the proper rate of nitrogen application.

Table 3. Average corn grain yield and NO3-N concentration in the drainage water as affected by nitrogen rate and time of application at Waseca. (From Randall).
Nitrogen Treatment 1978-82 Avg.
Corn Yield
(bu/A)
1978-82 Avg.
NO3-N Concentration
in tile lines (ppm)
Rate
(lb N/A)
Time of
Application
0 66 7
60 Spr. 110 8
120 Fall 131 15
120 Spr. 150 13
180 Fall 160 19
180 Spr. 168 15
240 Fall 163 26
240 Spr. 155 22

Results from these studies and others conducted in the Midwest clearly show that high ("insurance") rates of nitrogen should not be applied. Nitrogen applied at rates above optimum are costly in terms of return on the fertilizer investment and the environmental consequences of the higher nitrate losses.

• Plan nitrogen application timing to achieve high nitrogen-use efficiency.

Nitrogen application timing can significantly affect the efficiency of nitrogen use and the potential for nitrate contamination of groundwater. Generally, the greater the time between application and crop uptake, the greater the chance for nitrogen loss. On medium- to fine-textured soils a spring preplant application of nitrogen has been ideal. Yields with split or single sidedress treatments often are no better than with preplant application and sometimes are poorer. If sidedressed nitrogen is applied too late for crop use, or if dry conditions exist for an extended period of time after sidedress application, nitrogen will not be efficiently used by the crop. Instead it remains in the soil profile where it is susceptible to leaching in the fall or early the following spring. In Minnesota, the period of maximum nitrogen uptake by corn continues from late June through July. Therefore, fall nitrogen application—eight months before the start of nitrogen uptake—has the highest potential for nitrate loss. Even April applications of nitrogen on irrigated sandy soils exhibit a high potential for loss. Thus, region-specific BMP recommendations should be used to achieve high nitrogen-use efficiency.


Related Publications

FO-6126 Best Management Practices for Nitrogen Use in Southeastern Minnesota

FO-6127 Best Management Practices for Nitrogen Use in South-Central Minnesota

FO-6128 Best Management Practices for Nitrogen Use in Southwestern and West-Central Minnesota

FO-6129 Best Management Practices for Nitrogen Use in East-Central and Central Minnesota

FO-6130 Best Management Practices for Nitrogen Use in Northwestern Minnesota

FO-6131 Best Management Practices for Nitrogen Use on Irrigated, Coarse-Textured Soils

FO-5880 Fertilizing Cropland With Dairy Manure

FO-5879 Fertilizing Cropland With Swine Manure

FO-5881 Fertilizing Cropland With Poultry Manure

FO-5882 Fertilizing Cropland With Beef Manure

FO-5883 Self-assessment Worksheets for Manure Management Plans

FO-3790 Fertilizing Corn in Minnesota

FO-3813 Fertilizing Soybeans in Minnesota

FO-3814 Fertilizing Alfalfa in Minnesota

FO-3773 Fertilizing Barley in Minnesota

FO-3772 Fertilizing Wheat in Minnesota

FO-3770 Understanding Nitrogen in Soils

FO-3774 Nitrification Inhibitors and Use in Minnesota

FO-3769 Providing Proper N Credit for Legumes

FO-2392 Managing Nitrogen for Corn Production on Irrigated Sandy Soils

FO-0636 Fertilizer Urea

FO-3073 Using Anhydrous Ammonia in Minnesota

FO-6074 Fertilizer Management for Corn Planted in Ridge-till or No-till Systems

FO-3553 Manure Management in Minnesota

G. W. Randall
Soil Scientist
Southern Experiment Station
University of Minnesota
M. A. Schmitt
Extension Specialist
Department of Soil, Water and Climate
University of Minnesota


COLLEGE OF AGRICULTURE, FOOD, AND ENVIRONMENTAL SCIENCES

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Funding for this publication was by the U.S. Department of Agriculture, Extension Service, under project number 91-EWQI-1-9265.



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