BMP's For Nutrient Management

Illinois Fertilizer Conference Proceedings
January 29-31, 1996

Main Index 1996 Index Search

BMP's For Nutrient Management

G. W. Randall¹

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Introduction

Best Management Practices (BMPs) are a set of tools that can be used to manage nutrient inputs to crop production so as to minimize the degradation of water resources while maintaining farm profitability. In Minnesota BMPs are defined as "voluntary practices that are capable of preventing and minimizing degradation of groundwater, considering economic factors, availability, technical feasibility, implementability, effectiveness, and environmental effects". At present, BMPs in state statute focus on nitrogen (N) with particular emphasis on fertilizer; however, discussion on BMPs associated with phosphorus (P) and manure is currently occurring. BMPs recommended for "official" adoption must be based upon research, particularly that which has been conducted by the University of Minnesota and other land-grant universities, and upon practical considerations. This ensures that the BMPs are technically sound and, at the same time, likely to be adopted by growers.

Three-Tier Best Management Practice Strategy For Minnesota

A three-tier structure of BMPs was developed for N management in Minnesota. The first tier is a set of BMPs, which are not crop or region specific, to be adopted throughout the state. The second tier consists of five sets of regional BMPs, each designed to be adopted in one of the five general regions of the state. The third tier consists of BMPs for special situations that exist and present unique environmental or management concerns.

Statewide Best Management Practices (Tier 1)

Statewide BMPs can be considered to be "generic" in that they apply to all areas in the state. In general, statewide BMPs are applicable to all cropping systems and agronomic practices. The statewide BMPs were based upon the concept that accurate determination of crop N needs is essential for profitable and environmentally sound N management decisions. The statewide BMPs are:

Develop realistic yield goals.
Selection of yield goal and the subsequent N application rate has a profound effect on groundwater quality. Limited research has indicated that growers tend to set unrealistic goals, commonly missing them by 10 to 30% and, as a result, application rates are higher than necessary to maximize yields and maximize economic returns. The "running average" concept for yield goal selection is strongly recommended. Yield goals are based on the past 5-year average, excluding the worst year. This approach provides a sound basis for a field specific or soil specific N rate that is environmentally 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 since they are essential to the development of realistic yield goals and attainment of maximum profitability. This field or soil specific information should be used to evaluate past experience and plan future N management programs. At a minimum farmers are encouraged to accurately and systematically keep information on crop yields, fertilizer, and manure applications, and soil test results. The information can be used to monitor and adjust N management in a precise fashion for profit and environmental benefits
Adjust N rate according to soil organic matter content, previous crop, and manure application.
Mineralization of soil organic matter releases N that is useable by crops. Nitrogen recommendations in Minnesota should be adjusted for soil organic matter content as determined by a soil test. Legumes in a crop system can supply substantial amounts of N to subsequent crops. For example, first year credits for N can range from 40 lb/A for soybean to 1501b/A for alfalfa. Similarly, N application rates can be significantly adjusted to account for manure application. By failing to account for these sources of N in determining the correct application rate, a surplus of N may be created; this surplus can potentially leach to groundwater.
Use a soil nitrate test when appropriate.
The use of a deep soil test to measure residual soil nitrate in the root zone can substantially improve N recommendations. This test requires preplant samples to be taken from 0 to 24" for corn and small grains and a 0-48" sample for sugar beets. Based on the nitrate test, N credits are given to fine tune the rate of fertilizer N recommended. The test is not recommended for coarse-textured soils or when corn follows alfalfa. In a corn production system the greatest chance for success is found when continuous corn is grown, a manure history exists, or when high rates of carryover N may exist due to abnormally low yields from dry weather, etc.
Use prudent manure management to optimize N credit.
a. Test manure for nutrient content.
b. Calibrate manure application equipment.
c. Apply manure uniformly throughout a field and do not apply above the recommended rates.
d Injection of manure is preferable, especially on strongly sloping soils.
e. Incorporate broadcast applications whenever possible.
f. Avoid manure application to sloping, frozen soils.
Credit second year N contributions from alfalfa and manure.
Do not apply N fertilizer above recommended rates.
Nitrogen application rates higher than current University of Minnesota and other land-grant universities' 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 universities' recommendations because they are based on long-term field research. Environmental impacts will be reduced if recommendations are followed.
Nitrogen applications should be timed to achieve high N-use efficiency.
Nitrogen application timing can significantly affect the efficiency of N use and the potential for nitrate contamination of groundwater. Generally, the greater the time between application and crop uptake, the greater the chance for N loss. However, if sidedressed N is applied too late for crop use and/or to dry soils (wherein root activity is limited), N not used by the crop can leach to groundwater. Regional or soil specific recommendations should be used to achieve high N-use efficiency.

Regional Best Management Practices (Tier 2)

In order to achieve a goal of minimizing environmental impacts while optimizing agricultural profits, BMPs must account, to some extent, for local variation in soils, hydrogeologic conditions, and climatic conditions. In this interest, the state has been divided into five regions based upon general climatic conditions. The regional BMPs shown below refine the prescriptions of the statewide BMPs and can be adjusted/updated as new research-based information becomes available.

Southeastern Minnesota

Southeastern Minnesota is characterized by:

permeable, silt loam soils with underlying fractured limestone bedrock (Karst)
average annual precipitation is > 30 inches
area is very susceptible to groundwater contamination
primary crops are corn, forages, oats and soybeans
livestock are primarily dairy, beef, and hogs.

The BMPs are:

Do not apply fertilizer N in the fall.
The risk of leaching loss of nitrate from fall N application is heightened in southeastern Minnesota due to the high average annual precipitation, the well-drained and permeable nature of the soils, and the presence of karstic terrain. Spring preplant or sidedress N applications provide for more efficient use.
Anhydrous ammonia or urea sources of N should be used in spring preplant applications. Broadcast urea should be incorporated within 3 days of application. Consult the Soil Conservation Service for further information if soils are high erosive.
Sidedress applications to corn should be applied prior to the V4 stage of development.
Sidedress applications of urea and UAN-28 should be injected or incorporated to a minimal depth of 4 inches.
A nitrification inhibitor should be used with preplant N applications if soils are poorly drained and soil moisture levels are high in the upper portion of the profile. Check label for registered crops.
Minimize direct movement of surface water runoff to sinkholes.

Results from University of Minnesota trials in southeastern Minnesota reinforce the above BMPs. A continuous corn study started in 1987 clearly demonstrates that N should not be applied at above recommended rates and not in the fall (Table 1). Highest 4-yr average yields occurred with the 150-lb N rate; however, N0₃-N concentrations in the soil water at 5 feet also began to climb rapidly at this rate. Perhaps a N rate of about 120 lb/A would have optimized yield and profitability while minimizing nitrates in the groundwater.

Fall application (Nov. 13) of anhydrous ammonia with and without N-Serve gave yields in 1990 that were 7 to 10 bu/A less than with the same N rate applied in the spring before planting (Table 1). Moreover, N0₃-N concentrations in the soil water were 50 to 7036 higher with the fall applications. Split application of anhydrous ammonia (50% preplant + 5036 sidedress at 8- to 10-leaf stage) did not improve yields over the preplant treatments but did result in higher N0₃-N concentrations in the soil water.

South-Central Minnesota

South-central Minnesota is characterized by:

fine-textured soils with very poor to moderate internal drainage
the drainage systems installed in most fields
average annual precipitation is 25 to 30 inches
generally susceptible to surface water contamination
primary crops are corn and soybean with hog production

The BMPs are:

Spring preplant applications of N are highly recommended over fall applications.
If the N is fall-applied, delay application until the soil temperature is below 50°F at a 6-inch depth. Anhydrous ammonia should be used for fall applications.
Anhydrous ammonia or urea sources of N should be used in spring preplant applications. Broadcast urea should be incorporated within 3 days of application. Consult the Soil Conservation Service for further information if soils are highly erosive.
Sidedress applications of N to corn should be applied prior to the V4 stage of development.
Sidedress applications of urea and UAN-28 should be injected or incorporated to a minimal depth of 4 inches.
A nitrification inhibitor should be used with fall or preplant N applications in poorly drained soils that have high moisture levels in the upper portion of the profile.
Carefully manage N applications on soils characterized by a high-leaching potential.
a. Do not apply fertilizer N in the fall to coarse-textured soils.
b. When soils have a high-leaching potential, application of N in a sidedress or split application program is preferred. Use a nitrification inhibitor with early sidedressed N on labeled crops.

Results obtained at Waseca corroborate these BMPs. Fall application of ammonium sulfate in early November resulted in significantly lower corn yields than spring applications (Table 2). Moreover, N0₃-N lost out of the file lines was markedly higher with the fall applications.

Since the data in Table 2 were obtained using ammonium sulfate, many farmers questioned whether the same results would occur if anhydrous ammonia (AA) was used. Results obtained on a Webster clay loam at Waseca in 1991 show the highest N0₃-N concentrations in the tile water and greatest N0₃-N losses with the fall-applied AA without N-Serve (Table 3). The addition of N-Serve to the fall AA reduced N0₃-N concentrations slightly but not to the level of the spring and split applications without N-Serve. Corn yields were improved significantly by the spring applications or by including N-Serve with the late-fall (October 25) application.

Southwest and West-Central Minnesota

Southwest/West-central region of Minnesota is characterized by:

medium to fine-textured soils with naturally poor to moderate internal drainage
tile drainage systems
average annual precipitation is < 26 inches
less susceptible to surface and groundwater contamination than south-central Minnesota
primary crops are corn, soybean, and small grains with hog and beef production

The BMPs are:

Use a soil nitrate test with a 2 to 4 -foot depth to determine N needs. Soil samples should be taken in the early spring or in the fall, after the soil temperature is below 50°F at the 6-inch depth.
Anhydrous ammonia or urea are recommended in spring preplant applications. Broadcast urea and preplant applications of UAN-28 should be incorporated within 3 days of application. Consult the Soil Conservation Service for further information if soils are highly erosive.
In situations where fall N applications are used, delay application until the soil temperature is below 50° F at a 6-inch depth. Use AA or urea sources of N. UAN-28 should not be fall-applied.
Sidedress applications of N to corn should be applied prior to the V4 stage of development.
Sidedress applications of urea and UAN-28 should be injected or incorporated to a minimal depth of 4 inches.

Benefit from using the soil N0₃ test to provide fertilizer N recommendations in western Minnesota can be readily seen from data collected at Lamberton in 1990 (Table 4). Soil samples taken in the spring to a 5-ft depth and analyzed for N0₃-N show substantially more residual N0₃ in the profile following corn compared to following soybean. Based on the N0₃-N in the top 4 feet and a yield goal of 140 bu/A, fertilizer N recommendations of 31 and 116 lb/A for continuous corn and corn following soybean, respectively, were applied. This amount for continuous corn was 120 lb N/A less than the recommendation without the soil N0₃-N test. Corn yields for the two cropping systems were not different and no deficiency symptoms were seen. Nitrate-N concentrations in the tile water in May reflected the accumulation of N0₃ in the soil profile.

East-Central and Central Minnesota

The Central/East-central region of Minnesota is characterized by:

coarse to medium-textured soils that are moderately to excessively drained
average annual precipitation is 25 inches
cropping systems dominated by corn and forages with dairy production
crop production is less intensive than the southern regions and nutrient use is lower

The BMPs are:

Carefully manage N applications on soils that have a high-leaching potential.
Anhydrous ammonia or urea sources of N should be used in spring preplant applications on fine and medium-textured soils. Broadcast urea should be incorporated within 3 days of application. Consult the Soil Conservation Service for further information if soils are highly erosive.
Sidedress applications of urea and UAN-28 should be injected or incorporated to a minimal depth of 4 inches.

Northwest Minnesota

The Northwest region of Minnesota is characterized by:

fine-textured soils that are poorly drained
average annual precipitation of <24 inches
lower potential for groundwater contamination
cropping systems dominated by small grain, soybean, and sugar beet

The BMPs are:

Use a soil nitrate test to a 2- or 4 -foot depth to determine N needs. Soil samples should be taken in the fall after the soil temperature is below 50°F at the 6-inch depth or early spring.
Delay fall N application until the soil temperature is below 50° F at a 6-inch depth. Anhydrous ammonia or urea sources of N should be used for fall applications. UAN-28 should not befall applied.
Broadcast urea and spring preplant applications of UAN-28 should be incorporated within 3 days of application. Consult the Soil Conservation Service for further information if soils are highly erosive.
Nitrification inhibitors are not recommended on fine-textured soils but are recommended on coarse-textured soils with high-leaching potential.

Special Situation Best Management Practices

The third tier of BMPs are referred to as Special Situations BMPs The special situations are a result of certain combinations of management and environmental conditions that may render an area or site susceptible to groundwater contamination than would be predicted by the general characteristics of the surrounding region. The third tier accounts for those management situations or sites that are interspersed throughout the state.

Irrigated Soils

Irrigation, especially on coarse-textured (sandy) soils and shallow-rooted crops, may increase the leaching potential of applied N. Irrigation increases the soil water content of the root zone, thus enhancing transport past the zone of effective crop utilization.

Localized areas of irrigation occur throughout the state. Water use in these areas is variable depending upon soil and geologic conditions and average yearly precipitation. Commonly irrigated cropping systems include corn and potato.

Do not fall-apply fertilizer N to soils in the following textural classes: sandy loam, loamy sand, and sand.
Follow proven water management strategies to provide effective irrigation and minimize leaching.
Test irrigation water for N content and adjust N fertilizer rates accordingly.
Use sidedress or split applications of N on irrigated soils. Do not rely on fertigation for delivering more than one-third of the required N.
Use a nitrification inhibitor when the bulk of the N is applied in a single preplant or early sidedress application. For corn, N treated with a nitrification inhibitor should be applied prior to the V4 growth stage. Check label for registered crops.
Include a small amount of N in starter fertilizer in most situations (10 to 20 lb/A).
Do not delay N applications past optimum uptake period.
Establish a cover crop following early harvest of crops. Consult the Soil Conservation Service for further information if soils are highly erosive.

Coarse-Textured (Nonirrigated) Soils

Coarse-textured soils need special management to prevent leaching losses. Coarse-textured soils are present in many different regions and can be found throughout the state in outwash plains, alluvial river valleys, and ancient beach ridges. These soils have considerable leaching loss potential due to rapid infiltration characteristics and low water-holding capacities which can easily be exceeded. Furthermore, these soils are often associated with unconsolidated sand and gravel aquifers that may have water tables that are near the soil surface.

Do not apply N fertilizer in the fall to coarse-textured soils.
Apply N in a sidedress or split application program.
Use a nitrification inhibitor with early sidedressed N.

Summary

Statewide and region-specific BMPs developed for Minnesota should greatly improve fertilizer N efficiency, economical return, and environmental quality if they are implemented by crop producers. Some of these BMPs, e.g., realistic yield goals; record keeping systems; adjustment of nutrient application rate based on soil OM, soil test, previous crop and manure; and proper time of application, can be implemented into a soil specific farming system that will enhance profitability while minimizing negative impacts on water quality. Technology is now available to package these BMPs into profitable crop production systems and nutrient management plans for today's farmer.