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Extension > Agriculture > Nutrient Management > Nitrogen > Nitrogen application with irrigation water-chemigation

Nitrogen application with irrigation water: chemigation

Jerry Wright and Fred Bergsrud, Agricultural Engineering,
George Rehm, Carl Rosen and Gary Malzer, Soil, Water and Climate
Bruce Montgomery, Minnesota Department of Agriculture

Applying a portion of a crop's nitrogen (N) requirement with irrigation water is a recognized best management practice to reduce nitrate leaching losses for some crops grown on coarse textured soils.

This practice is called nitrogen chemigation but is more commonly referred to as fertigation. This practice has been widely used by irrigating farmers for several years.

Research in Minnesota and neighboring states has shown that splitting N applications on high N use irrigated crops such as corn, sweet corn, and potatoes grown on sandy soils generally benefits crop yield, N use efficiency and the environment. Minnesota's N management plan, developed by the Minnesota Department of Agriculture (MDA) and others, also encourages the use of this practice for some irrigated crops grown on sandy soils. Tables 1 and 2 show corn and potato yield response to different N management strategies for different years.

Table 1. Corn Yields Affected By N Management at Becker, MN.

N Treatments Corn Yield - bushels per acre
1980 1981 1982
Nitrogen pounds per acre
150 +IH 150 +IH 150 +IH
Pre-Plant 153 199 92 108 197 196
Split 2/3, 1/3 157 190 126 149 194 195
Split 1/3, 2/3 172 181 134 162 194 196
8 Leaf 188 193 157 177 193 196
12 Leaf 193 180 168 159 193 193
Split 1/6, 1/6, 3/6, 1/6 190 159 202
Leaching Events 5" early June +5" mid-June none
+IH = first N application contained a nitrogen inhibitor
2 split = preplant + 12 leaf; 4 split = preplant + 8 leaf + 12 leaf + tassel
Source: Malzer & Graff, 1980, 1981 and 1982, University of Minnesota Soil Science Series

Table 2. Potato Yields Affected By N Management at Becker, MN.

N Treatment Fresh Weight 3+ oz. Size
1991 1992
lbs/ac
cwt per acre
0 0 282 240
120 40, 40, 40 403 427
160 80, 80, 0 381 455
240 0, 120, 120 411 538
240 40, 100, 100 421 505
240 80, 80, 80 411 516
240 120, 60, 60 401 481
200 40, 40, 40, 40+40 435 455
Leaching Events +5" June 4 rains none
N splits - starter, emergence, hilling, post hilling w/sap test
Source: Rosen, 1991 and 1992, University of Minnesota Soil Science Series

This publication outlines the factors (irrigation systems, safety devices, calibrations, etc.) that a farm manager who irrigates needs to consider before starting nitrogen chemigation with one or more irrigation systems. A chemigation user permit must be obtained from MDA and safety devices installed to apply N after January 1994.

Advantages and benefits

  1. Provides the farm manager with an option to apply additional N to a rapidly growing crop needing extra nitrogen.
  2. Reduces the environmental risk associated with having a large portion of a crop's fertilizer N supply available for potential leaching into the ground water by major rains.
  3. Increases the crop's N use efficiency in most growing seasons.
  4. May reduce the total N application, especially in growing seasons without excessive spring rains.

Disadvantages and risks

  1. Uniformity of N application depends on the water uniformity from the irrigation system.
  2. Extra investment must be made for the chemigation system and safety equipment.
  3. Liquid N sources for injection are commonly more expensive than other forms.
  4. The farm manager/operator must take time to learn about chemigation safety devices, calibration and management practices.
  5. Potential risk for all or a portion of the N supply to flow back into the irrigation water source (ground or surface) if two or more of the required safety devices malfunctions while chemigating.
  6. Requires maintaining a chemigation user permit with the MDA.

Irrigation systems

Apply nitrogen with irrigation water only with systems that can provide a uniform water application over the entire field and at an application rate that does not exceed the infiltration rate of the soil. Distribution of injected N through an irrigation system is no better than the same system's distribution of water.

An irrigation system that causes water movement down the plant rows is exceeding the soil intake rate and will not provide adequate N distribution. This situation may cause some N to either leach into the ground water in the areas where the water ponds or move into surface water by runoff.

Sprinkler systems like the center pivot (electric or oil drive) and the linear move can provide a very uniform distribution of water and N if the sprinkler package is properly selected and maintained. However, most water driven center pivots should not be used since the application around each drive tower is usually much higher than between the towers. Center pivots and linears can be equipped with several types of sprinkler packages (10 to 60 psi) and each can provide adequate water distribution. The end gun on a center pivot should only be operated during chemigation if it can be controlled to spray water within the field property.

Center pivots and linears are continuous moving systems. The nitrogen supply must be injected into the irrigation water at a constant rate and concentration to produce a uniform N distribution. Use a liquid N source and an adjustable injection metering device (piston pump, diaphragm pump or venturi injector).

Traveling guns and set move sprinkler systems (side wheel roll, hand move lateral) do not distribute water as uniformly over the field as center pivots because of the overlaps between moves. These systems should only be used to apply N fertilizer when the wind is very low and no other N application method is available.

Solid set sprinkler or trickle systems can provide adequate distribution of water and N when properly designed and operated. These systems do not generally require constant N rate injection so some granular sources may be dissolved in water and batch loaded with a venturi injector or a mixing tank.

Nitrogen sources

Several sources of commercially prepared N fertilizer are available for supplementing a crop's N needs. However, not every source can be easily injected into irrigation water or mixes well with irrigation water. Some fertilizer sources, such as ammonium thiosulfate, may also have limitations on application rates to prevent plant damage.

Liquid urea-ammonium nitrate (28%) is the most common source of N injected into irrigation water. It maintains a constant concentration without agitation and is easy to transport and store. These are necessary characteristics for irrigation systems such as the center pivot that require continuous injection to produce a uniform distribution of the fertilizer. This N source supplies three (3) pounds of N fertilizer for each gallon applied.

Granular fertilizers like ammonium nitrate or sulfate can be used in batch loading situations with solid set sprinkler or trickle systems.

Anhydrous ammonia or any other N fertilizer that has free ammonia should not be applied through any sprinkler systems in Minnesota. Ammonia can react chemically with the salts in the water and form precipitates that coat the inside of pipelines and possibly plug sprinklers. Some of the ammonia can also be lost to the atmosphere due to volatization while water is sprayed into the air.

Nitrogen rate and timing

Timing of a N application and the N rates suggested for irrigated crop production depend on several factors. They include type of crop, soil texture, crop growth rate, yield goal and in-field N credits from previous crop and manure.

For corn, chemigation is best used to apply only the last 1/6 to 1/3 of the crop's N needs. This should be started at or just before the plant's peak N uptake time. Peak N use time for corn starts between the 12th and 16th leaf stage of development and slows down substantially by the time the silk turns brown.

figure-1

Figure 1.

Timing of a late N chemigation for potatoes generally should only be applied when based on petiole nitrate status. Nitrogen demand by potatoes is generally greatest between initial tuber growth and tuber enlargement (3 to 8 weeks after emergence). Figure 1 shows the rate of N uptake by potato vines and tubers as the plant matures.

The total amount of N fertilizer applied by chemigation typically may vary between 20 to 60 pounds per acre depending on the N requirements for the crop. With the larger amounts, the application should be split into two or three chemigation events.

The total N requirements for a given crop can be estimated from information published in:

Injection methods

Nitrogen can be injected into pressurized irrigation systems by several metering methods.

Continuous moving irrigation systems like the center pivot require a meter device that can provide a constant injection rate. The positive displacement pumps and the venturi injector are the most common injection equipment for moving systems like the center pivot. The metering device for a given system should be sized to have the ability to apply the desired N rate per acre (eg. 10 to 30 pounds) at a reasonable water depth (1/3 to 1").

Solid set sprinkler or trickle systems can be set up to apply N at a constant rate but both are more commonly operated with a batch loading technique. The venturi injector is suited for either injection approach. Batch loading can also be done with a pressure differential mixing tank for systems that cover small acreage.

Positive displacement pumps are available in two basic arrangements: piston and diaphragm. Both types are marketed by several manufacturers and are available in at least two or more injection rate ranges. A pump should be selected for a given field situation so that it will not have to be operated at the high or low end of the injection range. Some pumps may require a pair of injectors to provide the necessary injection rate.

Diaphragm pumps can be adjusted very easily during injecting while piston pumps need to be stopped. Most pump models can operate within a wide range of irrigation system pressures.

Three-phase electric motors are the most common source of power for piston and diaphragm pumps. Some injection pump models can be driven by belt power or a water motor.

Venturi injectors come in several sizes and can be operated under different pressure conditions. Most venturi systems are set up in a shunt pipeline parallel to the main irrigation pipeline since they require at least a 20% differential pressure to work properly. Venturi injectors do not require external power to operate but some chemigation units utilize a small booster pump in the shunt pipeline to produce a differential pressure. Venturi systems can be easily adjusted during operation to change the rate of chemical injection.

Pressurized differential mixing tanks are available in only a few sizes, with the largest tank suited for only a few acres at a time. Pressurized mixing tanks require diversion of some water from the main irrigation line into the tank; then returning the mixed solution back into the main line at a point of lower pressure. The rate of flow back into the main line is controlled by a regulating valve, but chemical concentration will be slowly reduced over the injection time period. Some mixing tanks are equipped with a collapsible bag that separates the chemical from the water. This modification allows the chemical to be injected at a more constant rate. Both systems may require repeated fillings to complete the application.

Chemigation safety measures

Nitrogen solutions used during chemigation have the potential of getting into the irrigation water source if proper operating procedures and safety devices are not installed or maintained by the operator.

The Minnesota Department of Agriculture's chemigation regulations require all irrigation systems applying nitrogen after January 1994 to contain several safety devices and measures. These measures will minimize the risk of accidentally contaminating the water source. MDA also requires each irrigation system to have a chemigation users permit.

Detailed information on chemigation user permits and safety equipment can be obtained from the Minnesota Department of Agriculture, Agronomy Services Office at 90 West Plato Blvd., St. Paul, MN 55107 (phone 612/297-2614).

Chemigation has three main ways of potentially polluting irrigation water sources if safety devices are not functioning correctly. These three are:

  1. The N solution in the supply tank and irrigation pipe line could flow back or be siphoned back into the water source when the irrigation system shuts down.
  2. The chemigation system could continue to inject N into the irrigation pipe line when the irrigation system shuts down, thus causing the N solution to spill onto the ground or flow back into the water source.
  3. The chemigation system could shut down while the irrigation system continues to operate and force water back into the N supply tank causing it to overflow and spill onto the ground.

Figure 2 shows a typical arrangement of safety devices and measures required by MDA regulations to protect an irrigation water well or surface water source from any of the problems described above. A general description of safety and management measures is described in Minnesota Extension article Chemigation Safety Measures.

figure-2

Figure 2.

MDA's basic safety devices and measures for N chemigation are described as follows:

  1. A single positive closing check valve (MDA approved) must be installed in the main pipeline between the point of chemical injection and the irrigation water supply pump. The check valve must contain an air vacuum relief valve and automatic low pressure release valve immediately upstream from the check valve flapper. The low pressure drain must be located on the bottom of the pipe and positioned to discharge flow away from an irrigation well head or surface body of water. The check valve must also have an inspection port that can be easily opened to inspect the check valve flapper and the low pressure drain when the irrigation system is shut down. If the chemigation system is connected to a potable well or a public water supply system the main pipeline must contain a Minnesota Department of Health (MDH) approved reduced pressure zone (RPZ) backflow preventer.
  2. The chemigation injection meter must be interlocked with the irrigation system's power source or water supply so it will shut down anytime the irrigation system or pumping plant stops running or the irrigation water flow is disrupted.
  3. The injector's discharge hose must contain a positive closing check valve that will not allow flow either way when the injection metering device is not operating.
  4. The irrigation system must contain a low pressure shut-down switch on the main pipeline that will shut down the irrigation system and the chemigation system if the operating pressure of the irrigation system drops to a pressure unsatisfactory for proper N and water distribution.
  5. The chemigation supply tank must not be located closer to an irrigation well than the distance specified in the MDH rules, chapter 4725. And, it must be safeguarded according to the MDA specifications described in the following discussion. The separation distance from a surface water source must likewise be no less than that specified for an irrigation well unless other state/federal regulations are more applicable.
  6. The chemigation supply tank must be housed in a secondary containment (dike) unit if the tank storage meets at least two of the following conditions: a) the supply tank has a rated capacity of more than 1500 gallons, b) the tank is located within 100 feet of a water supply, and c) the supply tank storage is located at the site for more than 30 consecutive days.

The minimum required capacity for a secondary containment unit is 125 percent of the tank capacity (110% if under a roof). The secondary containment unit must be constructed of solid reinforced masonry, reinforced concrete, metal or synthetic materials and be leakproof at all times.

Calibration

Chemigation calibration is very important to assure that the desired amount of N is distributed uniformly over the irrigated field. It is also very helpful in determining the best size of injection meter and supply tank needed to match the given irrigation system.

Several factors are involved in calibrating an injection meter (positive displacement pump or venturi) for a moving irrigation system like a center pivot. These factors are:

You can calculate the required injection rate of liquid N into the irrigation water for a center pivot by using the following equation:

GPH = A * V divided by T

where:

GPH = injection rate of liquid N in gallons per hour
A = area to be fertilized in acres
V = volume of N solution needed in gallons per acre
T = time to irrigate/chemigate the field in hours

The area (A) of the field can be estimated using the ASCS reported acreage, or calculated by using actual field dimensions.

The volume (V) of N per acre depends on the concentration of the N source and the desired N application rate. For example, to apply 20 pounds of liquid urea-ammonium nitrate (28-0-0) per acre divide 20 by 3 (3 lbs N per gallon), which means 6.7 gallons of 28% N is needed per acre. Amounts for other N rates are shown in Table 3.

Table 3. Amount of 28-0-0 N fertilizer required to give various rates of available N per acre.

N Rates lbs/ac 28% Gallons/ac.
5 1.7
10 3.3
15 5.0
20 6.7
25 8.4
30 10
35 11.7

Total time (T) to irrigate/chemigate the field should be estimated by measuring the actual time required to operate a complete pass over the field at the desired water depth. If time doesn't allow for a complete pass, determine the time in minutes the end tower takes to travel 100 feet when operated at the desired speed. For a center pivot, then convert the test time to hours and take it times the circumference of the end tower in feet and divide by 100 (= test run length). If the center pivot is not a full circle, estimate or measure the actual length of the end tower wheel path. Figure 3 shows an example of calculating the total travel time (T) for a center pivot.

figure-3

Figure 3.

To calibrate the injection meter, first set the meter at the manufacturer's suggested setting for the calculated injection rate (GPH). Then run an injection test (with only water) into the irrigation system while it is operating. While injecting, measure the amount of water that is being injected over a given time and compare the rate to the calculated rate. If the rates are not the same, readjust the injector meter and retest the system.

Once the actual chemigation operation is started, recheck the injection rate and readjust the meter if necessary. Recheck one or more times during the chemigation process.

For batch loading with a solid set sprinkler or trickle system, the volume of total N is the main calibration component. Total volume of N for batch loading is dependent on the area of the irrigated zone and the desired N rate. The injection rate does not need to be precisely controlled. However, the injector should apply the N in a time period which does not result in over-irrigation or leaching of any previously applied chemical and should not be damaging to any part of the irrigation system or crop.

If an irrigation system presents a more complicated calibration situation, assistance can be obtained from one of the authors of this publication, chemical injection representative or county extension staff.

Management tips

Safe and accurate N chemigation requires the following steps to be followed each time an application of N is made:

  1. Check travel time of the irrigation system at the desired water application depth and recalculate the chemical injection rate for the planned amount of nitrogen.
  2. Inspect performance of the check valves, low pressure drain, low pressure shutdown switch and all fittings on the chemical supply and discharge hose. Repair all malfunctioning parts before initiating the next chemigation.
  3. After the chemigation process has started, recheck the N injection rate and adjust if not equal to the planned application.
  4. Periodically revisit the irrigation system and recheck the operation of the injection meter, operating pressure of the system, and water distribution of the irrigation system including the end gun operation on center pivots.
  5. At the end of each N application continue running water through the irrigation system until all of the N has been discharged from the pipeline of the irrigation system. This may take 10 to 15 minutes. Also run clean water through the injection meter, chemical discharge hose and check valve. If you're not planning to use the chemigation system more this year, remove any left-over N from the supply tank or relocate the tank at least 150 feet from any water source (unless in the required containment unit).
  6. If an accident occurs, take action to keep potential spill to a minimum and report the incident to the MDA immediately (1-800-422-0798) for assistance.

References

Fanning, C. 1981. Fertigation. North Dakota Cooperative Extension Service. Water Spouts Newsletter. June issue.

Fischbach, P. 1973. Fertilizing Through Center Pivots. University of Nebraska Cooperative Extension Service. Fact Sheet 73-20.

Haman, D.Z., A. Smajstrla & F. Zazueta. 1990. Chemical Injection Methods for Irrigation. Florida Cooperative Extension Service. Circular 864.

1993. Minnesota Rules, parts 1505.2100-2800 (MDA chemigation) and 4725.3350 & 4450 (MDH water well cross connection and separation distances with pollution sources).

Wall, D.B. and B.R. Montgomery. 1991. Nitrogen in Minnesota Ground Water. Minnesota Pollution Control Agency and Minnesota Department of Agriculture.

Werner, H. 1991. Chemigation: Is it for you? South Dakota State University Cooperative Extension Service. Fact Sheet 861.

This material is based upon work supported by the U.S. Department of Agriculture, Extension, under special project number 91-EWQI-1-9265.

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