Applying nitrogen with irrigation water: Chemigation

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. It’s 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 using 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.

Yields are for corn grown in Becker. Key: +IH = first N application contained a nitrogen inhibitor; 2 split = preplant + 12 leaf; 4 split = preplant + 8 leaf + 12 leaf + tassel. Source: Malzer and Graff (1980, 1981 and 1982).

Yields are for potatoes grown in Becker. N splits: Starter, emergence, hilling and post-hilling with sap test. Source: Rosen (1991 and 1992).

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.

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 malfunction while chemigating.

6. Requires maintaining a chemigation user permit with the MDA.

Sprinkler systems like the center pivot (electric or oil drive) and the linear move can very uniformly distribute water and N if the sprinkler package is properly selected and maintained. However, most water-driven center pivots shouldn’t be used because the application around each drive tower is usually much higher than between the towers.

You can equip center pivots and linears with several types of sprinkler packages (10 to 60 psi), each of which can provide adequate water distribution. Only operate the end gun on a center pivot during chemigation if it can be controlled to spray water within the field property.

Center pivots and linears are continuous, moving systems. To produce a uniform N distribution, you must inject the nitrogen supply into the irrigation water at a constant rate and concentration. Use a liquid N source and an adjustable injection metering device such as a piston pump, diaphragm pump or venturi injector.

Traveling guns and set-move sprinkler systems (e.g., side-wheel roll, hand-move lateral) don’t distribute water as uniformly over the field as center pivots because of the overlaps between moves. Only use these systems 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 adequately distribute water and N when properly designed and operated. These systems don’t 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.

Several sources of commercially prepared N fertilizer are available for supplementing a crop's N needs. However, not every source can easily be 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

Liquid urea-ammonium nitrate (28 percent) is the most common N source 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 uniformly distribute the fertilizer.

This N source supplies three pounds of N fertilizer for each gallon applied.

Granular fertilizers

You can use granular fertilizers like ammonium nitrate or sulfate in batch-loading situations with solid-set sprinkler or trickle systems.

N fertilizers with free ammonia

Don’t apply anhydrous ammonia or any other N fertilizer that has free ammonia through any sprinkler systems in Minnesota.

Ammonia can chemically react with the salts in the water and form precipitates that coat the inside of pipelines and possibly plug sprinklers. Some ammonia can also be lost to the atmosphere due to volatilization while water is sprayed into the air.

Timing of N applications and suggested N rates for irrigated crop production depend on several factors including:

• Type of crop.

• Soil texture.

• Crop growth rate.

• Yield goal.

• In-field N credits from previous crop and manure.

Corn

For corn, chemigation is best used to apply only the last 1/6 to 1/3 of the crop’s N needs.

Start this 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 substantially slows by the time the silk turns brown.

Potatoes

Generally, only apply late N chemigation for potatoes based on petiole nitrate status.

Nitrogen demand by potatoes is generally greatest between initial tuber growth and tuber enlargement (three to eight weeks after emergence). Figure 1 shows the rate of N uptake by potato vines and tubers as the plant matures. 

Other crop-specific needs

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

Resources for estimating a given crop’s total N requirements:

• Managing nitrogen for corn production on irrigated sandy soils

• Nutrient management for commercial fruit and vegetable crops in Minnesota

• Potato fertilization on irrigated soils

You can inject nitrogen into pressurized irrigation systems by several metering methods.

Continuously moving irrigation systems like the center pivot require a meter device that can provide a constant injection rate. Positive displacement pumps and venturi injectors are the most common injection equipment for moving systems like the center pivot.

Size a system’s metering device so it can apply the desired N rate per acre (e.g., 10 to 30 pounds) at a reasonable water depth (1/3 to 1 inch).

You can set up solid-set sprinkler or trickle systems 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. For systems that cover small acreage, batch-loading can also be done with a pressure differential mixing tank.

Positive displacement pumps

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.

Select a pump for a given field situation so you won’t have to operate it at the high or low end of the injection range. Some pumps may require a pair of injectors to provide the necessary injection rate.

You can very easily adjust diaphragm pumps 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 power source for piston and diaphragm pumps. Some injection pump models can be driven by belt power or a water motor.

Venturi injectors

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 because they require at least a 20 percent differential pressure to properly work.

Venturi injectors don’t require external power to operate but some chemigation units use a small booster pump in the shunt pipeline to produce a differential pressure. Venturi systems can easily be adjusted during operation to change the rate of chemical injection.

Pressurized differential mixing tanks

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. A regulating valve controls the flow rate back into the main line, but chemical concentration will slowly reduce 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.

Nitrogen solutions used during chemigation can potentially get into the irrigation water source if the operator doesn’t install or maintain proper operating procedures and safety devices.

Regulations and permits

Since 1994, chemigation regulations from the Minnesota Department of Agriculture (MDA) have required all irrigation systems applying nitrogen to contain several safety devices and measures. Measures minimize the risk of accidentally contaminating the water source. MDA also requires each irrigation system to have a chemigation user’s permit.

You can get detailed information on chemigation user permits and safety equipment from:

Minnesota Department of Agriculture
Agronomy Services Office
90 West Plato Blvd.
St. Paul, MN 55107
612-297-2614 

Pollution risks

If safety devices aren’t correctly functioning, chemigation has three main ways of potentially polluting irrigation water sources:

1. The N solution in the supply tank and irrigation pipeline flow back or get siphoned back into the water source when the irrigation system shuts down.

2. The chemigation system continues to inject N into the irrigation pipeline when the irrigation system shuts down, causing the N solution to spill onto the ground or flow back into the water source.

3. The chemigation system shuts 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.

Safety device arrangements

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 each of these problems.

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

Closing check valve

• 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 reduced pressure zone (RPZ) backflow preventer approved by the Minnesota Department of Health (MDH).

Chemigation injection meter

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.

Discharge hose

The injector's discharge hose must contain a positive closing check valve that will not allow flow either way when the injection metering device isn’t operating.

Low-pressure shutdown switch

The irrigation system must contain a low-pressure shutdown 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.

Chemigation supply tank

• Don’t locate the chemigation supply tank closer to an irrigation well than the distance specified in the MDH rules, chapter 4725.

• It must be safeguarded according to the MDA specifications described in subsequent bullets.

• Likewise, the separation distance from a surface water source must be no less than that specified for an irrigation well unless other state or federal regulations are more applicable.

• 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 1,500 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 percent if under a roof). The secondary containment unit must be constructed of solid reinforced masonry, reinforced concrete, metal or synthetic materials, and be leak-proof at all times.

Use the following equation to calculate the required injection rate of liquid N into the irrigation water for a center pivot:

GPH = A x V / 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

Area to be fertilized

Estimate the field’s area (A) using the ASCS reported acreage, or calculate it using actual field dimensions.

Volume needed

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 pounds of N per gallon), which means 6.7 gallons of 28 percent N is needed per acre. Table 3 shows amounts for other N rates.

Time to irrigate/chemicate

Estimate the total time (T) to irrigate/chemigate the field 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, convert the test time to hours and multiply it by the circumference of the end tower in feet and divide by 100 (= test run length).

• If the center pivot isn’t 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.

To calibrate the injection meter, first set the meter at the manufacturer’s suggested setting for the calculated injection rate in gallons per hour (GPH). Then, using only water, run an injection test into the irrigation system while it’s operating.

While injecting, measure the amount of water that’s being injected over a given time and compare that rate to the calculated rate. If rates aren’t the same, readjust the injector meter and retest the system.

Once the actual chemigation operation has started, recheck the injection rate and readjust the meter if necessary. Recheck at least once more during the chemigation process.

Batch-loading

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 depends on the area of the irrigated zone and the desired N rate.

The injection rate doesn’t need to be precisely controlled. However, the injector should apply the N in a time period that doesn’t result in over-irrigation or leaching of any previously applied chemical. It also shouldn’t be damaging to any part of the irrigation system or crop.

If an irrigation system presents a more complicated calibration situation, you can get assistance from a chemical injection representative or county extension staff.