Compared to aerial and ground applications, delivering agrochemicals such as fertilizers, fumigants, and pesticides and their adjuvants through irrigation systems can mean a more uniform application and lower fuel costs. The chemigation method, however, can take additional technical expertise to maintain.
Chemigation in North and South Dakota is seen almost exclusively on center-pivot systems, and proper calibration of systems is imperative to successful chemigation.
Safe and effective chemigation is accomplished by both proper management and equipment, mainly accurate calibration of chemigation systems. Experts from South Dakota State University share the best way to calibrate systems.
To calibrate your system, all parts of the irrigation and chemical injection system must be functioning properly. Always check the system for problems and ensure uniformity of water distribution. Besides having proper calibration equipment, items such as a stopwatch, calculator, measuring tape, marking flags and a calibration tube are all helpful for calibration.
Do the math
Performing various calculations for coverage, duration, water depth and application rate should be done to chemigate effectively:
Area covered. Measure the sprinkler’s wetted coverage and add it to the center-pivot length. Once you have the total wetted coverage, square it and multiple by 0.00007208 — which is 3.14 (pi) divided by 43,560 (number of square feet in an acre). SDSU Extension supplies this formula for pivots that run full circle:
Area covered in acres (A) = (wetted radius in feet)2 × (3.14 ÷ 43,560)
Duration needed. One of two methods can be used to determine the time required to cover the field — running the system across the field or measuring the distance the pivot travels during a measured time period.
For the first method, you operate the system wet at the same speed you would while chemigating. Record the setting and the time it takes for one revolution, or to cover the targeted piece of the field.
For the second, you will measure the distance from a pivot point to the outer wheel track. Then, operate the system running wet at the same speed you would while chemigating. Choose two points to measure, such as a marking flags or fence posts, and the time it takes to move between the markers.
To calculate the time to cover a circle, SDSU provides these formulas:
Wheel track circumference in feet = 3.14 × 2 × distance from pivot to outer wheel track
Rotation time = wheel track circumference × time between stakes ÷ distance traveled
With a percent time setting difficult to reset exactly, SDSU recommends leaving this setting the same from calculations to actually chemigating.
Depth of water. Next up is to determine the depth of water applied, as many labels of certain chemicals will require a specific water depth. When calculating this, always keep the timer setting on the pivot the same as it will be when chemigating. Depth is the net depth applied while assuming an 85% application efficiency. The formula follows:
Depth (inches) = gallons per minute (gpm) × rotation time × (0.85 ÷ number of acres)
Application rate. Once your center pivot is properly calibrated, you can move on to calibrating your injection system. For this, you’ll determine the chemigation application rate, the amount of chemical and the injection rate.
With a properly calibrated center pivot and injection system, you’re set to start application of your field. Be sure to check the calibration system at regular times while chemigating, and monitor the chemigation setup to ensure its proper operation.
Find formula examples and other information from SDSU Extension or by contacting your county Extension office.