Fertigation is the process of adding fertilizer to irrigation water. Commonly 28 percent nitrogen injected into irrigation water in-place of, or addition to side dress nitrogen applications. Fertigation allows application of nitrogen closest to the plants uptake in need of nitrogen without extra field traffic. Fertigation is often used in conjunction with side dress to apply the additional nitrogen required for the higher expected yields in the irrigated portion of field that are only partially irrigated.
The equipment needed for fertigation is fairly simple and readily available at most irrigation dealers. A chemigation backflow valve is used to protect the water supply from contamination and an anti-backflow injection valve prevent the system from allowing irrigation water from entering the fertilizer supply tank if the injection pump stops.
A positive displacement injection pump pushes the fertilizer calibrated rate through the injector valve into the flow of irrigation water. The injection pump needs to be sized for the system and purpose. A pump capable of injecting from one to 100 gallon per hour is common for nitrogen application but micro pumps capable one to 100 ounces per hour may be used for micro-nutrients or acids used to neutralize the irrigation water.
A typical example would be an irrigation pump supplying 450 gallons a minute to a given field; this is equal to about 1 acre inch of water each hour. If the producer injects ten gallons of 28 percent during that hour into the irrigation water applied. This will provide 30 pounds of nitrogen to each acre. Since ten gallons per hour is equal to 6/10 of a gallon per minute the producer adjusts the injection pump to move 6/10 of a gallon each minute. A calibration tube temporally replaces the fertilizer supply tank to allow measuring the 6/10 gallon in a minute sucked from the tube or the injector valve can be removed from the water supply line and its output sprayed in to a graduate pail for measurement. Since the pumping rate of positive displacement pump will not change with supply tank level or pressure level the injector is spraying into either of the calibration measure will be accurate.
Interlocking the injection pump to the same power supply as the well or water pump allows safe shutdown of both pumps if power is interrupted or pump safeties out. Linking the pivot to the same system prevents over fertilizing a single spot if the pivot is shut off for any reason.
The uniformity of the nitrogen application is dependent on irrigation system uniformity. If a large portion of the nitrogen needs are being applied through irrigation, the irrigation system uniformity’s need to be at 90 percent or above. If the irrigation system is less uniform than 90 percent or is unknown nitrogen applications through the irrigation system need to be smaller portions of the nitrogen needs. Make sure all system repairs are made before you fertigate, end guns and cornering arms that fail to come on or shut off when they should are notorious for messing up fertilizer applications.
For fertigation to be part of a nitrogen management plan will also require a back up to the fertilizer application through irrigation. Self-propelled high crop applicators and areal application of urea is a couple of the options available to irrigator that are prevented from fertigation by continually wet weather.
In seasons that have a wet or rainy June there may not be enough soil water holding capacity left between rainfalls to allow the fertilizer water combination to be held in the root zone. Higher capacity injection pumps allow more concentrated applications of nitrogen in lower water, thus allowing for the nitrogen to happen between rain showers. In an extremely wet late June it may be necessary to use a backup plan. In 2014 many producers applied their last increment of nitrogen to corn crops by air allowing either rainfall or a light irrigation to incorporate the urea.
Source: Lyndon Kelley, Michigan State University Extension