Fertigation is the process of adding fertilizer to irrigation water. Commonly 28% nitrogen (N) injected into irrigation water in place of, or addition to, sidedress N applications. Fertigation allows application of N closest to the plants uptake without extra field traffic. It’s often used in conjunction with sidedress to apply the additional N 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 1-100 gallons per hour is common for N application, but micro pumps capable of 1-100 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 per minute to a given field; this is equal to about 1 inch per acre of water for each hour. If the grower injects 10 gallons of 28% during that hour into the irrigation water applied, this will provide 30 pounds of N to each acre. Since 10 gallons per hour is equal to 6/10 of a gallon per minute, the grower 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, and can be removed from the water supply line and its output sprayed into 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 measures 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 N 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% or above. If the irrigation system’s uniformity is less than 90% or unknown, N applications through the irrigation system need to be smaller portions of the N 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 N management plan will also require a backup to the fertilizer application through irrigation. Self-propelled high crop applicators and aerial 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 N in lower water, thus allowing for the N to happen between rain showers. In an extremely wet, late June it may be necessary to use a backup plan. In 2014, many growers applied their last increment of nitrogen to corn crops by air, allowing either rainfall or a light irrigation to incorporate the urea.