James Adkins, Associate Scientist-Irrigation Engineering; firstname.lastname@example.org
With the extreme wet weather holding up sidedress fertilizer applications, several farmers are looking towards fertigation to address immediate fertility issues. If you do not typically fertigate, or it been a few years since your fertigation system was used, you should be able to answer the questions below before considering injecting fertilizer through any irrigation system. If you decide to fertigate, turn off endguns where possible as the application rate ranges from 0.2 – 5 times what is applied under the main span. You should also check the pump frequently and mark the supply tank at regular interval to track the application rate and detect any injection pump calibration issues.
Here are some questions you should ask before fertigating:
Can you make a full circle with the pivot without getting stuck?
A stuck pivot while fertigating can be a disaster especially if the overwatering timer isn’t functioning the machine doesn’t shutdown automatically. If you typically have problems with machines getting stuck after an intense irrigation season an application on already saturated soil will only accelerate the rutting process, making the rest of the season more difficult.
Do you have the proper equipment?
Fertigation requires backflow prevention, calibrated injection pump(s), power supply at each pivot, and uniform irrigation application.
Is your fertigation pump and power supply functional?
Check valves stick, seals fail and electrical power supplies get bypassed with midseason emergency pivot repairs; check all your equipment before committing to fertigation.
Do you have an accurate pivot timer setting chart and good application uniformity?
In the over 780 center pivot evaluations performed by UD, 10-15% have either no timer setting chart that includes full circle time or have incorrect full circle estimates. Hitting your target application rate is not possible without accurate full circle time numbers. Furthermore over 50% of systems had a 20% or more variation of applied irrigation which will translate into a 20% +/- variance from the targeted fertilizer application.
Injection pumps should be calibrated with back pressure on the injection line. This is very easy with a venturi/mazzei type or small diaphragm pump that includes a built in flowmeter as the pump can be adjusted on the fly. Positive displacement pumps like the piston and diaphragm pumps made by Inject-o-Meter and AgriInject have a pulsed output and are nearly impossible to calibrate with a sight glass flowmeter (a few specific electronic flowmeters can read pulsed flow – $800+ each). The best way to calibrate a pulsed injection pump is with a graduated tube installed with proper valving on the suction side of the injection pump and measuring the injected volume over time with the irrigation pump running. In absence of other calibration methods, measuring the volume injected into a container from the pressure side of the injection will give a rough estimate of the flowrate but keep in mind that the flow will typically drop when injecting into pressurized water.
Use this equation when calibrating an injection pump to a target fertilizer rate at a given pivot timer setting:
Use this equation when adjusting the pivot timer setting to match a known injection pump rate:
Backflow Prevention – Chemigation Valve
Whenever any material is injected into an irrigation system a backflow prevention device is required by law. Backflow prevention is more complicated than just the basic check valve typically included on most irrigation pumps. A chemigation valve consisting of a spring loaded flapper type check valve, a low pressure drain and air/vacuum relief valve are all required in addition to an injection pump interlock switch (to prevent operation of the injection pump when the irrigation pump is off) and an injection line check valve (to prevent irrigation water from backfeeding through the injection pump into the supply tank).
More information regarding the calibration of fertigation systems can be found in this factsheet from South Dakota State: https://openprairie.sdstate.edu/cgi/viewcontent.cgi?referer=https://www.bing.com/&httpsredir=1&article=1045&context=extension_fact