Growers will need to implement practices to protect groundwater from nitrate contamination to comply with the water quality regulations in California. Many growers now use drip irrigation to achieve higher efficiency with water and nitrogen fertilizer applications. Fertigating through drip systems allows nutrients to be applied more frequently than with tractor applications so rates can be adjusted to match the N uptake rate of the crop as well as place nutrients in the root zone.
The right amount of N fertilizer to apply to a crop will depend on the application interval and N uptake rate, which varies at different growth stages. Recently established vegetable crops uptake much less N than maturing crops. Also, crediting for nitrate in irrigation water, soil and tissue testing can further refine fertilizer recommendations.
Once the amount of N has been determined, irrigators must accurately inject the correct volume of fertilizer through the irrigation system. To accomplish this, they need the right equipment and training. Farming operations that produce vegetables and row crops often use fertigation trailers for transporting fertilizer to the field and injecting liquid fertilizer into an irrigation system. A fertigation trailer usually consists of a nurse tank with a capacity of 500 to 1,000 gallons and a small gas or electric pump (Fig. 1). Fertilizer is usually transferred from large storage tanks into the nurse tank using small gas-powered pumps (Fig. 2).
Few fertigation trailers or large fertilizer tanks include a flowmeter to measure the volume of fertilizer transferred to a nurse tank or metered into the irrigation system. Irrigators often rely on markings on the side of the nurse tank to estimate the volume of fertilizer pumped (Fig. 1). These markings are usually not accurately calibrated nor have fine enough graduations to precisely meter out fertilizer. Furthermore, the markings can be hard to read, especially if the trailer is not positioned on a level surface.
A flowmeter could increase the accuracy of metering fertilizer into a nurse tank or for measuring the volume of fertilizer injected into the irrigation system. A flowmeter would also facilitate tracking the volume of fertilizer applied to each crop. Either an irrigator could manually record the readings, or the meter can be wired to a data logger or control system to automatically register volumes.
Accurately measuring fertilizer volume with a flowmeter can be challenging. Fertilizers can be corrosive to equipment and instrumentation, and they have a range of densities that can affect flow measurements. More than a decade ago, we tested several models of fertilizer flowmeters, which proved to be inaccurate. Since that time, many improvements have been made in flowmeter technology. Hence, it seemed worthwhile to test the accuracy and precision of a new generation of fertilizer flowmeters.
Evaluating Flowmeter Accuracy for Fertilizer Application
We evaluated three flowmeter models designed for metering liquid fertilizer:
• Banjo FM100 meter
• Dura-meter
• Blue White F-1000
Each model relies on a different mechanism to monitor fertilizer volume. The Banjo meter measures flow using a magnetic sensor, the Dura-meter uses a nutating disk and the Blue White meter (Fig. 3) uses a small propeller.
The accuracy of the flowmeters was tested using 25 gallons of either water, ammonium nitrate (AN20, 20% N), or urea-ammonium nitrate (UAN32, 32% N). These liquids have varying densities (water = 8.3 lbs/gal, AN20 = 10.5 lbs/gal, UAN32 = 11.1 lbs/gal). A testing manifold was set up in the UCCE greenhouse in Monterey County that pumped a calibrated volume of each fluid through the flowmeters using an electric diaphragm pump (Fig. 4). Five or more test runs were made for each meter and fluid. The average volume measured and the standard deviation from the mean volume were calculated.
Comparing Flowmeter Performance and Cost Efficiency
All three flowmeter models accurately measured water and fertilizer volumes (Table 1). Measurement errors were generally less than ±2% of the true volume. The Dura-meter was the most accurate flowmeter of the three models and had an overall average absolute error of -0.2 gallons per 25 gallons measured and a coefficient of variation of ±0.3%. The Blue White meter, which uses a paddle wheel to measure volume, was the least accurate and had an overall absolute error of 1 gallon per 25 gallons measured and a coefficient of variation of ±1.3%. The type of liquid metered affected the accuracy of the Banjo and Blue White meters more than the Dura-meter.
Although the Dura-meter was the most accurate of the three flowmeters, it did require an initial calibration before testing began. The other meters could not be manually calibrated. The nutating disk mechanism in the Dura-meter directly measures liquid volume, which may explain why the meter was not affected by the different densities of the liquids tested. Both the paddle wheel and the magnetic sensor mechanisms used in the Blue White and Banjo meters indirectly estimate flow rate.
Another advantage of the Dura-meter was that it was the cheapest of the three meters when tests were conducted. Another version of the Dura-meter can be used to turn off an injection pump when a specified volume of fertilizer has been injected. This version is available as part of the Auto Batch System (Dura-ABS™).
The Banjo meter is also available in a model (MFM100) that can output an electrical pulse proportional to flow rate so the volume of injected fertilizer can be recorded on a data logger or control system. It could also potentially be wired to cut off the injection pump when a desired volume of fertilizer is metered into the irrigation system.
Best Practices
Any of the tested flowmeters could help irrigators more accurately apply the intended volume of fertilizer to a crop as well as verify and maintain records of the applied fertilizer volumes. Depending on the preferences of the growing operation, it may be more efficient to install the meters on either the fertigation trailer or the main fertilizer tank.
If a fertigation trailer is used for injecting fertilizer at several fields during the day, installing the meter on the trailer would be a logical choice. However, if the nurse tank is filled for fertigating one block at a time, the flowmeter could be installed on the main fertilizer tank. Finally, the flowmeters tested are also helpful for monitoring tractor-sidedress applications of liquid fertilizers.
