- Author: Michael Cahn
- Contributor: David Chambers
- Contributor: Noe Cabrera
Growers will need to implement best management practices that reduce nitrate leaching losses on the Central Coast to comply with Agricultural Discharge Order 4.0. The use of drip irrigation has allowed many growers to be efficient with both water and nitrogen fertilizer. Fertigating through the drip system allows for spoon feeding nitrogen in amounts matching the pattern of crop N uptake, and to place fertilizer in the root zone. Tools like the soil nitrate quick test and the online irrigation and nutrient management platform, CropManage, can help farm managers accurately determine the right amount of fertilizer to apply to satisfy crop N requirements without jeopardizing production.
Once the right amount of fertilizer to apply has been determined, it is important that irrigators have the tools that they need to accurately inject the correct volume into the drip system. Fertigation trailers usually consist of a nurse tank that can hold a maximum volume of 500 to 1000 gallons of fertilizer and are equipped with a small gas or electric pump used to inject liquid fertilizer into the drip system. Often irrigators rely on markings on the side of the nurse tank to determine the volume of fertilizer that they are injecting. These markings are usually not accurately calibrated nor have fine enough graduations to precisely measure out fertilizer volume. Furthermore, tank markings can be hard to read, especially if the trailer is not level.
A flowmeter could increase the precision of metering fertilizer into a nurse tank or for measuring the volume of fertilizer injected into the drip system. Using a flowmeter for metering fertilizer would also facilitate tracking the volume of fertilizer applied to each crop by either noting the meter readings or by interfacing the flowmeter to a datalogger that can record the application volumes.
We evaluated the accuracy of three models of flowmeters designed for metering liquid fertilizer: 1. Banjo FM100 meter, 2. Dura-meter, and 3. Blue White F-1000 (Fig. 1). Each model relies on a different mechanism to monitor fertilizer volume. The Banjo meter measures flow using a magnetic sensor, while the Dura-meter uses a nutating disk, and the Blue white meter uses a small propeller. The accuracy of the flowmeters was tested using 25 gallons of either water, ammonium nitrate (20% N), or urea-ammonium nitrate (32% N). A testing manifold was set up in the UCCE Monterey greenhouse that pumped a calibrated volume of each fluid through the flowmeters using an electric diaphragm pump. Five or more test runs were made for each meter and fluid. The average volume measured and standard deviation from the mean volume was calculated.
All three models of flowmeters accurately measured water and fertilizer volumes (Table 1). Measurement errors were generally less than ±2% of the true volume. The Dura-meter which uses a nutating disk to measure volume 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 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.
Table 1. Accuracy of flowmeter measurements of water and two types of liquid fertilizer (AN20 and UAN32).
Although the Dura-meter was 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 directly measures volume of a liquid which may explain why the Dura-meter was not affected by the density of the liquid 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 the 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 the auto batch system (Dura-ABS™). The Banjo meter is also available in a model (MFM100) the can output an electrical pulse proportional to flow rate so that volume of fertilizer injected can be recorded on a datalogger.
Conclusions
Three commercially available flowmeters were demonstrated to accurately measure fertilizer. Either of these meters could help irrigators more precisely apply the intended volume of fertilizer to a crop as well as verify and maintain records of the fertilizer volumes used to grow each crop. Depending on the practices of the growing operation it may be more efficient to install the meters on either the nurse tank trailer or the main fertilizer tank. If the nurse tank is used for injecting fertilizer at several fields during the day, then installing the meter on the trailer would be logical, but if the nurse tank is only filled for a single field at a time, the flowmeter could be installed on the main fertilizer tank.
- Author: Michael Cahn, Irrigation and Water Management Advisor
- Contributor: Thomas Lockhart, Staff Research Associate
- Contributor: Laura Murphy, Staff Research Associate
An important benefit of drip irrigation is the ability to apply fertilizer through the irrigation water, permitting growers to spoon-feed nutrients, such as nitrogen (N), to their crops. By avoiding applications of large amounts of N fertilizer when the crop is small and uptake rates are low, losses of nitrogen by leaching can be minimized. Also, unlike furrow and overhead sprinklers, drip can deliver fertilizer in the zone where roots are most concentrated.
While drip fertigation offers several advantages for managing nitrogen fertilizer during the season, success depends on the management of the drip system and using best practices for fertigation. Drip systems with poor distribution uniformity may likely cause fertilizer to be unevenly distributed within a field. Also, the strategy of injecting fertilizer into a drip system can affect the distribution of fertilizer to the crop. Proper fertigation requires injecting at a steady rate and at a location that provides sufficient mixing of fertilizer with irrigation water. To assure that the fertilizer uniformly distributes within the field after an injection, sufficient irrigation time with clean water is needed so that all of the fertilizer is flushed out of the drip tape before the irrigation ends.
For drip to be economical for vegetable growers on the central coast, most farming operations retrieve drip tape after each crop is harvested and repair and reuse the tape for 8 to 12 crops. Breaks and leaks in the tape are repaired using a splicing machine (Figure 1). Growers have expressed concern that fertigating through their drip systems is not resulting in even applications of N fertilizer after the tape has been reused for multiple crops. Splicing machines often do not fully repair leaks in tape, and emitters tend to plug over time unless the tape was adequately maintained by flushing and chemical treatment.
In response to grower concerns, we evaluated the uniformity of applied water and nitrogen fertilizer for surface placed drip in 11 commercial lettuce fields during the fall of 2012 and during the spring of 2013.
Fig. 1. Splicing machines are used to repair leaks and breaks in drip tape
Procedures
All fields were planted with romaine or iceberg lettuce varieties on 40-inch or 80-inch wide beds. At each site irrigation, pressure, and fertilizer uniformity were evaluated during a single irrigation event. Field sizes ranged from 8 to 20 acres, and the maximum row lengths ranged from 600 to 1340 ft. Drip tape at all field sites was 7/8 inch diameter, medium flow tape (0.34 gpm/100 ft), but varied by manufacturer and age. The location where fertilizer was injected into the irrigation system, and start and end time of the fertigation, as well as the duration of the irrigation, were recorded. Before irrigating, couplers fitted with ¼ gallon per hour pressure compensating emitters that were spliced in to the drip tape at 24 locations within the field, representing the head, tail and middle areas. Water from these emitters was collected into 5 gallon containers during the entire irrigation (Figure 2) and analyzed for NO3-N and NH4-N. The discharge rate of 4 emitters and pressure of the tape was measured near each of the 24 fertilizer sampling locations (total of 96 emitters). Mass (lbs) of N applied at each of the 24 collection locations within a field was estimated by multiplying the measured discharge rate of the drip tape by the irrigation time and by the concentration of N in the collected water. Uniformity of applied water, tape pressure, and fertilizer was calculated by comparing the lowest 25% of measurements to the average of all 24 measurements. In addition to evaluating fertilizer distribution uniformity, we evaluated the time for fertilizer to travel to the furthest distance from the injection point by injecting food dye for a 5 minute period into the irrigation system and monitoring the water for color at the furthest point from the injection location.
Fig. 2. Low flow (1/4 gph) pressure compensating drip emitters were used to collect samples of irrigation water during the entire irrigation cycle.
Results
Distribution uniformity of applied water for the 11 fields averaged 73% and ranged from 38% to 88% (Table 1). The industry standard for irrigation uniformity of surface drip is 85%. Fertilizer application uniformity averaged 67% and ranged from 46% to 82%. The distribution uniformity of the drip systems of 7 fields evaluated was greater than 74% (avg = 82%) and fertilizer uniformity was greater than 72% (avg = 77%) (Figure 3).
One of the causes for poor distribution uniformity of some drip systems may have been related to pressure. Pressure uniformity averaged 80% and ranged from 43% to 99% (Table 1). Average pressures in the drip tape ranged from 3.5 to 13.8 psi (Table 2). Where the system pressure averaged 4.3 psi, the tape discharge rate was 30% less than the manufacturer's rating. Irrigation distribution uniformity decreased substantially when the average field pressure was less than 5 psi (Figure 4). Additionally, a substantial percentage of emitters of some drip systems were plugged (Table 2) which would reduce irrigation system uniformity. Leaks were evaluated in 5 fields and ranged from 1 to 5 leaks per 1000 ft of tape (Table 2). Significant leaks can potentially reduce drip uniformity by lowering the downstream pressure. Other limitations to good drip uniformity included mixing different types of tape in the same field, fluctuating pressure during the irrigation, and row lengths longer than 800 ft.
Field 8 had a high uniformity of pressure and irrigation distribution but a low fertilizer uniformity. We speculate that the fertilizer which was injected at a “T” connecting the valve in the field with the submain did not have sufficient time to mix with the irrigation water before the flow split into opposite directions. Hence, the average concentration of N on one side of the field was approximately half the concentration measured on the other side of the field. The distribution uniformity of fertilizer on individual sides of the fields was greater than 87%.
With the exception of field 8, fertilizer distribution uniformity was closely related with irrigation system uniformity (Figure 4). Fields with the lowest fertilizer uniformity were operated at the lowest average pressure and/or had the highest level of plugged emitters (Table 2).
Fertilizer was injected at the well in 4 of the fields and at the submain valve in the other 7 fields (Table 3). Injections were made simultaneously using 2 valves at 3 of the fields. Fertilizer was injected during an average of less than 30 minute period often at the beginning of the irrigation (Table 3). The time required for the fertilizer to travel to the furthest point of the irrigation system averaged 42 minutes but ranged from as short as 22 minutes to as long as 1 hour. Field size, row length, and injection location appeared to affect the travel time of the fertilizer. The average time for flushing the fertilizer was 3.75 hours, which was ample time to allow the fertilizer to completely flush from the system. The irrigation industry recommends that for long irrigations (> 4 hours), fertilizer should be applied in the middle of the irrigation cycle. Only at field 10 was the fertilizer applied during the middle of the irrigation. The long flush time after injecting could potential leach nitrate forms of fertilizer below the root zone of the crop. On average, half of the applied fertilizer N measured in the collection buckets was in the nitrate form.
Conclusions
This survey of commercial lettuce fields demonstrated that N fertilizer applied by drip has an average distribution uniformity of 77% when the injection is properly made and the drip system is operated and maintained to achieve an average distribution uniformity of 82%. The results also showed that N fertilizer applied by drip is frequently distributed to fields unevenly due to poor uniformity of the drip systems, or because proper injection procedures were not followed. Operation procedures observed at these sites would suggest that irrigators may need training to better understand the principles of fertigation so that fertilizer is applied at the highest uniformity possible, and in a manner that will prevent leaching losses of nitrate.
Acknowledgements
We thank the California Leafy Green Research Board for funding this project and the many growers that cooperated with the field trials.
Table 1. Summary of irrigation, fertilizer, and pressure uniformity of drip irrigated lettuce fields.
Table 2. Drip tape characteristics at commercial lettuce sites.
Table 3. Irrigation summary for drip irrigated lettuce fields.
Fig. 3. Relationship between distribution uniformity of retrievable drip systems and fertigation uniformity. Each symbol denotes a commercial lettuce field evaluated during the study.
Fig. 4. Effect of tape pressure on the distribution uniformity of retrievable drip systems. Each symbol denotes a commercial lettuce field.