Results

The original 4 hour testing procedure did not result in significant plugging with any tape. Water source or fertilizer combination, although there was a tendency for flows to slow where 10-34-0 was used. The four hour period with the fertilizer in the tape was apparently insufficient time to all the chemical precipitation to form or accumulate. The test procedure was modified to increase the time and add a second cycle to simulate a second irrigation with the fertilizer remaining in the tape between irrigations. Plugging was observed with this longer test procedure. Figure 2 and Table 2 show the Salinas Valley test with 10-34-0 and the tape was almost completely plugged by the end of the first run and did not recover at the beginning of the second irrigation cycle, the test equipment failed during the second cycle so the flushing with acid to test the recovery from the plugging could not be completed, Table 3 and Figure 3 show the same fertilizer with good water and the Central Valley tape. The results were similar to those in Table 2 in that complete plugging occurred at the end of the first irrigation and the interval before the second. In this case, flushing with acid was able to restore about 40% of the emitters to nearly their original flow rate. The organic based phosphate fertilizer, Structure®, caused some reduction in flow rate under the same conditions but the loss was about 10% compared to nearly 100% plugging with the 10-34-0. Flushing with just water in the second irrigation cycle restored the original flow rate of the tape for each the Salinas Valley simulation. Table 4 and Figure 4 and the Central Valley simulation. Table 5 and Figure 5. These simulations, made intentionally extreme, are intended to demonstrate that phosphate fertilizers can be safely applied through drip tape. Note that without proper precautions and care, chemical precipitates can still form and slow or stop tape flow under field conditions similar to these. The interesting results found in the tests were these:  

1. The chemical precipitation may require several hours to form and cause plugging. Short fertigation periods, as recommended by Actagro for use of Structure®, may be safer than long runs.
2. Phosphate fertilizer remaining in the tape because of insufficient post-application flushing can be responsible for additional plugging problems.
3. Some forms of phosphate fertilizers, such as Actagro Structure®, produce significantly less chemical precipitate than other forms and have demonstrated less plugging potential.
4. Tapes in current use are capable of being flushed after partial plugging with phosphate fertilizers to restore some, and perhaps all of the original performance.

It would appear that the “fatal plugging” by phosphate fertilizers of drip tape and emitters that was observed in the first years of the use of drip irrigation is not as big a problem as it once was. The use of phosphate and other low solubility fertilizers may be considered for fertigation programs with appropriate choice of material, testing and careful monitoring, flushing and maintenance of the system.

Additional Results

As an aside to the original study, Actagro asked CIT/CWI to evaluate the concern that lower flow tapes (like the “coastal” tape used here), might in some way harbor a larger residual quantity of Structure® organic-phosphorus fertilizer than other drip tapes. Fully eliminating the residual color from Structure® after application in the above testing was a concern from the start. It was observed that it took a considerable effort, more than 1 flushing, to fully eliminate all residual color from the test apparatus after Structure® application. As it was observed that no plugging resulted from normal use of Structure®, it was assumed that any remaining quantities of phosphorus were very low. It was decided to measure the product concentration at which no visible light passed through samples as an indicator of how well a known quantity of material “dyed” the water. There was significant dark color in the samples at 0.1% and 0.5% of the original application concentration. At just 0.3% of the original application concentration, there was no transmittance of visible light. This worked out to just 32 ounces product/100 gallons of water. The very dark color created by this very low level of fertilizer product eliminates concern that something inherent in the tape causes an accumulation of product in the tape. The concentration of Structure® in the tape rapidly decreases proportionate to the amount of water flowing through the tape after the fertilizer injection ceases. When the ends of a drip line are opened to flush the system with brand new tape, the material is first very dark, then lightens somewhat, but takes awhile to fully clear. This would also be the case in tape that has already been used and possibly exacerbated by any imperfections in filtration or tape maintenance to that point.