Can you identify the water problem in this case study, where overhead irrigation is staining poinsettia foliage? A brown residue formed on the outside of leaves, creating an aesthetic problem (Figure 1a, see slideshow).
The staining occurred regardless of fertilizer concentration. Lower leaves had to be manually removed or cleaned, increasing labor cost and causing delays during peak shipping. A bath-ring effect was even forming on greenhouse side walls from the overspray of irrigation booms (Figure 1b, see slideshow).
At the 20-acre facility, the labor cost to inspect, clean, and replace clogged emitters, filters, and irrigation lines was estimated at more than $148,000 annually. On opening a 2½ -inch-diameter pipe after a five-year lapse, the sludge that lined the inside greatly reduced the effective pipe area (Figure 2, see slideshow). The water mains that were cleaned by water-blasting remained free of sludge for about three years.
Although emitters could be cleaned in acid (Figure 3, see slideshow), the iron surrounding the rubber diaphragm was not removed, and the emitters still clogged. Some of the emitters and drip irrigation lines had to be replaced, at an additional cost of more than $4,000. The total annual cost therefore exceeded $152,000 (not including crop shrinkage and labor to remove stained leaves). A copper-ionization system being used for algae control was not controlling the issue. Something else had to be done.
Getting To The Root Of The Problem
The greenhouse was using well water, with a slightly high pH of 7.4, a low electrical conductivity of 0.1 mS/cm, and low alkalinity at 33 ppm CaCO3. A water test found that total suspended solids, a measure of particles in the water, equaled 6.2 mg/L (higher than the desired 5 mg/L limit for drip irrigation).
Lab analysis showed 1.3 ppm of dissolved iron. However, this iron concentration is deceivingly low, because most of the suspended iron precipitated out overnight (Figure 4, see slideshow). An iron test that was performed onsite immediately after the water came out of the well showed 8 ppm of iron.
Bacteria count in the source water was 253 colony-forming units per milliliter (CFU/mL), which increased to 573 CFU/mL at a far emitter. These levels are well below the biofilm risk level of 5,000 CFU/mL measured by plating bacteria with 3M Petrifilms.
A sludge sample from inside a pipe was sent to a commercial analytical laboratory. After dissolving the sludge in acid for analysis, the lab reported minerals including iron (58%), phosphorus (23%), calcium (11%), potassium (3%), and zinc (2%).
Page 2 – The Culprit and In Case You Missed It