Pinpoint Toxicity In Your Pond Water

This month’s case study highlights a coastal Florida producer who was starting to use pond water for irrigation (Figure 1, see slideshow). The pond water had a lower electrical conductivity (EC) than the operation’s well water, and the well water supply was running low. However, the grower realized that the open pond had a high level of algae and could possibly harbor pathogens, so he installed a water-treatment system.

Water pumped from the pond passed through a sand filter, and sodium hypochlorite (liquid bleach) was injected to chlorinate the water. The water was batch-treated in a holding treatment tank before being pumped to the greenhouse, to ensure there was adequate contact time to control algae and pathogens (Figure 2, see slideshow).




Despite this treatment, plant health problems occurred when the pond water was applied in the greenhouse. Some plants showed pitting and bronzing symptoms on foliage, and stunted growth (Figure 3a and 3b, see slideshow).

Water Analysis Helps Narrow Down The Problem

Given that the plant symptoms only occurred when the pond water was used, what could be the problem? A series of water tests were run on the untreated pond water (Table 1), but none of the results were far out of range:
• pH was slightly high, but plant symptoms were not consistent with iron deficiency at high pH.
• Concentration of dissolved salts (both electrical conductivity, and a detailed analysis of individual dissolved ions) was low. Applied fertilizer concentration in the greenhouse was also low, and pesticides were applied at normal label rates. Therefore, residue on foliage or high salts in the root substrate were not damaging plant health.
• Total suspended solids and bacteria counts were higher than desired, which is typical in pond water because of algae and microbial growth. However, the grower did not complain of clogged irrigation emitters after chlorination and filtration.
• No oomycetes (Pythium or Phytophthora) were found in the water sample, and plant symptoms were not consistent with root or crown rot.
That led us to focus on the chlorination treatment. The grower was applying 1.75 gallons of 10% chlorine in the 8,000-gallon tank. The test kit for free chlorine showed a reading of up to 3.5 ppm for the “residual” level coming out of the batch tank (see Table 2 for chlorine terms).

The Culprit: Overdosing Chlorine

Because the nutrients in the pond combined with the active ingredient of chlorine (hypochlorous acid), the total applied chlorine was much higher than the 3.5 ppm of free chlorine being read on the meter.

We calculated almost 22 ppm was being applied. This is much higher than the safe zone of 2 to 5 ppm of applied total chlorine, and 1 to 2 ppm of residual chlorine. To calculate the dose based on the gallons of bleach and tank size, go to our website, and under “Tools” use the “Recipe to ppm” tool (Figure 4, see slideshow).

We have run experiments in our greenhouse to test chlorine phytotoxicity. We have not seen damage when hypochlorous acid was applied overhead at 2 ppm with each irrigation to bedding plants. However, we have heard reports from commercial greenhouses where phytotoxicity occurs with low doses (2 ppm, especially in propagation). Chloramines, then hypochlorous acid, and then chlorine dioxide, in that order, have increasing reactivity and likelihood of phytotoxicity. We have also observed phytotoxicity from other sanitizing agents applied at high rates, including hydrogen dioxide-based products and copper ionization. Simply switching away from chlorine is therefore not always the answer.

Solution: Reduce Chlorine Dosage

The grower reduced the applied dose to no more than 3 ppm of total chlorine, and 1 to 2 ppm of residual chlorine. For safety, this was based on two ways to check dosage (a) measurements of total and free chlorine, and (b) calculated amount of chemical added per volume of irrigation water.

Page 2 – Take-Home Lessons and In Case You Missed It

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