The 2012 ASHS Annual Conference

Irrigation water treated with free chlorine (hypochlorous acid and hypochlorite ion) at 2 mg·L^-1 and pH range of 6 to 7.5 can control zoospores of Pythium and Phytophthora spp.  The objective was to quantify the effect of water soluble fertilizers on concentrations of free chlorine level in a sodium hypochlorite solution.  Chlorine from sodium hypochlorite (Clorox^® regular bleach) was applied at 2.6, 10, and 20 mg·L^-1 of free chlorine into deionized water only (control) and 11 fertilizer treatments with 200 mg·L^-1 of nitrogen.  Fertilizer treatments included reagent-grade ammonium sulfate (NH₄)₂SO₄, ammonium nitrate (NH₄NO₃), potassium nitrate (KNO₃), urea salts, and seven commercial blended N–P–K water-soluble fertilizers, with macro- and micronutrients.  Commercial fertilizers contained ammonium-N at 0 to 50% of total-N, urea-N at 0 to 14% of total-N, and nitrate-N at 50 to 93% of total-N.  Measurements of free chlorine (mg·L^-1), total chlorine (mg·L^-1), and oxidation-reduction potential (ORP, in mV) were recorded after 2 min and 60 min after chlorine was applied, to quantify the oxidative strength of the solution.  Combined chlorine was determined from the calculated difference between the total and free chlorine measurements.  All solutions were maintained at pH 6 at 25 °C during the analysis.  In the control solution, the applied 2.6 mg·L^-1 of free chlorine decreased to 2.5 mg·L^-1 after 2 min or 2.2 mg·L^-1 after 60 min.  Treatments of (NH₄)₂SO₄ and NH₄NO₃ had decreased free chlorine levels to below 0.1 mg·L^-1 after 2 min or 60 min.  Urea reacted more slowly than ammonium salts, whereby free chlorine decreased to 2.3 mg·L^-1 after 2 min and 0.4 mg·L^-1 after 60 min.  In contrast, KNO₃ had less impact on free chlorine, with 2.4 mg·L^-1 free chlorine available at both 2 min and 60 min.  With all commercial fertilizers tested, after 2 min, free chlorine decreased to below 0.1 mg·L^-1.  Total chlorine was above 2 mg·L^-1 after 60 min in all treatments, indicating that the majority of chlorine was in a combined form for ammonium and urea salts and commercial fertilizers.  The ORP of commercial fertilizer blends and ammonium-containing salts was lower than 600 mV, whereas deionized water, KNO₃, and urea treatments had ORP levels above 650 mV.  Fertilizer containing ammonium or urea required 20 mg·L^-1 or more of applied chlorine to maintain residual free chlorine above 2 mg·L^-1.  Research on the disinfestation strength and phytotoxicity risk of combined chlorine forms such as chloramines is needed.