Efficiency of Chemical and Physical Hard Water Softening Methods to Reduce the Incompatibility of Hard Water Cations with herbicide Glyphosate

 Water is the most frequently used carrier for herbicide applications. Thus, the physicochemical properties of water in spray mixture can affect the activity of herbicides. A high concentration of Na+, K+, Ca2+, Mg2+, Fe3+ and other cations in hard water can decrease herbicide efficacy. Weak acid herbicides that have been antagonized by one or more of the above cations include sethoxydim, 2,4-D, 2,4-DB, clethodim, imazethapyr, tralkoxydim, and glufosinate and glyphosate. Approaches to minimize hard water antagonism have included decreasing the spray carrier volume and using water-conditioning additives that have proven effective at ameliorating cation-caused antagonism include ammonium sulfate, ammonium nitrate, potassium phosphate, and citric acid. Passing hard water containing Ca2+, Mg2+ or Na+ through an external magnetic device results in the nucleation and crystallization of the respective carbonates. As a result, hard water can be softened for a period. Considering that the hardness of Iranian agriculture is increasing and adding an adjuvant to spray solution is also considered to be more environmental contamination, therefore, the physical conversion of hard water to soft water via its passage through a magnetic field is definitely a good alternative. The objectives of this research were to investigate the effect of adding CaCO3, MgCO3, Na2CO3, K2CO3, or Fe2(CO3)3 to distilled water on glyphosate efficacy to jimsonweed (Datura stramonium L.), and to compare the chemical hard water softening methods (ammonium sulfate, ammonium nitrate, citric acid and potassium phosphate) to a new physical hard water softening method (passing carrier through a magnetic field) to Reduce the incompatibility of hard water cations with glyphosate.

 The seeds of jimsonweed were collected from plants in the fields of Qazvin city, Iran. They were stored in the dark at room temperature until use. Bioassays were conducted in a greenhouse located on the Ferdowsi University of Mashhad, Iran. To increase seed germination before starting the experiment, the seeds were washed every 1 hour for 7 days to remove seed germination inhibitors. Twenty-five seeds were sown at 0.5 cm depth in 2 L plastic pots filled with a mixture of sand, clay loam soil, and peat (1:1:1 by volume). At cotyledon-leaf stage, the seedlings were thinned to four per pot. The pots were irrigated every four days with tap water. Treatments were sprayed at the four-leaf stage. The experiment was arranged as a completely randomized design with four replications as a factorial design with factors of carrier type (distilled water alone or containing 0.5 g L-1 of CaCO3, MgCO3, Na2CO3, K2CO3, or Fe2(CO3)3) and hard water softening method (ammonium sulfate, ammonium nitrate, potassium phosphate, citric acid, and passing through a magnetic field) and glyphosate dose (0, 12.81, 25.62, 51.25, 102.5 and 205 g a.i. ha-1). For magnetizing the carriers, it was passed 10 times through a magnetic treatment device modified from Rashed-Mohassel (30). The mixing order for treatment solutions was (i) adding CaCO3, MgCO3, Na2CO3, K2CO3, or Fe2(CO3)3 to distilled water, then (ii) adding/using water conditioning method, and after 15 min (iii) adding glyphosate. Then, the solutions were sprayed after about 5 min using a calibrated moving boom sprayer at 180 L ha-1 at 200 kPa with 11002 flat-fan nozzle. Shoots were harvested four weeks after treatment, dried for 48 h at 70°C, and dry weight was determined. The data of shoot dry weight were subjected to a non-linear regression analysis for determination of ED50 values (herbicide dose needed to obtain 50% reduction in dry weight) using the following logarithmic logistic dose-response model. The relative potency (R), the horizontal displacement between the two curves, was also calculated.

 As judged by the relative potency values given in Table 1, the hard water softening methods decreased the ED50 values when distilled water was used as the carrier. Therefore, the activity of glyphosate against jimsonweed was significantly increased in the presence of the hard water softening methods. There were significant differences in performance among hard water softening methods as ammonium sulfate was the most effective method. Glyphosate activity was not decreased when applied in a K2CO3 solution but it was decreased when applied in Na2CO3, MgCO3, CaCO3 or Fe2(CO3)3 solutions. Except potassium phosphate which had only a significant effect at reducing the antagonism in the CaCO3 carrier; all hard water softening methods could restore glyphosate activity in hard water contaminated carriers to efficacy levels comparable to glyphosate alone in distilled water. There was no statistical difference in response between the magnetized carrier and ammonium sulfate when they were used in Na2CO3, MgCO3, or Fe2(CO3)3 solutions. It is reported that hard water softening methods may adjust the spray solution pH so that more active ingredient can transport across the leaf surface into the plant via ion trapping phenomenon. Ammonium sulfate was the most successful method to ameliorate the decreased glyphosate activity due to antagonism with Na+, Mg2+, Ca2+, or Fe3+ in the spray solution. By adding ammonium sulfate, the sulfate ion () conjugates with the hard water cations and removes free cations from solution by forming cation-SO4 molecule, allowing ammonium ion () to form glyphosate-NH4 molecule. A glyphosate-NH4 molecule diffuses across the cuticle easier and quicker. A mechanism for physical hard water softening method is illustrated in Fig. 2.

 Although the physical hard water softening method was not effective as compared to some chemical hard water softening methods (ammonium sulfate and citric acid), from the point of view of economical and agricultural, applying the physical hard water softening method will be benefit because it needs no chemical.