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Fluid Journal : Spring 2016
18 The Fluid Journal Spring 2016 to take advantage of winter rains for leaching of sodium followed by in- season fluid calcium fertilizer injections to lower the Sodium Adsorption Ratio (SAR) of the irrigation water. This two- tier approach uses gypsum to remediate a sodic soil and fluid calcium fertilizers to reduce the effects of sodium carried in the irrigation water in-season. A second trial conducted by the University of California Cooperative Extension in 2007 that compared the effects of water-run calcium thiosulfate (“CaTs”) through the drip irrigation system to broadcast applied gypsum. The calcium thiosulfate was applied at 10 gallons per acres for five successive irrigations. Gypsum was applied at 500, 1,000 and 2,000 lbs./A. followed by five successive irrigations. Results presented in Figure 1 show parts per million (ppm) of sodium leached after each respective calcium product application/irrigation. Figure 1 shows measurable amounts of sodium were leached with successive irrigations for all treatments, including the control. The data also indicate that the fluid calcium thiosulfate treatment was as or more effective than the gypsum treatments. This is due to the superior solubility of calcium thiosulfate compared to gypsum, which provides for more immediate results. In the long run, it would be expected the higher rates of gypsum would leach more sodium simply due to the higher load of soluble calcium that gypsum provides, but over a much longer period of time and with more water. Gypsum has a very low level of solubility (2.05 gm/liter) and over 100,000 gallons of water would be needed to dissolve one ton of pure gypsum. Common sources The most common fluid calcium fertilizers found in the market are calcium ammonium nitrate, calcium nitrate and calcium thiosulfate. Their properties are presented below in Table 2. Calcium chloride and calcium polysulfide are commercially available products but are used less frequently due to the addition of chloride and handling issues, respectively. The most commonly used acid fertilizers today are sulfuric acid, phosphoric acid, and urea sulfuric acid products. All are highly effective for water treatment but due to the handling risk that phosphoric and sulfuric acid present, only the much safer urea sulfuric acid products will be presented here. USA Urea-sulfuric acid (USA) fertilizers are a unique class of products with a variety of useful applications, from cleaning of drip irrigation systems to wheat stubble burn-down. USA chemistry was developed to provide acidifying benefits of sulfuric acid without the personnel hazard that is associated with the transportation and handling of concentrated sulfuric acid. The heat of reaction and the extremely corrosive nature of sulfuric acid have been removed during the manufacturing process of USA products. Never attempt to mix sulfuric acid with urea as this is a very violent reaction that requires specialized equipment and procedures! USA products are available from two basic suppliers: Agrium and Verdegaal Brothers, and are available in three grades having the properties (Table 3}. USA products are ideally suited for treating irrigation water that is high in bicarbonate, particularly in low volume irrigation systems such as drip or micro-sprinklers. The general chemical equation when injecting USA into irrigation water containing bicarbonates is: H2SO4 + 2HCO3͢SO42- + 2CO2 +2H2O. The bicarbonate is digested, releasing carbon dioxide gas and water. At a pH of 6.5, about 50 to 60 percent of the bicarbonate ions carried in the irrigation water is consumed by this reaction. So, the benefits of reduced bicarbonate content are: • Reduced plugging pressure on drip system • Reduced formation of excess lime (calcium carbonate) in the soil • Higher levels of soluble and exchangeable calcium in the soil • Reduced soil pH in some situations • Reduced opportunity to form sodic soils • Maintain or improve soil porosity and structure. Techniques Techniques for using USA in low volume irrigation systems are important in the following ways: Clean out. Periodic clean out is required to remove calcium carbonate deposits, loosen suspended solids that may have passed through the filter and consume biological matter. This is commonly referred to as “shock treatment” because it requires injection of USA at rates that cause a drastic drop in water pH. Target pH for shock treatments is in the 3.0 to 3.5 range. Shock treatments should be used only in systems that are composed of 100% acid tolerant materials. Always verify with manufacturers or dealers before acidifying irrigation systems. Most systems should receive a shock treatment at least annually. Injection may be made upstream or downstream from the filter station, depending on materials used in the filtration system. Contact your drip system dealer for specifics. Run injection for two hours, flush system with clean water, and openendsof8to10linesatatimeto slush dislodged solids until clear water flows. Repeat treatment if needed but always flush with clean water after acid injection. Maintenance. Continuous, or near continuous, maintenance applications are needed to achieve a water pH of 6.0 to 6.5. This improves water quality and lessens the opportunity for drip system plugging by mineral deposits. Maintenance applications may not eliminate the need for periodic shock treatments, but will lengthen the time required between such treatments. Maintenance applications are particularly beneficial with water having high bicarbonate and calcium levels (high plugging potential). Shock treatments should be performed during dormancy on perennial crops or post- harvest on annual crops to prevent accidental root injury. Use of USA may provide partial to moderate relief from root intrusion, depending on the degree of intrusion and the size of intruded roots. Larger roots are far more difficult to dissolve than are finer roots. Follow instructions above for shock treatment but turn the system off and allow the system to stand 4 to 6 hours before turning the system back on and flushing with clean water. Open 8 to 10 lines at a time to flush dislodged solid matter until clear water flows. Rate What rate of USA should be used to adjust pH to the maintenance range or for a shock treatment? The dominant ions found in irrigation water that determine the rate of USA needed to