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Fluid Journal : Early Spring 2013
11 The Fluid Journal Early Spring 2013 injected in microirrigation systems. Foremost of these is gypsum, but other materials such as potassium sulfate and other granular fertilizers are also being injected. The injection rate of the solutionizer machine should be adjusted to ensure that the injected material goes into solution prior to reaching any of the emitters. While the use of solutionizer machines has been an advance in chemigation, some caution must be used. Three issues are of concern: chemical precipitation, injected material solubility, and impurities in the injected material. As with liquid materials, solid materials containing calcium should be injected with caution if the irrigation water contains substantial bicarbonates (greater than 2 meq/1, or 120 parts per million) and the water pH is greater than 7.5. The jar test should be performed before injecting any new product. Remember that gypsum, which is calcium sulfate, is a ready source of calcium. Some of the materials injected by the solutionizer machine, including gypsum, are not readily soluble in the irrigation water. The concentrations at which these materials are injected should be monitored to ensure that the material is going into solution. In addition, adequate time should be allowed for the materials to go into solution once they are injected into the irrigation system. Impurities in the injected material are also a major consideration when using a solutionizer machine. Even the "pure," finely ground gypsum materials prepared for use with solutionizer machines contain some impurities. If the material is 95 percent gypsum, 5 percent of it is still "foreign" material. Thus, for every 100 pounds injected, there will be 5 pounds of the foreign material, some of which may not go into solution. Injecting granular fertilizers using solutionizer machines should be done with even greater caution. Many of these may have been meant to be land-applied, and no particular care was taken in the production to minimize the impurities. Some have oil or wax coatings to prevent water absorption, and introducing insoluble materials such as these into a microirrigation system may lead to emitter clogging. One option for dealing with suspended impurities is to inject the slurry from the solutionizer machine upstream of the microirrigation system's filter(s). A filter cannot be located on the line from the solutionizer machine since the output from the machine is a slurry, and the injected material (e.g., gypsum) does not go into solution until it is injected into the main irrigation line. Use caution if the microirrigation system has sand media filters, an automatic backflush screen, or disk filter system. Any material injected while the filters are backflushing goes out with the flush water---a potential environmental concern when injecting fertilizers or certain irrigation system maintenance products.System management Injection point. The location in the microirrigation system at which material is injected should depend on the type of material being injected. Readily soluble materials, such as fluid fertilizers and chlorine, should be injected downstream of the system's main filters. This will prevent the injected material from being part of the filter backflush water if filter cleaning occurs during injection. Also, a small screen or disk filter should be installed in the line from the storage tank to the injector to catch any impurities that may be in the material or tank. Acidic products for microirrigation system maintenance should not be injected where low water pH may damage metal components (e.g., some sand media filter tanks). Most plastic components will not be affected by low water pH. Materials injected by solutionizer machine should be injected upstream of the microirrigation system's main filters to remove any impurities in the injected material. Ideally, the injection should not occur while the filters are being cleaned. To ensure that the injected material and the irrigation water are mixed, materials should be injected into the middle of the water stream rather than at the pipe wall. Commercial devices are available that thread into the pipeline through a fitting and extend into the pipe, allowing injection directly into the fast-moving irrigation stream. It is also possible to make such a device using PVC fittings and pipe. Timing/duration. When injecting materials through a microirrigation system, two objectives should be kept in mind. First, the irrigation amount applied should be correctly determined so that the applied water and injected material remain in the plant's root zone. Applying more irrigation water than the plant's root zone can hold (over-irrigation) causes water to percolate below the root zone. Over-irrigation can also cause the injected material to percolate out of the root zone if the injected material travels through the soil easily with the water (e.g., nitrates). Good irrigation scheduling techniques can minimize this hazard and optimize the efficiency of chemigation. A good time to inject material is in the middle of an irrigation set (assuming that the irrigation set is long enough to allow a choice of when to do the injection). This makes it more likely that the injected material will stay in the root zone if over-irrigation occurs. Second, the duration of injection should provide a uniform application of injected material throughout the microirrigation system. It is important to remember that the injected material does not immediately reach all the emitters as soon as injection begins. A period of time is required for the water and injected material to move through the system to the emitters. This travel time depends on the design and layout of the microirrigation system. In the following discussion of travel times, it should be assumed that injected materials travel through the microirrigation system at the same speed as the irrigation water. Water travels through a microirrigation system's mainline and submain pipelines quite quickly. These pipelines are typically sized so that the flow velocity is less than 5 feet per second (fps) to minimize frictional pressure losses. Flow velocities of 1 to 3 fps are quite common in mainline and submains. Some pipeline systems are long, so movement of water and injected materials through them may take a while: travel times of 20 to 30 minutes are common, and travel times as long as 65 minutes have been observed (Table 1). Irrigation water flows slower in microirrigation lateral lines than it does through the mainline and submains. The flow velocity is particularly low at the tail end of the lateral lines. Understanding how water flows in drip lateral lines helps explain this. At the inlet of a drip lateral, the flow rate is that of all the combined downstream emitter discharges. For example, if 60 1-gallon-per-hour (gph) emitters were installed in the lateral line, the flow rate at the head of the drip
Late Spring 2013