Calculating Chemigation Injection Rates

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alulating hemigation njetion Rates WASHNGTON STATE UNVERSTY EXTENSON FAT SHEET FS035E hemigation is the appliation of agriultural hemials (e.g., fertilizers, pestiides, and soil onditioners or amendments) into or through an irrigation system with the applied irrigation water.

1 alulating hemigation njetion Rates WASHNGTON STATE UNVERSTY EXTENSON FAT SHEET FS035E hemigation is the appliation of agriultural hemials (e.g., fertilizers, pestiides, and soil onditioners or amendments) into or through an irrigation system with the applied irrigation water. t is an effiient and ost-effetive method of applying hemials. Vol = (Equation 1) T This fat sheet desribes how to alulate injetion rates for hemigation. English units are used elusively. Many of the equations an be solved with the irrigation alulators provided at alulators.php. Pestiide Labels The pestiide label is a legal and binding doument and must be read arefully, understood, and adhered to. n the event of disrepanies between the pestiide label and information ontained in this publiation, follow the pestiide label. A pestiide must be speifially labeled for hemigation to be legally used for this purpose. Appliation rates that eeed the labeled rate are not only unlawful, but will result in higher osts for hemials than neessary, potentially illegal pestiide residue levels, and possible damage to the rop, irrigation system, and/or the environment. Appliation rates that are too low may not ahieve the desired results. Therefore, it is in a grower s interest to omply with label instrutions, orretly alulate injetion rates, and properly set up and maintain irrigation and hemigation systems. t is also important to follow state laws and rules, loal ordinanes, and safety preautions for hemigation. (Antipollution devies and safety preautions are listed on the label.) njetion Rates by Fluid Volume The basi method of alulating injetion rates by fluid volume uses the following equation: = hemial injetion rate (gallons per hour) Vol = Total volume of hemial to injet (gallons) T = njetion time (hours) Eample: 350 gallons of hemial needs to be applied in 22 hours. The appliation rate is = 15.9 gallons/hour. The total volume of hemial (Vol) needed is alulated by multiplying the desired appliation rate (Q v, in gallons per are) by the area overed (A, in ares) by the irrigation system during the hemigation appliation, or: Vol = Q A (Equation 2) v A = Area (ares) Q v = Quantity of hemial to apply per are by volume (gallons per are) Eample: A boron supplement is being applied at 2.2 gallons per are onto a 125-are enter pivot field. The total volume of applied hemial is = 275 gallons of hemial. Note: t is important to trak and to use the orret omputational units. For eample, if injetion time (T) in minutes is used, then the hemial injetion rate ( ) result will ome out in gallons per minute instead of gallons per hour. f the appliation rate (Q v ) is reported on the label in quarts per are, divide the answer by 4 to get Vol in gallons. f the labeled appliation rate (Q v ) is speified in pints per 1

2 are, divide the answer by 8 to get Vol in gallons. See Table 1 for other ommon unit onversions. To simplify, Equations 1 and 2 an be ombined into the following equation: Qv A = (Equation 3) T Eample: A fungiide is to be applied at 2 pints per are to a 125-are enter pivot of potatoes. t takes 21 hours for the pivot to make a omplete revolution. Sine Equation 3 requires the units to be in gallons per are, 2 pints per are must be divided by 8, or 0.25 gallons per are. Now alulate the injetion rate using Equation 3: = 1.5 gallons of produt per hour 21 This injetion rate is far below what most pumps an aurately injet at. onsequently, water should be added to the tank to dilute the hemial. To determine how muh water to add and what the new injetion rate should be, see the setion Diluting hemial onentrations to Math nfleible njetion Rates. njetion Rates by Mass Sometimes, instead of fluid volume per are (Q v, in gallons per are), the appliation rate is speified in weight or mass per unit area (Q m ) suh as pounds per are. This is ommon when applying fertilizers (fertigation). However, sine hemials are injeted in a liquid form, the speified mass (Q m ) must be onverted to volume per are (Q v ). This is aomplished by dividing the desired appliation rate of the produt by the onentration of the injeted solution () in pounds/gallon. The onentration () is an inherent property of the fertilizer or pestiide, and an be found on the label. Qv = (Equation 4) Q m = Quantity of hemial to be applied by mass (pounds per are) = onentration (e.g., ative ingredient) of injeted solution (pounds per gallon) f the onentration () is given in ounes per gallon, then divide the answer (Q v ) by 16 to onvert the value to pounds per gallon. For simpliity, Equations 3 and 4 an be ombined into the following equation: A = (Equation 5) T Eample: Based on a leaf petiole analysis, 9 pounds per are of nitrogen is reommended. A grower plans to apply this using 32% urea-ammonium nitrate (UAN 32; Solution 32) fertilizer. There are 3.54 pounds of nitrogen in a gallon of 32% UAN. The fertilizer will be applied through a drip irrigation system to 45 ares during the last 6 hours of the irrigation set. (Remember to leave time to flush out the system.) The injetion pump should be set to: 9 45 = gallons per hour njetion Rates for Water hemistry ontrol n ertain irumstanes it is desirable to injet hemials to adjust water hemistry. This is ommon in drip irrigation to prevent the formation of mineral preipitates (iron, alium, or manganese), to manage algal or baterial growth, or to prevent or mitigate root intrusion. n these ases instead of gallons per are or pounds per are, the goal is a hemial onentration in the water ( w ), usually speified in parts per million (ppm). The injetion rate may be determined using Equation 6: Flow = P nt w (Equation 6) = hemial injetion rate (gallons per hour; gal/hr) Flow = Flow rate of the irrigation water (gallons per minute; gpm) w = Desired hemial onentration (parts per million; ppm) P nt = Perentage of hemial in solution (%) Eample: Sodium hypohlorite (12.5% solution) is to be injeted to ontrol algae and bateria growth in drip lines. The desired onentration is 3 ppm and the flow rate at the injetion site is 460 gpm. The injetion rate using Equation 6 would be: = 0.66 gallons per hour

3 Maimum onentration Sometimes a pestiide label will speify a maimum hemial onentration in the irrigation appliation system, speified as a perent (%). One the proposed hemial injetion rate ( ) is alulated as shown above, the onentration an be heked to ensure it is below the maimum. The perent onentration is alulated as: 100 s = 60 Flow (Equation 7) s = Solution onentration in the irrigation lines (%) = hemial injetion rate (gallons per hour; gph) Flow = rrigation water flow rate (gallons per minute; gpm) Eample: The alulated injetion rate is 1.73 gph, and the flow rate is 350 gpm. The label-speified maimum onentration in the irrigation water lines is 2%. The maimum line onentration using Equation 7 is: = 0.008% This is far less than the 2% maimum, so the injetion rate is fine. Miing Dry hemials Although less ommon, sometimes dry fertilizers or dry pestiide formulations (e.g., dry flowable, soluble powder, or wettable powder) are mied with water so that they an be used in a hemigation appliation. When applying hemials this way, two main questions must be solved: 1) how muh dry hemial will be needed and 2) how muh water should it be mied with to ensure the hemial s suspension? The total dry hemial to be applied an be alulated as: A Q m W t = (Equation 8) Pnt W t = Mass (weight) of the hemial to be applied (lbs) A = Area (ares) Q m = Rate to apply by mass (pounds per are; lb/are) P nt = Perent onentration in mi (epressed as a deimal), suh as the perent of elemental nitrogen in alium ammonium nitrate (AN-17; a) The minimum volume of water required for miing dry hemials an be alulated as: Wt Volmin = (Equation 9) S Vol min = Minimum volume (gal) W t = Weight of hemial to be applied (pounds; lbs) S = Solubility of hemial (pounds per gallon; lb/ gal), whih is obtained from the pestiide label or fertilizer solution harts. Note: Solubility is a funtion of solution temperature. Eample: Urea (46% nitrogen, with a solubility of 7.5 lb/gal) is to be dissolved and applied at 30 pounds of nitrogen/are to 25 ares. The mass or weight of hemial to apply (refer to Equation 8) is: = 1630 pounds 0.46 The minimum volume of water required to dissolve the urea is: = = gallons 217 gallons 7.5 of water of water More water an, and probably should be used to ensure that the dry hemial is ompletely dissolved. The total volume of the miture then beomes Vol, whih is used in Equation 1 to alulate the injetion rate ( ). Bath/Bulk Appliations (Drip, Hand-lines, Wheel-lines, Solid Set) Bath or bulk appliations are for set irrigation systems that don t move in the field while irrigation water is being applied. When applying fertilizers, timing during the irrigation set is the primary onsideration and the injetion rate is less ritial. Therefore, less epensive venturi-type injetors an be used. The injetion rate is set to limit hemial onentration ( s ) in the irrigation line. The injetion time (T) is hosen to set the hemial loation within the rop anopy or in the soil profile. t is usually best to injet fertilizers towards the end (i.e., during the last half) of the irrigation event or set so that the hemial remains in the upper soil profile where the majority of the plant roots are loated and where there is less hane of the hemial being leahed out of the root zone by subsequent irrigations. However, injetion should be finished in suffiient time 3

4 before the end of the set to ensure that all of the hemial has been flushed from the lines. Diluting hemial onentrations to Math nfleible njetion Rates Sometimes a hemial injetion system annot aurately injet at the low injetion rates required, or it has less fleibility in ontrolling the injetion rates suh as with a venturi or another pressure differential type method. n these ases water an be added to the hemial solution (diluted) to ahieve the needed appliation rate. Equation 5 an be solved for the required diluted hemial onentration ( dil ): A dil = (Equation 10) T This number is then used to alulate the amount of water to add to the tank miture (V add ) using the following equation: V V 1 ( dil add = 1 (Equation 11) dil V add = Amount of lean water to add to the miture (gallons) V 1 = Original volume of hemial at onentration 1 before miing (gallons) 1 = onentration of the premied solution (pounds per gallon; lb/gal) ) dil = onentration of the diluted solution as alulated in Equation 10 (pounds per gallon; lb/gal) The subsequent total volume of hemial (Vol) will be V 1 + V add. Obviously the onentration of the solution an be dereased by adding water (diluted), but not inreased. f the alulated dil is greater than 1, then a different injetion system must be used. Eample: 50 lbs of nitrogen per are is to be applied to a wheel line set of 1.6 ares/set during the last 6 hours of irrigation. A venturi injetor is being used that injets at a 50:1 ratio into a pipe line flowing at 150 gpm. Therefore, the infleible venturi injetion rate is 150 gpm/50 = 3 gpm. The required onentration of injeted solution ( dil from Equation 10) would be: = 4.4 pounds per gallon 3 6 The injetion is from a tank ontaining 300 gallons of a nitrogen solution with a onentration of 7 pounds per gallon, so the amount of water to add (V add from Equation 11) would be: 300(7 4.4) = 177 gallons 4.4 This means a 477 gallon ( ) solution of 4.4 lbs of nitrogen/gallon should be injeted with the 50:1 venturi injetor. Table 1. Unit onversions*. To onvert onvert to multiply by gallons per hour (gph) gallons per minute (gpm) gallons per hour (gph) fluid ounes per hour (oz/hr) 128 gallons per hour (gph) fluid ounes per minute (oz/min) 2.13 gallons per hour (gph) milliliters per minute (ml/min) 63.1 gallons per hour (gph) milliliters per seond (ml/se) 1.05 gallons per hour (gph) ounes per seond (ml/se) gallons per minute (gpm) gallons per hour (gph) 60 fluid ounes per hour (oz/hr) gallons per hour (gph) fluid ounes per minute (oz/min) gallons per hour (gph) milliliters per minute (ml/min) gallons per hour (gph) milliliters per seond (ml/se) gallons per hour (gph) ounes per seond (ml/se) gallons per hour (gph) 28.1 *Unit onversions an also be alulated at 4

5 Further Reading Burt,., K. O onner, and T. Ruehr Fertigation. San Luis Obispo: alifornia Polytehni State University. Hanon, B., N. O onnell, J. Hopmans, J. Simunek, and R. Beede Fertigation with Miroirrigation. Oakland: University of alifornia Agriulture and Natural Resoures. hemigation and Fertigation Assistane The Washington State Department of Agriulture offers individuals help to ensure that hemigation and fertigation appliations do not harm human health or the environment. See or all for details. By R. Troy Peters, Ph.D., P.E., Washington State University, Prosser, WA; Tom Hoffmann, Washington State Department of Agriulture, Moses Lake, WA; and W. Howard Neibling, Ph.D., P.E., University of daho, Twin Falls, D. Use pestiides with are. Apply them only to plants, animals, or sites as listed on the label. When miing and applying pestiides, follow all label preautions to protet yourself and others around you. t is a violation of the law to disregard label diretions. f pestiides are spilled on skin or lothing, remove lothing and wash skin thoroughly. Store pestiides in their original ontainers and keep them out of the reah of hildren, pets, and livestok. opyright 2011 Washington State University WSU Etension bulletins ontain material written and produed for publi distribution. Alternate formats of our eduational materials are available upon request for persons with disabilities. Please ontat Washington State University Etension for more information. You may order opies of this and other publiations from WSU Etension at or ssued by Washington State University Etension and the U.S. Department of Agriulture in furtherane of the Ats of May 8 and June 30, Etension programs and poliies are onsistent with federal and state laws and regulations on nondisrimination regarding rae, se, religion, age, olor, reed, and national or ethni origin; physial, mental, or sensory disability; marital status or seual orientation; and status as a Vietnam-era or disabled veteran. Evidene of nonompliane may be reported through your loal WSU Etension offie. Trade names have been used to simplify information; no endorsement is intended. Revised July FS035E

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