How to Calculate Fertigation Injection Rates for Commercial Blueberry Production^{1}
Introduction
Florida's commercial blueberry industry has increased significantly in both acreage and value, expanding from approximately 1,000 acres in 1993 to more than 5,500 acres in 2015 (USDANASS, 2017), and from $39 million in 2007 to $82 million in 2015 (USDANASS, 2016). Per capita consumption of blueberries has increased by 400% during the last decade (Braswell 2010). Florida produced approximately 25.3 million pounds of fresh blueberries in the 2015 season. However, Florida only contributed 4.4% of total US fresh blueberry production, which was more than 565.5 million pounds (USDANASS 2017). The value of blueberries destined for processing has also increased. The total US value of processed blueberries grew from $62.3 million in 2009 to $136.0 million in 2010 (USDANASS 2011).
Florida's subtropical climate allows for early blueberry production during a historically profitable marketing window. The industry is expected to continue to grow and expand. To increase nutrient and water use efficiencies and reduce nutrient leaching and environmental concerns, fertigation is recommended for commercial blueberry production. In fact, successful fertigation can enhance sustainability and maximize profitability for commercial blueberry enterprises. One of the key factors in fertigation is the correct calculation of fertilizer injection rate and time for the acreage. This publication helps blueberry growers correctly calculate fertigation injection rates and times and provides reference tables for checking injection rates and times needed for a variety of production scenarios.
Calculation of Injection Rate and Time
Fertigation rate and time depend on the irrigation water flow rate and fertilizer application rate. To simplify the calculation, we will use nitrogen (N) fertilizer as an example. For other nutrients, such as phosphorus, potassium, and the like, the principle is the same. For most cases, 0.1% of the irrigation flow rate is a proper injection rate (Burt, O'Connor, and Ruehr 1995). Six steps are needed to calculate the injection rate and time for fertigation if a solution N fertilizer is used.
Step 1: Determine the total amount of N needed for the fertigation event. Calculate the total amount of N needed by multiplying the farm size in acres by the N rate to be applied in pounds per acre. If you know your total linear feet of bed and width of row spacing, the linear bed feet can also be used. For example, for a blueberry farm with a 4foot bed and 8foot row spacing, the actual linear feet of bed per acre can be calculated as follows (Equation 1):
In other words, the rate of N divided by the linear bed feet gives the amount of N per linear bed foot to be applied. Multiplying the total linear feet of bed and the amount of N per linear foot of bed provides the total amount of N for the fertigation event. For calculating recommended fertilizer rates in raisedbed, mulched cultural systems, see https://edis.ifas.ufl.edu/ss516.
Step 2: Calculate the total weight of liquid fertilizer needed for fertigation. The total weight depends on both total N to be applied and the grade of the selected N fertilizer solution. For instance, UAN32 (ureaammonium nitrate solution, 3200) contains 32% N by weight. The total weight of the fertilizer solution to apply is equal to the total N needed (from Step 1) divided by the N concentration (0.32 in this example). For example, if you need 100 pounds of N for a particular fertigation event, how much UAN32 do you need? Divide 100 pounds by 0.32. You need 312.5 pounds of UAN32.
Step 3: Calculate the number of gallons of liquid N fertilizer. This number is determined by the density of liquid N fertilizer. Every solution fertilizer has a density listed on the fertilizer label. For example, 1 gallon of UAN32 weighs 11.05 pounds; thus, the density of this particular fertilizer is 11.05 pounds per gallon. The amount of fertilizer in gallons to apply is calculated by dividing the total weight of solution fertilizer (from Step 2) by the density.
Step 4: Calculate the dilution factor. The dilution factor is determined using the N concentration of the solution N fertilizer and the target N concentration in the irrigation water (parts per million [ppm]) for the fertigation event. The fertilizer grade (e.g., 32%) should first be converted into ppm by multiplying it by 1,000,000. The fertilizer concentration in ppm (e.g., 320,000 ppm) is then divided by the target N concentration (e.g., 150 ppm). The result is the dilution factor.
Step 5: Calculate the injection rate. The injection rate is determined by dividing the irrigation water flow rate (e.g., 1,000 gallons per minute) by the dilution factor (from Step 4).
Step 6: Calculate the injection time. Injection time is determined by dividing the number of gallons of solution N fertilizer needed (from Step 3) for the fertigation event by the injection rate (from Step 5).
Practical Example
We will use UAN32 (32% N, 11.05 pounds per gallon) to apply 5 pounds N per acre to a 5acre blueberry field with 8foot spacing. The irrigation flow rate is 1,000 gallons per minute and the target N concentration in the irrigation line is 150 ppm. To calculate the injection rate and time:
Step 1: Total N: 5 lb/acre N × 5 acres = 25 lb N
Step 2: Pounds of UAN32: 25 lb N ÷ 0.32 = 78.1 lb UAN32
Step 3: Gallons of UAN32: 78.1 lb ÷ 11.05 lb/gal = 7.0 gal
Step 4: Dilution factor: 0.32 × 1,000,000 ppm ÷ 150 ppm = 2,133.3
Step 5: Injection rate: 1000 gal/min ÷ 2133.3 = 0.47 gal/min
Step 6: Injection time: 7.0 gal ÷ 0.47 gal/min = 15 min
Therefore, in this particular case, 7.0 gallons of UAN32 are needed for the fertigation event.
For differentsized blueberry farms with 1,000 gallons per minute irrigation flow rate at target N concentration of 150 ppm N and using UAN32 as the N source, the corresponding gallons of UAN32 and injection time can be found in Table 1. Here, 150 ppm N is recommended because if the N concentration is too low, the plants may not be able to get sufficient N. The fertilizer should not exceed 200 ppm in fertigation (Krewer and NeSmith 2012). This example uses UAN32. Other soluble fertilizers can also be used in fertigation. If using double drip tape per bed, the injection time can be shortened by up to 50%.
For a 10acre field using UAN32 with a target N concentration of 150 ppm but with different water flow rates, the corresponding injection rate and time are shown in Table 2.
Why is it important to calculate the fertigation rate correctly? Because we need to make sure that blueberry plants receive sufficient—but not excessive—nutrients. We must avoid plant damage by not introducing too much salt at one time. We want to avoid overapplying fertilizer to save money in fertilizer cost, thus maximizing profitability. We should prevent or minimize potential nutrient contamination of nearby water resources.
References
Braswell, B. 2010. "There's Value in Growing Together: The Florida Blueberry Industry Continues to Take on the Many Challenges Any Agriculture Endeavor Faces." Growing Produce. Accessed October 31, 2011. http://www.growingproduce.com/article/18510/theresvalueingrowingtogether
Burt, C., K. O'Connor, and T. Ruehr. 1995. Fertigation. San Luis Obispo: California Polytechnic State University.
Krewer, G. and D. S. NeSmith. 2012. Bluberry Fertization in Soil. University of Georgia Ext. Fruit Publication 011. Accessed January 19, 2015. http://www.smallfruits.org/Blueberries/production/blueberryfert.pdf
Strange, C. 2007. "FDACS Funds Pojects to Enhance Florida Specialty Crop Competitiveness." Florida Department of Agriculture and Consumer Services. Accessed October 31, 2011. http://www.freshfromflorida.com/press/2009/03302009.html
USDANASS (United States Department of AgricultureNational Statistics Service). 2011. "Noncitrus Fruits and Nuts 2010 Summary (July 2011)." Accessed December 5, 2011. http://usda01.library.cornell.edu/usda/nass/NoncFruiNu/2010s/2011/NoncFruiNu07072011.pdf
USDA NASS (2017). National Agricultural Statistics Service (NASS). Washington, DC: US Department of Agriculture. Accessed March 29, 2018. http://usda.mannlib.cornell.edu/usda/current/NoncFruiNu/NoncFruiNu06272017.pdf
Tables
Injection rate and time needed to apply 5 pounds of N per acre (in a 150 ppm N solution made from UAN32 liquid fertilizer with 32% N) to a 1 to 100acre farm with a 1,000gallon per minute irrigation water flow rate.
Farm size 
Total N needed 
UAN32 fertilizer needed 
Dilution factor 
Fertilizer injection rate 
Fertilizer injection time 

(acres) 
(lb) 
(lb) 
(gal) 
(gal/min) 
(min) 

1 
5 
15.6 
1.4 
2133.3 
0.5 
3 
2 
10 
31.3 
2.8 
2133.3 
0.5 
6 
3 
15 
46.9 
4.2 
2133.3 
0.5 
9 
4 
20 
62.5 
5.7 
2133.3 
0.5 
12 
5 
25 
78.1 
7.1 
2133.3 
0.5 
15 
Note: To prevent any nutrient contamination of nearby water sources and maximize profitability of commercial blueberry production, fertigation time should NOT be more than 15 minutes. If you have a blueberry farm with more than 5 acres, you may need to use a pump with large capacity to perform your fertigation. 
Calculation of injection rate if applying 5 pounds of N per acre (in a 150 ppm N solution made from UAN32 liquid fertilizer with 32% N) to a 10acre farm with 500, 1,000, 1,500, or 2,000gallon per minute flow rate from the pump.
Water flow rate 
Total N 
UAN32 
Dilution factor 
Injection rate 
Injection time 

(gal/min) 
(lb/acre) 
(lb) 
(gal) 
(gal/min) 
(min) 

500 
50 
156.3 
14.1 
2133.3 
0.2 
60 
1,000 
50 
156.3 
14.1 
2133.3 
0.5 
30 
1,500 
50 
156.3 
14.1 
2133.3 
0.7 
20 
2,000 
50 
156.3 
14.1 
2133.3 
0.9 
15 