Fertilizers are designed to provide the elements necessary for plant growth. About 90% of the plant weight is made up of water. The remaining mass constitutes the plant dry weight, which is made up primarily of 17 elements that are required for plant growth. Let’s define some terms.
Different plants may show different sensitivities to the presence or absence of these nutrients, especially the micronutrients, but they are all required for specific plant functions.
Water soluble, commercial fertilizers used in the greenhouse come in a wide variety of formulations and nutrient analyses. A commercial fertilizer may contain one or all of the essential elements but the percent of each will be listed on the fertilizer label. Micronutrients may or may not be included in the formulation. If included they also will be listed in the “guaranteed analysis” section on the fertilizer bag.
Fertilizer recommendations are generally given in parts per million of nitrogen (ppm N) in the final solution being applied to the crop. Most growers use fertilizer injectors for applying fertilizers to greenhouse crops (Fig 1). These devices “inject” a specific amount of concentrated fertilizer solution (stock solution, Fig 1A) per increment of irrigation water that passes through the injector (Fig 1B). An important attribute of each fertilizer injector is the injector ratio, which is defined as volumetric ratio of stock solution to dilute fertilizer solution.
A 1:200 injector will deliver 200 gallons of dilute fertilizer per gallon of stock solution. If we wanted to mix stock solutions for each of these injectors to deliver 200 ppm of nitrogen using the same fertilizer formulation, the stock solution for the 1:200 injector would have to be twice as concentrated as the one for the 1:100 injector. Thus, the injector ratio determines the concentration of the stock solution that is needed to apply a specific rate of fertilization.
Growers need to know the injector ratio in order to prepare fertilizer stock solutions. Commercial injectors range from 1:5 to 1:500, but most small growers are using 1:16, 1:100 or 1:200. Injectors should be calibrated monthly during the growing season. The injector ratios can change over time due to wear of injector parts. Most injectors do not have calibration adjustments. Therefore, it is necessary to periodically check an injector to determine its actual ratio. Adjustments to obtain a given fertilizer concentration at delivery is done by adjusting the concentration of the fertilizer stock solution. The following procedure can be used to determine the actual injector ratio.
Injector ratio = Gallons of water collected x 128 ounces/gallon
Ounces of water depleted from measuring cup
Example 1. A grower used the procedure described above for testing a fertilizer injector. Ten gallons of water were collected in a barrel, and 13 fluid ounces were depleted from a measuring cup. What is the injector ratio for this fertilizer injector?
1. List all the variables:
2. Set up and solve the problem:
x = 10 gallons x 128 oz/gal
= 98.46 (about 98)
3. Answer: the fertilizer injector ratio is approximately 1:98.
Checking the fertilizer EC. Another way to determine the output of your fertilizer injector is by measuring the electrical conductivity (EC) of the dilute fertilizer solution. You can draw a fertilizer sample and have it analyzed for the concentration of N by a reputable testing laboratory. Alternatively, you can get a good estimate of the fertilizer concentration by measuring the EC of the solution yourself using a portable EC meter (Fig. 2). Be sure to calibrate the EC meter prior to each use and be sure to subtract the EC of your clear water from the fertilizer reading. Compare the corrected value for the EC of your fertilizer solution to that listed on the fertilizer bag for the ppm N you intended to be applying (Fig. 3). During the crop cycle, especially for short-term crops like bedding plants, you should measure the EC of your fertilizer solution on a weekly basis to check that the injector is working properly.
Growers must accurately determine the amount of fertilizer needed to mix stock solutions of fertilizers. Most of the manufacturers of commercial fertilizers and fertilizer injectors have produced tables that simplify this task. Information is also provided on fertilizer bags (Fig. 3). Without recourse to tables or bags, growers can use formulas to calculate the amount of fertilizer needed. If you know the rate of fertilization (in ppm N) that you want to apply to the crop, the percentage of the nitrogen in the fertilizer, and the injector ratio, then calculations are simplified by the following formula:
Amount of fertilizer to make 1 volume of stock solution
= Desired conc in ppm x Dilution factor
% of element in fertilizer x C
where the dilution factor is the larger number of the fertilizer injector ratio and the conversion constant (C) is determined by the units in which you want to measure the fertilizer:
|Unit||Conversion constant (C)|
|Ounces per U.S. gallon||75|
|Pounds per U.S. gallon||1200|
|Grams per liter||10|
This formula allows you to easily calculate the amount of fertilizer needed to mix stock solutions. Generally, the calculations are based on the percentage of nitrogen, but using this formula, you can make up solutions for any given element. This formula can be used with any fertilizer injector and any unit of measurement. Most growers in the United States prefer ounces (or pounds) of fertilizer per U.S. gallon. A conversion constant is also given for those who use the metric system.
Example 2. You have a 1:98 fertilizer injector and a fertilizer with an analysis of 20-10-20 (%N-%P2O5-%K2O). You want to apply a 200 ppm solution of nitrogen at each watering. How many ounces of fertilizer would you have to weigh out to fill a 20-gallon stock tank with concentrate?
1. List all the variables:
2. Set up and solve the problem:
x oz Fertilizer / gal = 200 ppm N x 98
20% N x 75
= 13.1 oz 20-10-20 /galx oz Fertilizer / stock tank = 13.1 oz/gal x 20 gal
Or, divide by 16 oz/lb = 16 lb 6 oz of 20-10-20 for 20 gal of stock solution
3. Answer: add 16 lb 6 oz of 20-10-20 to the stock solution bucket and fill to the 20 gallon mark.
Example 3. By the 25-lb bag. Many growers do not have access to an accurate scale for weighing fertilizers. Since most commercially formulated N-P-K fertilizers are packaged in 25-pound bags, we can easily determine how many gallons of stock solution to mix up from one bag of fertilizer to apply 200 ppm N using a 1:98 injector:
25 lbs/bag x 16 oz/lb = 400 oz/bag
400 oz/bag ÷ 13.1 oz/gallon = 30.5 gal/ bag
It is important to remember that the final volume of stock solution should be 30 gal. This means we add the fertilizer first, and then add water (warm water works best) for a final volume of 30 gallons. Adding the bag of fertilizer to 30 gallons of water will give us more than 30 gallons of stock and thus a more dilute stock solution than desired. Of course, you must make sure that you have a storage tank (barrel or trash can) large enough to accommodate your stock solution. Be sure to keep the stock solution dark to prevent nutrient degradation. Always verify that the fertilizer you are preparing has sufficient solubility to dissolve in the amount of water you intend to use for your stock solution volume. Maximum solubility is listed on the bag. For most soluble fertilizers, it ranges from about 2.5 to 4 lbs per gallon of water.
We can also use these calculations to determine the content of other fertilizer components in our fertilizer solution or we can develop a fertilizer solution based on an element other than nitrogen. This is important when using incomplete fertilizers to develop your own tank mixes or to supplement a complete fertilizer. When calculating ppm of P or K we must account for the oxide form of the elements in the fertilizer. We can use the simple rhyming rule “%K and %P equals 1.2 and 2.3” to convert from the oxide to the elemental forms for phosphorus and potassium, that is, from %P2O5 to %P and from %K2O to %K.
Example 4. You have a fertilizer with an analysis of 20-10-20. What is the percentage of phosphorus and potassium in the elemental form?
1. List all the variables to find out what is known and unknown:
2. Set up the problem:
%P = % P2O5
= 4.3% P in 20-10-20
%K = % K2O
= 16.7% K in 20-10-20
3. Answer: 20-10-20 contains 4.3% elemental phosphorus and 16.7% elemental potassium.
The conversion rule is useful when we desire to fertilize with simple fertilizers such as potassium nitrate (13-0-44). When plants are grown in media which contain adequate levels of phosphorus (for example, from a pre-plant addition of superphosphate), it is often recommended that 200 ppm of N and K be applied at each watering. We can achieve this fertilization program using potassium nitrate and calcium nitrate (15.5-0-0). We can use the formula previously given to calculate ppm K if we first convert from the oxide to the elemental form.
Example 5. You have a 1:100 injector and want to use potassium nitrate (13-0-44) and calcium nitrate (15.5-0-0) to supply 200 ppm of N and K with each watering. How many ounces of each fertilizer would you have to weigh out to make 1 gallon of concentrate?
%K = % K2O
= 36.7% K in 13-0-44
x = 200 ppm K x 100
36.7% K x 75
= 7.27 oz 13-0-44 /gal
13% N = ppm N
36.7% K 200 ppm K
36.7x = 2600
x = 2600 ÷ 36.7
= about 71 ppm supplied by potassium nitrate
200 ppm N - 71 ppm N = 129 ppm N needed from calcium nitrate
x = 129 ppm N x 100
15.5% N x 75
= 11.1 oz 15.5-0-0 /gal
With the aid of a hand-held calculator, you can easily determine the proper amount of fertilizer for making stock solutions. Remember, always recheck your calculations to ensure they are correct: errors may be very costly! Better yet, have someone else check your calculations. Keep records of your calculations for future reference. Check the EC of the final fertilizer solution against that given on the bag or fertilizer label to verify that your calculations were correct (remember to subtract the EC of your clear water) and monitor the EC of your diluted fertilizer solution on a weekly basis during the crop cycle. Check the calibration of your injector on a monthly basis during crop production or whenever you suspect a problem.
North Carolina State University Horticultural Substrates Lab (includes links to publications listed below and a PowerPoint presentation of the PourThru Method of monitoring media fertility) www.ncsu.edu/project/hortsublab/
Alkalinity Control for Irrigation Water Used in Greenhouses (NCSU) www.ces.ncsu.edu/depts/hort/hil/hil-558.html
Greenhouse Substrates and Fertilization (NCSU) www.ces.ncsu.edu/depts/hort/floriculture/plugs/ghsubfert.pdf
Substrate pH and Water Quality www.ces.ncsu.edu/depts/hort/floriculture/plugs/ph.pdf
Adapted from “Fertilizer Calculations for Greenhouse Crops,” Thomas H. Boyle, University of Massachusetts Extension Service.
Virginia Cooperative Extension materials are available for public use, re-print, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.
Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Jewel E. Hairston, Administrator, 1890 Extension Program, Virginia State, Petersburg.
June 30, 2009