Greenhouse and Nursery - Know your Fertilizer Calculations

When using liquid feed for greenhouse and nursery plants, it is important to know how to prepare stock solutions for your fertilizer injector. The following is an example of how to go about these calculations.

You have a 1:200 fertilizer injector and a fertilizer with an analysis of 15-16-17 (%N-%P2O5-%K2O). You want to apply a 250 ppm solution of nitrogen at each watering. How many ounces of fertilizer would you have to weigh out to make 1 gallon of concentrate?

A. To solve the problem:

1. List all the variables:
a. Desired concentration in parts per million (ppm) = 250.
b. Injector ratio = 1:200; dilution factor = 200.
c. Fertilizer analysis = 15-16-17 (15% N).
d. Ounces of fertilizer to make 1 gallon of concentrate = X (unknown). Use 75 as the conversion constant C.

Conversion constants
Ounces per U.S. gallon = 75
Pounds per U.S. gallon = 1200
Grams per liter = 10


2. Set up and solve the problem

Amount of fertilizer to make 1 volume of stock solution =
(Desired concentration in parts per million x Dilution Factor) divided by
(% of element in fertilizer x Conversion constant)

X= (250 ppm N x 200)/(15% N x 75)= 50,000/1,125 = 44.44 (about 44½ oz./gal.)


B. Answer: add 44½ ounces of 15-16-17 to a stock solution bucket and fill to the 1 gallon mark.



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 1 bag of fertilizer:

1. Convert 25 pounds into the equivalent amount of ounces:

25 pounds/bag x 16 ounces/pound = 400 ounces/bag

2. Using the information in Example 1, we then divide 400 by 44½ to get the number of gallons of stock needed:

400 ounces/bag 44½ ounces/gallon = 8.99 (about 9 gallons/bag)

Thus, one 25-pound bag of 15-16-17 fertilizer will make 9 gallons of stock for a 250 ppm N solution when using a 1:200 injector.

Example from "Fertilizer Calculations for Greenhouse Crops" by Thomas H. Boyle, Dept. Plant and Soil Sciences, University of Massachusetts, Amherst