Reading your rates – Part 2

Fertilizer Rates & Concentration Values

Last post, we outlined the connection between electrical conductivity and the fertilizer concentration in your nutrient solution. Here, we outline the missing link – the relation between parts per million of Nitrogen (ppm N) and actual feeding rates (g/L, oz/gal).

In the horticultural industry, rates of fertilizers have traditionally been given in ppm N. This is because nitrogen is most commonly found nutrient in fertilizer formulations, and because it allows us to state the concentration of a fertilizer solution independent of the fertilizer analysis. You can read more about this from UMass Extensions here. It is important to distinguish ppm N from total ppm, since total ppm typically refers to total dissolved solids, including those that are not coming from the fertilizer. When we refer to ppm N, we are referring to nutrient concentration coming from the fertilizer only.

Calculating Rates Based on ppm N

When preparing a fertilizer stock solution, it is important to know the injector ratio of your system. This refers to the amount of stock solution that is delivered for each increment of irrigation water that passes through the injector. For instance, a 1:100 injector ratio will deliver a final 100 gallons of dilute fertilizer solution for each 1 gallon of concentrated stock solution.

Other important factors include:

  • Desired ppm you wish to feed of a specific element guaranteed in the fertilizer
  • % of the element guaranteed in fertilizer
  • Conversion constant determined by the units desired for your rate (see table below)

Example 1

You want to feed Plant-Prod MJTM Bloom 10-30-20 at 150 ppm N, and your injector is set to 1:200. Your tank is 500 L and you want to know how many grams of fertilizer to add to your stock tank.
Let’s list all the factors you know:

  • Desired ppm N = 150
  • Injector ratio = 200
  • % of the element guaranteed in fertilizer = 10
  • Conversion constant for g/L (see table above) = 10

Now let’s put all that into the equation:

Now you know that you need to add 150 kg of Plant-Prod MJTM Bloom to your 500 L stock tank to have a final fertilizer solution of 150 ppm N. Note that the ppm N in your stock tank will be 200 x this concentration since you have an injector system set to 1:200.

Example 2

What if you have a rate in oz/gal that you’ve been using and want to compare it to a recommendation that is in ppm N? All you have to do is rearrange the equation.
Say you’ve been diluting 14 oz/gal of Plant-Prod 20-10-20 and using an injector set to 1:100.
Again, let’s list all the factors you know:

  • Rate in ounces per US gallon = 14
  • Injector ratio = 100
  • % of the element guaranteed in fertilizer = 20
  • Conversion constant for oz/gal (see table above) = 75

Now let’s put all that into the equation:

Now you know that you are feeding 210 ppm N when mixing a stock solution of Plant-Prod 20-10-20 at a rate of 14 oz/gal and using an injector ratio of 1:100.

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