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Competency Area 4: Nutrient Sources, Analyses, Application Methods

PO 32. Calculate fertilizer application rates from fertilizer analysis information.

Before using any fertilizers, it is important to understand how to read a fertilizer label.  All fertilizers are labeled as %N - % P2O5 - %K2O.

For example, a fertilizer labeled as a 20-5-10 means that the product contains, by weight: 20 percent N, 5 percent P2O5, and 10 percent K2O.

The nature of the fertilizer (liquid or dry) must also be taken into account.  Generally there is no measurable difference in crop response, as long as they supply the same amount of soluble nutrient.  From a plant's perspective, a pound of N from urea is equal to a pound of N from UAN.  The agronomic differences among N fertilizers are the risk for N losses due to volatilization, denitrification and leaching.  Ultimately, the choice between wet and dry P fertilizers is based on economics and farmer preference.  Note that for liquid fertilizers where the rate is given in gallons per acre, the density of the fertilizer must be known to determine the amount of nutrients applied per acre.  For example:


Weight (lbs/gal)

10-34-0 11.7
9-18-9 11.7
28-0-0 10.7

Fertilizer blends are developed by combining basic fertilizers to get nutrient concentrations appropriate for a given crop, soil, or application method.  Some common blends include:


20-10-10 8-32-16 20-20-20
6-24-24 10-20-20 0-10-40
5-15-15 0-20-20 19-19-19


21-17-0 10-34-0 7-21-7  

The basic formula for calculating how much fertilizer to apply to a given area for a specific amount of nutrient is:

Fertilizer "rate" often refers to two things:

  1. The desired lbs of actual nutrient per acre
    • e.g. a crop nutrient goal of 100 lbs N/acre, 25 lbs P2O5/acre, and 50 lbs K2O /acre
  2. The desired lbs of fertilizer material per acre necessary to achieve the lbs of nutrient per acre goal
    • e.g. given a 20-10-0 fertilizer, how many lbs of fertilizer material do we need to apply 25 lb N/acre?  How much P2O5 and K2O tag along?

Example 1

  • Needed: 45 lb N, 45 lb P2O5, 45 lb K2O per acre
  • Determine ratio of N : P2O5: K2O: (45:45:45 = 1:1:1)
  • Apply: for example, 300 lb of 15-15-15 (because 45 lb needed / 15% in fertilizer = 300 lb fertilizer)

Example 2

  • Needed: 150 lb N, 30 lb P2O5, 120 lb K2O per acre for corn
  • Apply: for example
K preplant (broadcast) 150 lb/ac 0-0-61 = 0 0 91
Starter 200 lb/ac 15-15-15= 30 30 30
Sidedress UAN 37 gal/ac 30-0-0 = 120 0 0
  Total 150 30 121
  • In this example, UAN 30-0-0 weighs 10.85 lb/gal; therefore, 37 gal/ac = 37 x 10.85 = 401 lb of UAN/acre.  At 30% N, this equals 401 x 0.30 = 120 lb N/acre.