Northeast Region Certified Crop Adviser (NRCCA) Study Resources

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In This Section

• Introduction
• PO 25 Plant nutrient sources
• PO 26 Form and analysis of nitrogen sources
• PO 27 Form and analysis of phosphorous sources
• PO 28 Form and analysis of potassium sources
• PO 29 Form and analysis of calcium or magnesium sources
• PO 30 Commercial fertlizer terms
• PO 31 Nutrient terms
• PO 32 Calculating fertilizer application rates
• PO 33 Calculating manure application rates
• PO 34 Fertlizer placement methods
• PO 35 Certifiable organic nutrient sources
• Summary

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:

Material	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:

Solid
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
Liquid
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.