Four Steps in Fertilizer
Calculations
• Determine the area
• Determine the rate per area
• Determine the amount of nutrient (area X
rate)
• Convert amount of nutrient to amount of
fertilizer
Two Basic Types of Calculations
(nutrient vs. fertilizer)
• You have a given amount of fertilizer and
you need to calculate how much nutrient is
in it
• You have a given amount of a nutrient to
apply and you need to calculate how much
fertilizer to use
Memorize and Understand This!
• Weight of fertilizer X % of Nutrient =
Weight of nutrient
• # of urea X 46% N = # of N
• Weight of fertilizer = (Weight of
nutrient)/(% of nutrient)
• # of urea = (# of N)/(46% N)
Converting From Fertilizer to
Nutrient :
• How many pounds of N are in a 50# bag of
ammonium nitrate (35-0-0)?
• 50# fert X 0.35 = 17.5 #N
• 50# fert X (0.35#N/#fert) = 17.5 #N
Converting From Nutrient to
Fertilizer :
• How many pounds of ammonium nitrate
(35-0-0) do you need to give you 5# of N?
• x# fert X 0.35 = 5 #N
• x# fert X (0.35#N/#fert) = 5 #N
• x# fert = (5 #N)/(0.35#N/#fert) =
14.3 # fertilizer
Example 1
• The area of a home lawn is 15,000 ft2
• You want to apply 1#N per 1000 ft2 using
ammonium sulfate (21-0-0)
• Total amount of N = (1#N/K) X (15K) =
15#N
• Total amount of fertilizer = 15#N/0.21 =
71# ammonium sulfate
Example 1, more difficult
• You want to apply 1#N per 1000 ft2 to the same
lawn, using 75% IBDU and 25% urea
• 15#N X 75% = 11.25#N from IBDU (31%N)
• x# IBDU = (11.25#N)/(0.31#N/#IBDU) =
36.3# IBDU
• 15#N X 25% = 3.75#N from urea (46%N)
• x# urea = (3.75#N)/(0.46#N/#urea) =
8.2# urea
Example 2, Acres
• 1 acre = approx. 43,000 ft2 = 43K
• 1#N/K X (43K/acre) = 43#N/acre
• How much ammonium nitrate (33-0-0) would
you buy for 75 acres if you want to apply a
total of 4#N/K during the next year, in 4
separate applications? It comes in 50 # bags.
How many bags would you order?
Continued
• 1#N/K = 43#N/acre
• 4#N/K = ?#N/acre = 172#N/acre
• For 75 acres, need: 172#N/acre X 75 acres =
12900#N
• x # AN = (12900#N)/(0.33#N/#fert)
=
39091 #AN
• 39091 #AN/(50 #AN/bag) = 782 bags
Example 3
• You are growing 25 acres of tall fescue and
plan on applying a total of 4 #N on the
following schedule:
2# N as MU (39-0-0) on May 1
1# N as 26-4-8 on Mar. 1
1# N as 26-4-8 on Oct. 1
• Question 1: How much of each fertilizer do
you need to order?
Question 1 Calculations:
•
•
•
•
Will apply 2#/K of both fertilizers
(2#N/K)(43K/acre)(25 acres) = 2150#N
(2150 #N)/(0.39 #N/# MU) = 5513 # MU
(2150 #N)/(0.26 #N/# 26-4-8) = 8269 # fert
Example 3 continued
• You also want to apply 1.5 # actual P/1000
and 3 # actual K/1000 ft2.
• Question 2: How much actual P and K are
applied in the 26-4-8 applications?
Question 2 calculations
• There are a couple ways to approach this.
• Figure out how much fertilizer is applied
per 1000 ft2:
• (8269 # fert)/(25 acres) = 331 # fert/acre
(331 # fert/acre)/(43K ft2/acre) = 7.7 #fert/K
• Now calculate how much actual P and K are
applied in 7.7 # fert/1000 ft2.
Continued...
• 7.7 # fert/1000 ft2 (26-4-8)
• For P: (7.7 #fert)(.04 #P2O5/#fert) = 0.3#
P2O5/1000 ft2
• (0.3# P2O5)(0.44# P/#P2O5) = 0.14 # P/1000
• To apply a total of 1.5 actual P/1000, need
1.36# P/1000 (1.5-0.14) from some other
source. Let’s use super phosphate (0-20-0).
Continued...
• Need 1.36 #P/1000 from 0-20-0 for 25 acres
• How much total P?
(1.36#P/K)(43K/acre)(25 acres) = 1462
#P
• How much 0-20-0 is needed? First calculate
the amount of P2O5 equal to 1462 #P:
(1462#P)/(0.44#P2O5/#P) = 3323 # P2O5
• How much 0-20-0 is needed?: (3323 #
P2O5)/(0.20# P2O5 /# fert) = 16614 # 0-20-0
Continued...
• Now figure out how much K must be applied:
• Remember we applied 7.7 #26-4-8/1000
• How much actual K is applied in 7.7 #
fert/1000 ft2?
(7.7#fert/1000)(0.08#K2O/#fert) = 0.62# K2O
• (0.62# K2O)(0.83#P/# K2O) = 0.51# K/1000
Continued...
• To apply a total of 3 actual K/1000, need 2.5#
K/1000 (3-0.5) from some other source.
Let’s use KCl (0-0-60)
• How much extra K is needed?
(2.5#K/1000)(43K/acre)(25 acres) = 2688
#K
• Convert from actual K to K2O:
(2688#K)/(0.83#K2O/#K) = 3238 # K2O
Continued...
• How much 0-0-60 is needed?:
(3238#K2O)/(0.60#K2O/#fert) = 5397 # of
KCl