Robert Mullen
Director of Agronomy
July 25, 2012
PotashCorp.com
Overview
•  Soil concepts
–  Nutrient movement
–  Critical levels
•  Fertilizer decisions
–  Philosophy of fertilization
–  Economic considerations
Soil Concepts
Nutrient Movement
•  Phosphorus and potassium are typically considered relatively immobile
–  Due to their nature and behavior in soils
Fertilizer
Manure
Plant uptake
Labile
Nonlabile
Adsorbed
P
Secondary
Minerals
Fe/AlPO4
CaHPO4
Adsorption
Desorption
Mineralization
Precipitation
Dissolution
Immobilization
Leaching
Primary
Minerals
Dissolution
Organic
Matter
Bound
P
Total P in soil – 50-1500 ppm ~
100-3000 lb/acre
Nutrient Movement
•  Potassium cycle
Fertilizer
Manure
Adsorption
Exchangeable
K
Desorption
Plant uptake
Fixation
Release
Nonexchangeable
(occluded)
K
Fixation
Total K in soil – 5,000-25,000 ppm
~ 10,000-50,000 lb/acre
Feldspars
Micas
Weathering
Leaching
Nutrient Movement
•  Soil textural influence on K movement
Claasen and Jungk,
SSSAJ, 1982
Nutrient Movement
•  So, P and K do not move much, so what; how does that influence
things like soil testing?
•  Before we go there, let’s see how these immobile nutrients are taken
up.
Nutrient Movement and Uptake
•  Nutrient mobility and competition
Root
system
sorption
zone
Root
surface
sorption
zone
Nutrient Uptake
•  Since they are available from a relatively small volume of soil, is there
much competition between plants for these nutrients?
•  There may be some competition, but not like for a mobile nutrient like
nitrogen
•  This is important, due to a lack of competition between plants, the
amount of nutrient required is not related to yield level
•  All we need to do is achieve a nutrient concentration to ensure
adequate availability!
Soil Potassium
•  Occluded K
–  Potassium trapped between clay particles
•  2:1 clay minerals like mica, vermiculite, and chlorite
10
Soil Potassium
•  Exchangeable K
–  Remember that clays are 3-dimensional structures
11
Soil Potassium
•  Occluded K is not necessarily unavailable to the plant, nor is supplied
K necessarily plant available, why?
12
Potassium
•  Adequate potassium nutrition increases water use efficiency and
reduces drought stress
•  Opening and closing of stomatal pores in plant leaves is regulated by K
concentration in the guard cells regulating transpiration
Guard
Cell
Potassium Deficiency
Critical Levels
•  Iowa State data – relative yield versus STP
Critical Levels
•  Iowa State data – relative yield versus STK
Critical Levels
•  Yield response as a function of soil test - P
Critical Levels
•  Yield response as a function of soil test - K
Critical Levels
•  It is not just the relative magnitude of the yield response, but also the
probability of response
STP (ppm)
Probability of response (%)
STK (ppm)
Probability of response (%)
<9
80
< 90
80
9-15
65
90-130
65
15-20
25
130-170
25
20-30
5
170-200
5
30 +
<1
200 +
<1
Illinois– P2O5
P2O5 removal
P2O5 balance
P2O5 fertilizer
60
50
Pounds /acre
40
30
20
10
0
-10
-20
-30
1985
1990
1995
2000
Year
Source: USDA-NASS, AAPFCO, IPNI
2005
2010
Illinois – K2O
Pounds /acre
K2O removal
70
60
50
40
30
20
10
0
-10
-20
-30
1985
1990
K2O balance
1995
2000
Year
Source: USDA-NASS, AAPFCO, IPNI
K2O fertilizer
2005
2010
Declining Soil Test Levels
•  “A random survey from 2007 and ’08 found 45% of Illinois fields
checked were below critical potassium levels needed for maximum
yields.” – Fabian Fernandez, University of Illinois
Declining Soil Test Levels
Soil test P
Soil test K
Both P and K soil test
levels are trending down.
Summary of Soil Concepts
•  P and K are relatively immobile
•  Soil testing can be used as a management tool
•  Soil testing is not perfect
–  Spatial variability, error in sampling/analysis, and temporal variability in analysis
(conditions at sampling time)
–  It is, by far, our best tool
Fertilizer Decisions
Philosophy of Fertilization
•  What is the goal of fertilization…
•  To maximize net return on inputs each year?
•  To assure that fertility limitations do not exist within a production year/
rotation?
•  To maximize short-term or long-term productivity?
Philosophy of Fertilization
•  Let’s revisit this data and put some economics to it
Philosophy of Fertilization
•  Net return to P application
Fertilizing Rotations
•  What if you are fertilizing rotations, do you have adequate P and K for
a soybean crop after a good corn crop?
•  It depends…
•  Upon your starting soil test level
•  For every bushel of corn you harvest you are removing 0.37 pounds of
P2O5 and 0.27 pounds of K2O
–  So a 200 bushel yield will remove 76 pounds of P2O5 and 54 pounds of K2O
–  A 250 bushel yield will remove 95 and 68 pounds of P2O5 and K2O, respectively
Fertilizing Rotations
•  If your starting soil test was near the critical, you will likely come up
short on your P and K for the soybean crop
•  For every 18 pounds of P2O5 removed, soil test will change by 1 ppm
•  For every 8 pounds of K2O removed, soil test will change by 1 ppm
Summary
•  Soil testing is our best tool, it is not perfect
•  You (and your clients) need to figure out the approach that best fits
their goals and economic desires
•  Applying same maintenance rates as practiced historically is likely not
maintaining current soil test levels
Thank You!
Questions?