"Are Your Soybeans Coming Up Short on P and K?"

1. Are Your Soybeans
Coming Up Short of
P and K?
Robert Mullen
Director of Agronomy
July 25, 2012
PotashCorp.com

2. Overview
• Soil concepts
– Nutrient movement
– Critical levels
• Fertilizer decisions
– Philosophy of fertilization
– Economic considerations

3. Soil Concepts

4. Nutrient Movement
• Phosphorus and potassium are typically considered relatively immobile
– Due to their nature and behavior in soils
Fertilizer
Manure
Plant uptake
Adsorption
Labile Adsorbed
P
Desorption
Solution P
H2PO4- Mineralization Organic
Precipitation
Secondary Matter
Minerals Dissolution HPO42- Bound
Fe/AlPO4 Immobilization
Nonlabile P
CaHPO4 <0.3 ppm
Leaching Total P in soil – 50-1500 ppm
Primary Dissolution ~ 100-3000 lb/acre
Minerals

5. Nutrient Movement
• Potassium cycle
Fertilizer
Manure
Adsorption
Exchangeable
K
Desorption Plant uptake
Fixation Solution K
Release
K+
1-10 ppm
Nonexchangeable Fixation
(occluded)
K Total K in soil – 5,000-25,000 ppm
~ 10,000-50,000 lb/acre
Leaching
Feldspars
Micas
Weathering

6. Nutrient Movement
• Soil textural influence on K movement
Claasen and
Jungk, SSSAJ, 1982

7. 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.

8. Nutrient Movement and Uptake
• Nutrient mobility and competition
Root
system
sorption
zone
Root
surface
sorption
zone

9. 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!

10. Soil Potassium
• Exchangeable K
– Remember that clays are 3-dimensional structures
11

11. Soil Potassium
• Occluded K is not necessarily unavailable to the plant, nor is supplied
K necessarily plant available, why?
12

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

13. Potassium Deficiency

14. Critical Levels
• Iowa State data – relative yield versus STP

15. Critical Levels
• Iowa State data – relative yield versus STK

16. Critical Levels
• Yield response as a function of soil test - P

17. Critical Levels
• Yield response as a function of soil test - K

18. 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

19. Illinois– P2O5
P2O5 removal P2O5 balance P2O5 fertilizer
60
50
40
Pounds /acre
30
20
10
0
-10
-20
-30
1985 1990 1995 2000 2005 2010
Year
Source: USDA-NASS, AAPFCO, IPNI

20. Illinois – K2O
K2O removal K2O balance K2O fertilizer
70
60
50
40
Pounds /acre
30
20
10
0
-10
-20
-30
1985 1990 1995 2000 2005 2010
Year
Source: USDA-NASS, AAPFCO, IPNI

21. 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

22. Declining Soil Test Levels
Soil test P
Soil test K
Both P and K soil test
levels are trending down.

23. 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

24. Fertilizer Decisions

25. 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?

26. Philosophy of Fertilization
• Let’s revisit this data and put some economics to it

27. Philosophy of Fertilization
• Net return to P application

28. 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

29. 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

30. 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

31. Thank You!
Questions?