Are Your SoybeansComing Up Short ofP and K?Robert MullenDirector of AgronomyJuly 25, 2012          PotashCorp.com
Overview• Soil concepts  – Nutrient movement  – Critical levels• Fertilizer decisions  – Philosophy of fertilization  – Ec...
Soil Concepts
Nutrient Movement     • Phosphorus and potassium are typically considered relatively immobile          – Due to their natu...
Nutrient Movement  • Potassium cycle                                Fertilizer                                            ...
Nutrient Movement• Soil textural influence on K movement                                          Claasen and             ...
Nutrient Movement• So, P and K do not move much, so what; how does that influence  things like soil testing?• Before we go...
Nutrient Movement and Uptake• Nutrient mobility and competition  Root  system  sorption  zone                             ...
Nutrient Uptake• Since they are available from a relatively small volume of soil, is there  much competition between plant...
Soil Potassium• Exchangeable K     – Remember that clays are 3-dimensional structures11
Soil Potassium• Occluded K is not necessarily unavailable to the plant, nor is supplied  K necessarily plant available, wh...
Potassium• Adequate potassium nutrition increases water use efficiency and  reduces drought stress• Opening and closing of...
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 responseSTP (pp...
Illinois– P2O5                          P2O5 removal   P2O5 balance         P2O5 fertilizer                60             ...
Illinois – K2O                          K2O removal    K2O balance         K2O fertilizer                 70              ...
Declining Soil Test Levels• “A random survey from 2007 and ’08 found 45% of Illinois fields  checked were below critical p...
Declining Soil Test Levels                               Soil test P                                             Soil test...
Summary of Soil Concepts• P and K are relatively immobile• Soil testing can be used as a management tool• Soil testing is ...
Fertilizer Decisions
Philosophy of Fertilization• What is the goal of fertilization…• To maximize net return on inputs each year?• To assure th...
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 goo...
Fertilizing Rotations• If your starting soil test was near the critical, you will likely come up  short on your P and K fo...
Summary• Soil testing is our best tool, it is not perfect• You (and your clients) need to figure out the approach that bes...
Thank You!Questions?
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Soybean College - Robert Mullen, Ph.D.

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Soybean College - Robert Mullen, Ph.D.

  1. 1. Are Your SoybeansComing Up Short ofP and K?Robert MullenDirector of AgronomyJuly 25, 2012 PotashCorp.com
  2. 2. Overview• Soil concepts – Nutrient movement – Critical levels• Fertilizer decisions – Philosophy of fertilization – Economic considerations
  3. 3. Soil Concepts
  4. 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 ImmobilizationNonlabile P CaHPO4 <0.3 ppm Leaching Total P in soil – 50-1500 ppm Primary Dissolution ~ 100-3000 lb/acre Minerals
  5. 5. Nutrient Movement • Potassium cycle Fertilizer Manure Adsorption Exchangeable K Desorption Plant uptakeFixation 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. 6. Nutrient Movement• Soil textural influence on K movement Claasen and Jungk, SSSAJ, 1982
  7. 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. 8. Nutrient Movement and Uptake• Nutrient mobility and competition Root system sorption zone Root surface sorption zone
  9. 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. 10. Soil Potassium• Exchangeable K – Remember that clays are 3-dimensional structures11
  11. 11. Soil Potassium• Occluded K is not necessarily unavailable to the plant, nor is supplied K necessarily plant available, why?12
  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. 13. Potassium Deficiency
  14. 14. Critical Levels• Iowa State data – relative yield versus STP
  15. 15. Critical Levels• Iowa State data – relative yield versus STK
  16. 16. Critical Levels• Yield response as a function of soil test - P
  17. 17. Critical Levels• Yield response as a function of soil test - K
  18. 18. Critical Levels• It is not just the relative magnitude of the yield response, but also the probability of responseSTP (ppm) Probability of response (%) STK (ppm) Probability of response (%)<9 80 < 90 809-15 65 90-130 6515-20 25 130-170 2520-30 5 170-200 530 + <1 200 + <1
  19. 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. 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. 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. 22. Declining Soil Test Levels Soil test P Soil test KBoth P and K soil testlevels are trending down.
  23. 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. 24. Fertilizer Decisions
  25. 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. 26. Philosophy of Fertilization• Let’s revisit this data and put some economics to it
  27. 27. Philosophy of Fertilization• Net return to P application
  28. 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. 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. 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. 31. Thank You!Questions?

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