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Are Your Soybeans Coming Up Short of P and K?

Robert MullenDirector of AgronomyJuly 25, 2012

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

Solution P

H2PO4-

HPO42-

<0.3 ppm

Solution P

H2PO4-

HPO42-

<0.3 ppm

AdsorbedP

Secondary MineralsFe/AlPO4

CaHPO4

Primary Minerals

OrganicMatterBound

P

Fertilizer

Manure

Adsorption

Desorption

Dissolution

Precipitation

Dissolution

Mineralization

Immobilization

Plant uptake

Leaching

Labile

Nonlabile

Total P in soil – 50-1500 ppm ~ 100-3000 lb/acre

Nutrient Movement

• Potassium cycle

Solution K

K+

1-10 ppm

ExchangeableK

Nonexchangeable(occluded)

K

FeldsparsMicas

Fertilizer

Manure

Adsorption

Desorption Plant uptake

FixationRelease

Fixation

Weathering

Leaching

Total K in soil – 5,000-25,000 ppm ~ 10,000-50,000 lb/acre

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

• 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 (%)

< 9 80

9-15 65

15-20 25

20-30 5

30 + <1

STK (ppm) Probability of response (%)

< 90 80

90-130 65

130-170 25

170-200 5

200 + <1

Illinois– P2O5

1985 1990 1995 2000 2005 2010-30

-20

-10

0

10

20

30

40

50

60

P2O5 removal P2O5 balance P2O5 fertilizer

Year

Po

un

ds

/acr

e

Source: USDA-NASS, AAPFCO, IPNI

Illinois – K2O

1985 1990 1995 2000 2005 2010-30

-20

-10

0

10

20

30

40

50

60

70

K2O removal K2O balance K2O fertilizer

Year

Po

un

ds

/acr

e

Source: USDA-NASS, AAPFCO, IPNI

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

Both P and K soil test levels are trending down.

Soil test K

Soil test P

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?