Potassium for Corn:Soil testing and its

relationship to yield

John S. Breker, M.S. student

Dave W. Franzen, Extension Soils Specialist

NDSU Dept. of Soil Science

Cayuga, ND 6/27/2015Normal plants: 2.2% KDeficient plants: 1.0% K

Milnor, ND 8/10/2015

Assessing plant-available K

• NCREA-13 standard method for North Central region

• 20 mL 1.0 M NH4OAc at pH 7

• 2 g air-dry soil

• Shaken (200 cycles per min) for 5 minutes and filtered

• Atomic absorption spectroscopy for K determination

• Provides estimate of solution and exchangeable K

• Assumptions for interpretation:

• Air-drying does not change field conditions

• Solution and exchangeable K are the available pools

• Snapshot at sampling time indicative for growing season

Soil potassium surprises, 2015

13 sites in southeastern ND

• Cass, Barnes, Richland, and Sargent Co.

Seven sites with spring K levels below 150 ppm critical level

• Only 3 of 7 sites had yield increase to K fertilizer

• One site had yield increase above critical level

Site Initial K

ppm

Yield increase

bu/ac

Significance

%

Yield max.

bu/ac

Absaraka 113 ---- NS 165

Arthur 125 ---- NS 164

Barney 170 ---- NS 219

Casino 120 ---- NS 226

Dwight 110 ---- NS 212

Fairmount 1 190 19 5 210

Fairmount 2 119 28 10 209

Leonard N 380 ---- NS 174

Leonard S 191 ---- NS 169

Milnor 118 31 5 194

Prosper 205 ---- NS 193

Valley City 202 ---- NS 120

Walcott 107 14 5 197

Relative yield of check compared to maximum yield with dry K test, late May

y = -3E-06x2 + 0.0015x + 0.725

R² = 0.0705

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

0 20 40 60 80 100 120 140 160 180

Rel

ativ

e g

rain

yie

ld

Dry test K (ppm)

p>F 0.259, no significant relationship

Relative yield of check compared to maximum yield with dry K test, early September

y = -2E-06x2 + 0.0013x + 0.7192

R² = 0.0686

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

0 50 100 150 200 250

Rel

ativ

e g

rain

yie

ld

Dry test K (ppm)

p>F 0.269, no significant relationship

Relative yield of check compared to maximum yield with %K of CEC

p>F = 0.4329, no significant relationship

y = -205.66x2 + 11.704x + 0.7756

R² = 0.1979

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

0.0% 1.0% 2.0% 3.0% 4.0%

Rel

ativ

e g

rain

yie

ld

% K (of CEC)

Revisiting soil potassium(if ever we visited it before…)

From http://extraterrestrials.wikia.com/wiki/File:Alien_Crash_Landing.jpg

Sample drying effect and K release

Sample drying changes the clay layer arrangement• Exposes new regions of clay mineral for release of K ions

• Drying redistributes interlayer cations, allowing Ca ion competition for wedge sites, releasing K

Drying can oxidize Fe2+ to Fe3+ within clay lattice, decreasing clay layer charge

• Weakening bonds to interlayer K ions, resulting in more K release

• Wet soil conditions can cause Fe reduction and clay layer collapse, trapping K

Some soils contain hydroxy-Al interlayers, keeping clay layers propped open

Soil test comparison:Air-dry soil (DK) vs. field-moist soil (MK)

y = 0.9322x - 12.716

R² = 0.896

0

100

200

300

400

500

600

700

0 100 200 300 400 500 600 700

MK

(p

pm

)

DK (ppm)

DK/MK ratio (all sites)

y = 3.9095x-0.245

R² = 0.3461

0.0

0.5

1.0

1.5

2.0

2.5

0 100 200 300 400 500 600 700

DK

/MK

MK (ppm)

MK vs. DK/MK

Average DK/MK = 1.26

DK/MK ratio

y = 6.1422x-0.355

R² = 0.562

y = 17.509x-0.523

R² = 0.5828

y = 20.013x-0.487

R² = 0.5102

0.00

0.50

1.00

1.50

2.00

2.50

0 100 200 300 400 500 600 700

DK

/MK

MK

Norm F1,P LN

Norm: Sandy to coarse-loamy to fine-loamy soil texture families

F1, P, & LN: Fine to fine-silty soil texture families

STK correlations, n=868

H2O K-feldspar Muscovite Smectite

(clay fraction)

Illite

(clay fraction)

Smec-to-Illite

ratio

DK 0.24 -0.22 0.78 -0.17 0.16 -0.29

MK 0.11 -0.30 0.70 -0.30 0.29 -0.42

DK/MK 0.33 0.37 0.00 0.50 -0.50 0.53

(DK-MK) 0.38 0.24 0.28 0.39 -0.40 0.39

Drying effect greater in smectite-

rich soils.

Drying effect greater at higher

soil water contents.

Relative yield of check compared to maximum yield with soil K test, late May

y = -3E-06x2 + 0.0015x + 0.725

R² = 0.0705

y = -7E-06x2 + 0.0026x + 0.6505

R² = 0.1866

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

0 50 100 150 200 250

Rel

ativ

e g

rain

yie

ld

STK (ppm)

DK MK Poly. (DK) Poly. (MK)

DK: p>F 0.259, no significant relationship

MK: p>F 0.022, significant relationship

y = -2E-06x2 + 0.0013x + 0.7192

R² = 0.0686

y = -6E-06x2 + 0.0026x + 0.638

R² = 0.2163

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

0 50 100 150 200 250 300

Rel

ativ

e g

rain

yie

ld

STK (ppm)

DK MK Poly. (DK) Poly. (MK)

Relative yield of check compared to maximum yield with soil K test, early September

DK: p>F 0.269, no significant relationship

MK: p>F 0.011, significant relationship

Drying effect significantly increases NH4OAc-extractable K

• No simple relationship between DK and MK, occasional decrease

• Dependent on soil K level, water content, soil texture, and clay type

Field-moist K test better predictor of relative yield than the current air-dry K test

• Both field-moist and air-dry K tests are still poor predictors for K fertilization

From Sparks and Huang, 1985

From McLean and Watson, 1985

What about mineral K?

Dr. Donald Sparks, Univ. of Delaware

“Historical perspective on the chemistry and mineralogy of soil potassium” at 2014 ASA meetings

Worked in coastal plain soils of Virginia and Delaware, sandy with low organic matter and CEC

• Low exchangeable bases, incl. K

• K applications failed increase corn yields

From Sparks et al., Agron. J. 1980

From Sparks et al., Agron. J. 1980

From Parker et.al., SSSAJ 1989b

From Parker et.al., SSSAJ 1989a

From Sparks and Huang, 1985

From Sparks et al., SSSAJ 1980b

From Sparks and Huang, 1985

From Martin and Sparks, SSSAJ 1983

Integrating mineralogy

Table X. Soil mineral composition for K-bearing and other relevant minerals.

Whole soil < 2μm-fraction

Site Quartz Plagioclase K-feldspar Muscovite/Illite Smectite Illite Kaolinite

-------------------------------------------- % ------------------------------------------------------

AB 42.0 28.4 9.9 1.8 84 14 2

AR 37.9 27.5 9.5 3.0 85 12 3

B 36.5 18.3 6.3 3.8 79 16 5

C 41.6 22.7 6.4 2.6 85 12 3

D 45.8 21.9 6.0 2.3 82 15 3

F1 38.5 15.4 5.6 3.0 87 10 3

F2 38.2 18.3 7.4 1.9 79 14 7

LN 33.4 21.6 6.9 6.6 70 25 5

LS 52.4 17.8 5.5 <IDL† 52 41 7

M 39.7 17.6 8.6 3.4 74 20 6

P 34.0 17.4 9.2 3.6 83 14 3

V 36.5 18.2 5.6 1.7 65 30 5

W 39.4 20.7 6.2 1.8 47 48 5

† <IDL, below instrument detection limit

Response surface of relative yield to field-moist K and K-feldspar content

5L

Total model: p>F 0.0054, r2 = 0.37

MK: p>F 0.0614

K-spar: p>F 0.0283

Total model: p>F 0.0054, r2 = 0.37

MK: p>F 0.0604

K-spar: p>F 0.0210

7L

Indicates some maintenance of yield at low MK levels

at both low and high K-feldspar levels.

7L

Total model: p>F 0.0058, r2 = 0.37

MK: p>F 0.0023

Muscovite: p>F 0.0130

Total model: p>F 0.0015, r2 = 0.42

MK: p>F 0.0005

Muscovite: p>F 0.0171

9E

Indicates MK and muscovite related, does not

describe yield maintenance at low MK levels.

Response surface of relative yield to field-moist K and muscovite content

Plant-available K is a combination of solution and readily exchangeable K plus contributions from K-feldspars and “nonexchangable” K that cannot be assessed with a rapid soil test.

We do know that the current test methodology on air-dry soil samples is not satisfactory (correct about 50% of the time).

Seasonal STK variation

• Highest in spring, decreases throughout season

• STK increases after leaf senescence, K leach from tissue

• Freeze/thaw cycles over winter

• 11 of 13 sites follow sinusoidal pattern

• Relationship with MK better than DK (drying effect adds more variability)

• Intra-seasonal range: 44-86 ppm MK (one to two interpretation classes)

• Specific pattern is site dependent and difficult to predict

Sinusoidal STK sites (11 of 13)

MK = 113.7+30.1*sin(0.628*t+1.342)

p>F <.0001 r2=0.18

Conclusions from 2015

• Sample drying effect• Significant differences from field-moist K

• Field-moist K test better predictor of yield than standard air-dry K test

• Yield responses to fertilizer inconsistent• STK alone, not a great predictor

• Some low K sites fail to respond to K application

• Mineralogy plays some role

• Temporal STK variation• Highest in spring, lowest in fall

• Great enough to change interpretation classes

• Field-moist K less variability than air-dry K

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