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
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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