Soil Test Correlation and Calibration for Recommendations
C.C. Mitchell, Rao Mylavarapu, and Nathan Slaton
Three terms often used in soil testing circles are “correlation”, “calibration”, and
“interpretation”. While these have been used and defined over the years (Brown, 1987), they
continue to form the basis of our soil testing programs. The Glossary of Soil Science Terms
(1996), defines these terms:
Soil test correlation. The process of determining the relationship between plant nutrient uptake
or yield and the amount of nutrient extracted by a particular soil test method.
Soil test calibration. The process of determining the crop nutrient requirement at different soil
test values.
Soil test interpretation. The process of developing nutrient application recommendations from
soil test concentrations, and other soil, crop, economic, environmental and climatic information.
Without some correlation and calibration, interpretation would be impossible. Therefore,
putting soil test values to practical use by growers involve all three processes. These processes
have been described in detail in previous publications (Corey, 1987; Cope and Rouse, 1973;
Eckert, 1987; Evans, 1987; Olson et al, 1987).
Most soil test correlation and calibration data in the southern U.S. were collected in the
1950s, 60s, and 70s by researchers working at state agricultural experiment stations. Some states
are able to continually update and re-visit their calibration through research verification trials,
new soil fertility research, or long-term research. However, public support for this type of
research is not readily available. Results from earlier research were often published as state
experiment station or extension circulars and may be difficult to access outside of the state where
it was published. Also, data were often extracted from other soil fertility research projects and
may not appear simply as “soil test calibration” research. An example is J.T. Cope’s Alabama
Agricultural Experiment Station Bulletin 561 (1984) which summarized several long-term soil
fertility experiments. These experiments also provided important data for soil test calibration in
Alabama but soil testing is not mentioned in the title. Also, most state laboratories have
summarized their correlation and calibration research and included relevant information from
adjacent states into publications or web sites summarizing their soil test calibration and
interpretation. Today, most of these are on-line documentations or are part of a laboratory’s web
site, e.g. Oklahoma’s web site (http://www.soiltesting.okstate.edu/). This information is
accessible through the SERA-6 website, http://www.clemson.edu/sera6/state_labs.htm.
Two southern cooperative series bulletins initiated by SERA-6 attempted to compile
correlation, calibration and interpretation for peanuts and cotton on Coastal Plain soils (Mitchell,
1994; Mitchell, 2010). However, because of the diversity of soils, crops, and climates in the
southern region and because of the autonomy of state agricultural experiment stations and state
Land Grant Universities, soil test interpretations remain largely a state-by-state function. Table 1
1
(Savoy, 2009) documents soil test calibration used by public soil testing laboratories in the
Southern Region.
References
Adams, J.F., C.C. Mitchell, and H.H. Bryant. 1994. Soil test fertilizer recommendations for
Alabama crops. Agron. & Soils Dep. Ser. No. 178. Ala. Agric. Exp. Stn. Auburn University, AL.
Brown, J.R. 1987. Soil testing: sampling, correlation, calibration, and interpretation. SSSA
Spec. pub. No. 21. Soil Science Soc. Amer. Madison, WI.
Cope, J.T., Jr. and R.D. Rouse. 1973. Interpretation of soil test results. In L.M. Walsh and J.D.
Beaton (editors) Soil testing and plant analysis. Revised edition. Soil Sci. Soc. America,
Madison, WI.
Cope, J.T. 1984. Long-term fertility experiments on cotton, corn, soybeans, sorghum, and
peanuts, 1929-1982. Ala. Agric. Exp. S WI.tn. Bul. 561. Auburn University, AL
Corey, R.B. 1987. Soil test procedures: correlation. In J.R. Brown (editor) Soil testing:
sampling, correlation, calibration, and interpretation. SSSA Spec. Pub. No 21. Soil Sci. Soc. of
America. Madison, WI.
Evans, C.E. 1987. Soil test calibration. In J.R. Brown (editor) Soil testing: sampling, correlation,
calibration, and interpretation. SSSA Spec. Pub. No 21. Soil Sci. Soc. of America. Madison, WI.
Eckert, D.J. 1987. Soil test interpretations: basic cation saturation ratios and sufficiency levels.
In J.R. Brown (editor) Soil testing: sampling, correlation, calibration, and interpretation. SSSA
Spec. Pub. No 21. Soil Sci. Soc. of America. Madison, WI.
Glossary of Soil Science Terms. 1997. Soil Sci. Soc. Amer. Madison, WI.
Mitchell, C.C. 1994. Research-based soil testing interpretation and fertilizer recommendations
for peanuts on Coastal Plain soils. Sou. Coop. Ser. Bul. No. 380. Ala. Agric. Exp. Stn. Auburn
University, AL
Mitchell, C.C. 2010. Research-based soil testing interpretation and fertilizer recommendations
for cotton on Coastal Plain soils. Sou. Coop. Ser. Bul. No. 410. Ala. Agric. Exp. Stn. Auburn
University, AL. http://www.clemson.edu/sera6/sera6-cotton%20pub.pdf
Olson, R.A., F.N. Anderson, K.D. Frank, P.H. Grabouski, G.W. Rehm, and C.A. Shapiro. 1987.
Soil test interpretaions: sufficiency vs. build-up and maintenance. In J.R. Brown (editor) Soil
testing: sampling, correlation, calibration, and interpretation. SSSA Spec. Pub. No 21. Soil Sci.
Soc. of America. Madison, WI.
Savoy, H.J. 2009. Procedures used by state soil testing laboratories in the southern region of the
United States. Sou. Coop. Ser. Bul. 409.
http://www.clemson.edu/agsrvlb/sera6/srbull409aug09versionupdatedISBNbull.pdf
2
Table 1. Soil test calibrations by extractant for phosphorus and potassium by the 13 Southern States
and PR (from Savoy, 2009)..
State
AL
Method a
Mehlich 1
Soil
Crop
All
Peanuts
All
Peanuts
Phosphorus, lb./acre
VL
L
M
H
0-4
5-10
11-19
20-50
Soil
Text.
VH
51200d
CEC
meq/
100g
0-9
0-4.5
4.69.0
>9.0
Lancaster
All CEC<
9
All except
peanuts
O-12
13-25
26-50
51100
101200
All CEC
9+
All except
peanuts
0-7
8-15
16-30
31-60
61-120
>9.0
All
All
0-18
19-36
37-72
73144
145
>9.0
Black
Belt
Potassium, lb./acre b
VL
L
M
H
VH
0-20
21-28
29-40
41-100
101+
21-30
31-42
43-60
61-150
151+
31-40
41-57
58-80
81-200
201+
61-120
121-240
241+
0-9.0
288
Clays
Mehlich 1
Cotton,
0-4.5
0-30
0-4.5
0-20
21-40
41-80
81-160
161+
Cotton,
4.6
0-45
46-90
91-180
181-360
361+
legumes,
9.0
0-30
31-60
61-120
121-240
241+
>9.0
0-60
161120
121-240
241-280
481+
>9.0
0-40
41-80
81-160
161-320
321+
>9.0
0.80
81160
161-240
241-480
481+
>9.0
0.50
51120
121-190
191-320
321+
181-260
261-350
> 350
legumes,
and
vegetables
Grasses,
corn,
peanut,
and soybean
and
vegetables
Grasses,
4.6
corn,
peanut,
9.0
and soybean
Cotton,
legumes,
and
vegetables
Grasses,
corn,
peanut,
and soybean
Lan-
Cotton,
caster
legumes,
and
vegetables
Grasses,
corn,
peanut,
and soybean
AR
Mehlich 3
All
Cotton
< 32
32-50
51-70
71100
> 101
< 120
121180
3
State
Method a
Soil
AR
Corn
< 32
32-50
51-70
71100
> 101
< 120
121180
181-260
261-350
> 350
Soybean
< 32
32-50
51-70
71100
> 101
< 120
121180
181-260
261-350
> 350
Crop
Phosphorus, lb./acre
VL
L
M
Wheat
< 32
32-50
51-70
Rice
< 32
32-50
51-70
Grain sorghum
Forages for
< 32
< 32
32-50
32-50
Soil
Text.
H
51-70
51-70
pasture
(including
legumes)
VH
71100
71100
71100
71
CEC
Potassium, lb./acre b
VL
L
M
H
VH
> 101
< 120
121180
181-260
261-350
>
350
> 101
< 120
121180
181-260
261-350
>
350
121180
181-260
261-350
>
350
meq/
100g
> 101
< 120
> 101
100
Forages for
pasture
< 32
Turf grasses
< 32
Commercial
< 40
vegetables
32-50
32-50
51-70
51-70
>
350
< 120
121180
71100
71100
> 101
< 120
> 101
<42
181-260
261-350
121180
181-260
261-350
>
350
42-80
81-120
121-200
>200
3516000
>151
40-60
61-80
81
<122
122180
181-260
261-350
0-40
41-70
71-120
121-250
251+
150
FL
GA
Mehlich 1
All
All
Mehlich 1
Coastal
Plain
All except
peanut,
legumes, cotton,
stone fruits, nuts,
0-20
21-30
31-60
0-30
31-60
61100
101+
0-60
61-150
151-250
251+
0-30
31-60
61
101+
0-70
71-170
171-275
276+
61+
0-30
31-75
76-125
125+
61119
120+
lawns,
ornamentals,
and
vegetables
Coastal
Cotton, legumes,
Plain
stone fruits, nuts,
100
and vegetables
Coastal
Plain
Peanut
0-15
16-30
Piedmont
All except
peanut,
legumes, cotton,
stone fruits, nuts,
0-20
21-40
4175
76+
0-100
101-200
201-350
351+
0-20
21-40
41
76+
0-120
121-250
251-400
401+
35+
0-50
51-100
101-175
175+
201+
0-150
151-250
251-450
451+
3160
lawns,
ornamentals,
vegetables
Piedmont
Cotton, legumes,
stone fruits,nuts,
75
vegetables
Piedmont
Peanut
0-10
11-20
0-50
51100
GA
Mehlich 1
All
Golf greens and
tees,
ornamentals,
and
flowers
State
Method a
Soil
Crop
KY
Mehlich 3
All
Corn, soybean
2135
101200
Phosphorus, lb./acre
VL
L
M
H
0-5
6-27
28-60
61+
Soil
Text.
VH
CEC
meq/
100g
Potassium, lb./acre
b
VL
L
M
H
0-99
100-190
191-300
301+
VH
4
LA
Mehlich 3
All
Burley tobacco
0-6
7-28
29-57
58-79
0-96
96-205
206-303
304450
451+
Mehlich 3
All
Alfalfa
0-8
9-27
28-60
61+
0-97
97-203
204-296
297447
448+
Mehlich 3
Coastal
Plain
All
0-10
11-40
41-80
81+
Sandy
loams
4
0-90
91-136
137-227
228+
Flatwoods
All
0-10
11-35
36-70
71+
VFsan
6
0-113
114-182
183-273
274+
Silt
loams
8
0-136
137-227
228-318
319+
Silt
loams
10
0-182
183-273
274-364
365+
Very
8
0-136
137-227
228-318
319+
Silt
loams
10
0-182
183-273
274-364
365+
Clay
loams
15
0-227
228-364
365-455
456+
Loamy
sands
4
0-90
91-136
137-227
228+
Very
8
0-136
137-227
228-318
319+
Silt
loams
10
0-182
183-273
274-364
365+
Silt
loams
15
0-227
228-364
365-500
501+
Clays
20
0-318
319-454
455-682
683+
81
+
dy
loams
Miss.
Terraces
Coastal
All
All
0-10
0-10
11-35
11-30
36-70
31-70
71+
71+
prairies
fine
sandy
loams
Alluvial
All
0-40
41-60
61120
121+
fine
sandy
loams
Table 7….
Continued
State
MS
Method a
Lancaster
Soil
All
Group 1 c
Crop
Phosphorus, lb./acre
VL
L
M
All except rice
0-18
19-36
37-72
Rice
0-9
10-18
19-36
H
73144
3745
Soil
Text.
VH
CEC
meq/
100g
VL
L
<7
0-40
41-80
7-14
0-50
51-110
0-60
61-130
25+
0-70
71-150
<8
0-50
51-110
8-14
0-60
61-140
0-70
71-160
0-80
81-180
M
H
VH
145+
46+
1525
Group 2
Potassium, lb./acre b
1525
25+
81120
121210
211+
111160
161280
281+
131180
181315
316+
151200
201350
351+
111160
161280
281+
141190
191335
336+
161210
211370
371+
181240
241420
421+
5
Group 3
<8
0-70
71-150
8-14
0-90
91-190
15
0-120
25
25+
NCa
Mehlich 3
All
All
0-21
22-54
55107
108
215+
0-150
0-34
151200
201350
351+
191240
241420
421+
121
241
291510
240
290
151260
35-87
-
261320
321560
88
175348
511+
561+
349+
174
214
OK
Mehlich 3
PR
Bray P1
State
21-40
41-65
65+
0-10
11-20
21+
<156
Bray P2
0-10
21-40
41+
<156
Olsen
0-12
13-35
36+
<156
Method a
All
All
0-20.
All
Soil
Crop
Phosphorus, lb./acre
VL
SC
TN
Mehlich 1
Mehlich 1
0-50
L
M
H
Soil
Text.
VH
CEC
meq/
100g
VA
a
b
d
156312
156312
156312
251350
351+
313+
313+
313+
VL
L
M
H
VH
0-70
71-156
157-235
236+
0-70
71-156
157-235
236+
Coastal
Plain
All except peanut
0-30
31-60
61-120
Piedmont
All except peanut
0-20
21-40
41-80
All
Peanut
0-10
11-19
20-50
50+
0-28
29-40
41-100
100+
All
All except cotton
0-18
19-30
31-120
121+
0-90
91-160
161-320
321+
0-140
141180
181-319
320+
16-75
76-175
176-310
311+
Mehlich 3
Mehlich 1
151250
Potassium, lb./acre b
121240
81240
cotton
TX
51-150
All
All
All
0-3
4-11
12-35
36-110
111+
0-15
Extractants listed for phosphorus.
Extractants listed for potassium. Groups are combinations
of soil types and crops.
In Alabama only this is termed Extremely High (EH) instead of Very High (VH) (for
phosphorus only) to indicate that the supply of phosphorus is more than five times the critical
value.
6