1. Corn Suitability Ratings
An Index to Soil Productivity
76th Annual Soil Management and Land Valuation Conference
Gerald “Jerry” Miller
Iowa State University
COLLEGE OF AGRICULTURE

2. Outline Background and History The algorithm Index versus yields
Correction factors
Uses
Questions

3. “The aim of the appraiser, as he sets out to make a physical examination of a farm, should be to DETERMINE VARIATIONS IN THE SOIL which indicate DIFFERENCES in crop-producing ability.”
W.G. Murray, 1969, p. 291

4. The task of a soil scientist is to identify soil variability across the landscape and record these differences to produce a soil map.

5. North Central Iowa Landscape

6. N
Story Co NE ¼ Sec 14
T83N, R21W
5/21/76

7. Unit of Soil Interpretation A Soil Map Unit
Symbol on map
Soil Series Clarion
Soil Type Clarion Loam
Slope Class % slope gradient C
Erosion Class Moderately eroded

10. Productive Capacity of Land
Soil differences
Weather differences
Response to technology
Soil differences-the kind and distribution of the soils on the landscape I.e. slope, erosion class, size and shape of the soil mapping unit. Soil homogeneity vs. soil heterogeneity. Soil differences address the frequency and intensity of crop production.
Technology-
1 fertilizer, pesticides, tillage, irrigation, drainage, timeliness.
2 field design and shape as related to machining site and efficiency
3 these parameters:
Response to technology—inputs such as commercial fertilizers, pesticides, large machinery, irrigation, and other management tools. These parameters may have as great or greater influences on the crop production potential of a soil as do soil qualities such as natural fertility.

11. Corn Suitability Rating (CSR)
History
Late 1940’s – Taylor county soil survey
Late 1960’s – refinement and CSR list developed for each completed county soil survey
1971 – special report 66 “Productivity Levels of Some Iowa Soils” defines the algorithm
1990’s – website

12. Corn Suitability Rating
Figure between 5 and 100.
A figure of relative productivity value of corn for different soils-taking into account other chemical and physical properties, degree of erosion, location on the landscape, and location within the state.
CSR’s can be used to rate one soil against another. For one soils potential yield production.
Assumes:
1 Adequate management
2 Natural weather (no irrigation)
3 Artificial drainage where required
4 No land leveling
5 Protection from flooding

13. Factors Considered in CSR System
Benchmark soil
Slope class
Erosion class
Drainage class
Depth phases
Native vegetation
Parent material
Precipitation
Temperature
Special soil conditions

14. Corn Suitability Ratings Assumptions
Adequate management
Natural weather conditions (no irrigation)
Artificial drainage where required
Bottomland soils protected from frequently flooding
No land leveling or terracing

15. Productivity Index Index Versus Predicted Yield
Accounts for predicted yields as well as frequency and intensity
Allows for ranking one soil map unit against other soil map units

16. Corn Suitability Ratings and Estimated Yields for Major
Soils in Eastern Iowa
Soil type
Soil map unit symbol
CSR
Est. corn yield
Est. soybean
yield
Muscatine sicl
119
100
170 57
Tama sicl
120
120B 95
167 56
120C 80
162 54
120C2 78
158 53
120D2 68
149 50

17. Productivity Index Index Versus Predicted Yield
Should be fairly constant over time while technology may cause changes in predicted yields

18. Corn Suitability Ratings
Iowa system: for use with modern county soil survey maps
Supplements listing CSR’s available in most counties with modern soil survey reports
CSR’s included on crop yield table in published soil survey reports starting in 1995

19. Corn Suitability Ratings Adjustment Factors
Drainage (tile and surface)
Flooding (bottomland soils)
Contrasting soil inclusions within a map unit
Rainfall and temperature

21. Clarion-Nicollet-Webster Soil Association Area75
0.862 79
0.908 74
0.850 81
0.931 85
0.977 82
0.943 83
0.954 87
1.000 89
1.023 86
0.989 To 88
1.011 91
1.046
Reference: Clarion loam, A slope, CSR 87
Average annual precipitation, 30 inches
Average annual temperature, 48oF

22. Galva-Primghar-Sac Soil Association Area78
1.040 74
0.987 75
1.000 71
0.947 72
0.960 76
1.013 73
0.973 80
1.067 79
1.053
Reference: Galva silty clay loam, A slope, CSR 75
Average annual precipitation, 27 inches
Average annual temperature, 47.5oF

23. Marshall Soil Association Area82
0.911 86
0.955 84
0.933 90
1.000 87
0.978 88
Reference: Marshall silty clay loam, A slope, CSR 90
Average annual precipitation, 30 inches
Average annual temperature, 49.5oF

24. Ida-Monona Soil Association Area Weather Adjustment Factor by County
0.953
1.000
0.976
0.918
0.882
0.929

25. Calculation of weighted average CSR from the 80-acre field shown.
Soil map number
Soil type name
Acreage
CSR X Acreage CSR 55
Nicollet loam
5.9 90
531.0
62D
Storden loam
6.4 45
288.0
62E
1.9 35
66.5 95
Harps clay loam
8.0 63
504.0
107
Webster clay loam
13.8 85
1173.0
135
Coland clay loam
2.0 80
160.0
138B
Clarion loam
36.1 82
2960.2
138C2
3.9 65
253.5
201B
Coland-Terrill complex 40
80.0
6016.2
6016.2
80.0
= 75.2 =
weighted average CSR for field

26. Corn Suitability Ratings
Provides for calculation of a quantitative value of crop productivity for a tract of land
Independent of size of tract

27. County Weighted Average Corn Suitability Rating
Weighted Average CSR
59.5
65.5
68.0
66.9
64.8
69.2
69.6
71.8
55.8
72.4
74.0
62.7
49.4
70.9
77.4
60.1
49.9
76.4
54.4
60.5
55.7
58.2
60.8
56.9
53.6
61.8
64.2
56.8
55.2
63.5
65.1
52.7
45.7
63.2
70.4
71.4
75.5
69.4
77.7
74.8
76.8
72.5
73.2
75.9
84.7
72.3
70.7
77.6
72.8
61.2
47.3
65.8
73.6
64.1
65.0
74.2
54.7
68.2
58.8
56.1
40.1
40.6
46.8
39.7
35.1
41.3
39.9
43.1
71.6
63.0
51.3
71.1
67.5
75.2
59.6
62.6
60.4
66.6
47.8
56.6
40.9
62.4
48.1
64.3
41.5
73.4
Under 60
60-70
70-75
Over 75
Calculated from acreages and CSR’s contained in ISPAID (Iowa Soil Properties and Interpretations Database)
As of August 15, 2001
Prepared by Gerald A. Miller and Thomas E. Fenton, Professors of Agronomy, and Brian Tiffany, System Support Specialist
Department of Agronomy, Iowa State University, Ames, Iowa 50011

28. Use of Corn Suitability Ratings
“No”
Are not equal to yields
Weighted averages not comparable among major soil areas

29. Use of Corn Suitability Ratings
“Yes”
Determining productivity potentials
Tax assessment of agricultural lands
Land values
Comparable land for estate taxes
“Prime Agricultural Land”

30. Questions
Thank you!
COLLEGE OF AGRICULTURE