Corn Suitability Ratings An Index to Soil Productivity

1. Corn Suitability Ratings An Index to Soil Productivity 76th Annual Soil Management and Land Valuation Conference Gerald “Jerry” Miller Iowa State University soil@iastate.edu 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. Symbol on map Soil Series Clarion --- Soil Type Clarion Loam 138 Slope Class 5-9% slope gradient C Erosion Class Moderately eroded 2 Unit of Soil Interpretation A Soil Map Unit

10. Productive Capacity of Land • Soil differences • Weather differences • Response to technology

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 – websitehttp://extension.agron.iastate.edu/soils

12. Corn Suitability Rating

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 RatingsAssumptions • Adequate management • Natural weather conditions (no irrigation) • Artificial drainage where required • Bottomland soils protected from frequently flooding • No land leveling or terracing

15. Productivity IndexIndex 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

17. Productivity IndexIndex 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 RatingsAdjustment Factors • Drainage (tile and surface) • Flooding (bottomland soils) • Contrasting soil inclusions within a map unit • Rainfall and temperature

21. 74 0.850 75 0.862 79 0.908 85 0.977 85 0.977 81 0.931 To 83 0.954 79 0.908 81 0.931 85 0.977 85 0.977 85 0.977 81 0.931 83 0.954 85 0.977 86 0.989 82 0.943 85 0.977 87 1.000 87 1.000 89 1.023 85 0.977 88 1.011 89 1.023 91 1.046 91 1.046 88 1.011 89 1.023 91 1.046 91 1.046 Clarion-Nicollet-Webster Soil Association Area Reference: Clarion loam, A slope, CSR 87 Average annual precipitation, 30 inches Average annual temperature, 48oF

22. 71 0.947 74 0.987 75 1.000 72 0.960 75 1.000 78 1.040 73 0.973 76 1.013 79 1.053 76 1.013 78 1.040 80 1.067 Galva-Primghar-Sac Soil Association Area Reference: Galva silty clay loam, A slope, CSR 75 Average annual precipitation, 27 inches Average annual temperature, 47.5oF

23. 82 0.911 84 0.933 86 0.955 88 0.978 88 0.978 88 0.978 90 1.000 88 0.978 90 1.000 88 0.978 90 1.000 87 0.978 90 1.000 Marshall Soil Association Area Reference: Marshall silty clay loam, A slope, CSR 90 Average annual precipitation, 30 inches Average annual temperature, 49.5oF

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

25. weighted average CSR for field 6016.2 80.0 = 75.2 = Calculation of weighted average CSR from the 80-acre field shown.

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. 59.5 68.0 65.5 66.9 68.0 74.0 77.7 69.4 56.9 41.5 64.8 71.8 69.2 69.6 70.4 71.4 71.4 75.5 69.4 64.3 48.1 73.4 74.8 73.2 76.8 72.5 55.8 62.7 72.4 74.0 71.6 71.1 63.0 51.3 84.7 49.4 60.1 70.9 77.4 74.0 75.5 75.9 47.3 61.2 49.9 59.5 68.0 76.4 72.3 77.6 75.9 70.7 72.8 65.8 62.4 75.2 54.4 58.2 60.5 55.7 73.6 74.2 64.1 65.0 59.6 67.5 74.2 65.0 60.8 61.8 56.9 53.6 54.7 56.1 68.2 58.8 66.6 62.6 64.2 63.5 56.8 55.2 40.1 39.7 40.6 46.8 56.6 60.4 63.0 40.9 65.1 63.2 52.7 45.7 35.1 43.1 41.3 39.9 47.8 County Weighted Average Corn Suitability Rating Weighted Average CSR 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