Agronomy 212 - Grain and Forage Crops

Calculate average weighted corn suitability rating for a field using a soil map

Based on Miller, G.A. 1988. Corn suitability ratings - An index to soil productivity. Iowa State University Extension publication Pm-1168.

Corn Suitability Rating (CSR) is an index procedure developed in Iowa to rate each soil for its potential row-crop productivity. CSRs provide a relative ranking of all soils mapped in the state of Iowa. The CSR is very useful for rating one soil's potential yield production against others over time. While predicted yields are expected to change with time because of updates in crop production technologies. CSRs are expected to remain relatively constant in relation to one another.

The CSR index rates soils on a scale of 0 to 100. A rating of 100 is given to soils than have no physical limitations, occur on minimal slopes, and can be continuously row-cropped. Ratings near 0 have severe limitations for row crops and would best be left in permanent cover, such as grassland or forest.

Corn suitability ratings can be used for many practical purposes, such as quantification of productivity potential, calculation of land values, evaluation of farmland quality, and equalization of tax assessment. They can be used to quantify the productivity potential for individual soils, fields, farms, and larger tracts of land.

Because CSR values vary with soil type, you must be able to calculate a weighted average CSR for a tract of land to get the most benefit from the index. The weighting is required because soil types with greater acreage will have more influence on the productivity of the tract than those with lesser acreage. Determination of the weighted average CSR is demonstrated in the following example problem.

Example problem

You want to calculate the average weighted CSR for the 80-acre field marked in gray on the soil map.

To do this you must first determine the acreage of each soil mapping unit in the 80-acre field. A ruler and the scale on the map or a GIS program can be used to estimate the acreage for each soil type (Note: The acreage of each soil type will be given to you on the Agronomy 212 exam).

The CSR's are weighted for the field by multiplying the acreage of each by its CSR. These acreage x CSR values are added together to get a summed acreage x CSR value for the entire field. Dividing this summed acreage x CSR value by the total acres in the field results in the weighted average CSR for the field. The table below contains the calculation of the weighted average CSR for the 80-acre field.

Table 1. Calculation of the weighted average CSR for an 80-acre field using the soil map above.
Soil map number Soil type name Acreage CSR Acreage x CSR
55 Nicollet loam
5.9
90
531.0
62D Storden loam
6.4
45
288.0
62E Storden loam
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 Clarion loam
3.9
65
253.2
201B Coland-Terrill complex
2.0
40
243.5
         
  Total
80.0
 
6016.2

Weighted average CSR for field = 6016.2/80.0 = 75.2

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Agronomy 212 - Grain and Forage Crops
Authored by Dr. Lance R. Gibson
Copyright 1998-99, 2000-01 Iowa State University. All rights reserved.
Revised: February 11, 2005.