Soil Productivity Potential

Soil productivity indexes or soil ratings have become widely used in several Midwest states. Productivity indexes reflect the physical and chemical properties of the soil in terms of soil productivity for commonly grown crops such as corn and soybeans. In Iowa, the productivity index is called Corn Suitability Rating (CSR). The Corn Suitability Ratings reflect the integrated effects of numerous factors that influence the yield potential and use of the soil for row-crop production. Soil properties, topography, and weather conditions are the dominant factors that affect yield potential. Topographic characteristics are major factors that determine the intensity of land use for row-crop production. Slope gradient and length affect erosion rates, water infiltration, and ease and efficiency of machine operation. Other factors that affect the productivity of a tract of land are the size and shape of the individual soil map units and the combination of these units within a given tract. Corn Suitability Ratings are designed for rating the inherent productivity potential of each soil map unit identified in the soil survey. A soil map unit identifies the soil name to include the texture of the surface horizon and the slope and erosion class. An example of a soil map unit is: Tama silty clay loam, 5-9% slopes, moderately eroded.

An index system for rating soil productivity has several advantages over estimated yields because the index system:

A. Accounts for the estimated yield as well as the intensity and frequency that a given soil map unit may be used for row crop production.
B. Provides a system for ranking a soil map unit against other soil map units within the confines of the state for which the index system was designed.
C. Provides for the index of a given soil map unit to be relatively constant over time. New technology may make estimated yields obsolete, but the index value of a given soil map unit and its relationship to other soil map units should remain constant.
D. Provides for a method to evaluate the productivity of a tract of land and compare the tract of land against other tracts of land. The productivity of a tract of land can be calculated by determining the acreage of each soil map unit and multiplying the acreage by the productivity index of the soil map unit. The sum of the products for all soil map units in the tract divided by the total acreage results in an average productivity index for the tract.

Corn Suitability Ratings assume an adequate level of management. The Corn Suitability Rating scale ranges from 100 (the best) to 5 (the poorest). An index of 100 is reserved for soils that:

A. Are located in areas of most favorable weather conditions for Iowa
B. Have high yield potential
C. Can be continuously row cropped without degradation due to erosion
D. Have adequate surface, subsurface drainage, or have drainage installed that allows for timely field operations to occur throughout the growing season

In Iowa the assignment of Corn Suitability Ratings for soil map units specifies the following conditions:

A. Natural weather conditions (not irrigated)
B. Surface or subsurface drainage installed where needed
C. Soils on lower landscape positions not subject to frequent damaging floods
D. No land leveling or terracing

In the soil judging contest, the soil will be rated for its productivity potential in terms of how intensively the land can be row cropped without causing excessive erosion or other soil degradation. Basic physical properties such as slope, texture, distribution of texture in the soil profile, topsoil thickness, soil depth, internal drainage, and parent material will be considered in this classification. The soil will be rated in one of four classes based on its potential for intensive row cropping. Adequate management includes tillage management, crop sequences, and drainage practices if needed for intensive row cropping. These practices include crop residues on the ground surface following planting of the current crops, contouring, strip cropping, sod-based crops used in rotation with corn and soybeans, and surface and/or subsurface drainage that allows for conducting timely field operations.

Terraces and other structural practices on upland soils are considered as part of a high level of management and are not included in consideration of adequate management. Structures to prevent flooding and overflow for soils on floodplains or terraces are considered as part of average management. The four classes indicating the potential of land for the continuous growing of row crops are:

HIGH
Soils can be used for growing continuous row crops such as corn and soybeans. Soil degradation is not expected to occur due to erosion with use of adequate management practices. Adequate surface and/or subsurface drainage has been installed, if needed, to allow for timely field operations to occur throughout the growing season. Most soils with 0-5% slopes may be included in this class except those soils with excessive or very poor internal drainage.

MEDIUM
Soils can be used for growing row crops at least half of the time but not continuously with use of adequate management practices. Small grains or sod-based crops such as alfalfa or other legumes or legumes in combination with grasses are needed to maintain potential soil losses at or below the established levels of tolerance. In a 5-year rotation the growing of corn and soybeans 3 of the 5 years would be acceptable for soils in this class. Most soils with 5-9% slopes may be included in this class and nearly level soils in Land Capability Class IIIw. Included are those soils occurring on 0-5% slope that do not qualify for the High class.

LOW
Soils can be used for growing row crops less than half of the time with use of adequate management practices. Small grains or sod-based crops such as alfalfa or other legumes or legumes in combinations with grasses are needed to maintain potential soil losses at or below the established levels of tolerance. In a five-year rotation the growing of corn and soybeans two of the five years would be acceptable for soils in this class. Soils with 9-18% slopes may be included in this class.

UNSUITED
Soils are not suited for row crops. Soils should be limited to sod-based crops, permanent pasture, or woodland in order to maintain soil losses at or below the established levels of tolerance. Soils in the class have physical properties that prevent the growing of corn or soybeans to be established as a viable crop. Soils on slopes greater than 18% may be included in this class