Soil Productivity Potential

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.
20
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