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We at AgriData have received multiple reports of certain soil productivity values in North Dakota dropping substantially with the last couple Soil Data refreshes from the NRCS/USDA. Since AgriData, Inc. does not have any control over why and how the values are updated, we requested an explanation from Wade D. Bott, the North Dakota State soil scientist on the differences. Below is the entirety of his response to the question of why some soils have drastic drops of their Soil Productivity Indexes. 

"In the past, the poorly drained and wetter soils on slopes less than or equal to 9 percent were correlated as drained phases. Meaning that man has artificially lowered the water table, either by using subsurface drain tile or surface ditches, in an effort to produce agricultural commodity crops. This broad correlation decision was made by the correlator at the time with the assumption that many of the interpretations we were providing were agronomic and not possible without drainage. The soil survey is now capable of designing, developing, and delivering soil interpretations for a much broader spectrum of land uses and management practices.

During the soil survey update process, we found that less than 20 percent of the poorly drained and wetter soils on less than or equal to 9 percent slopes were actually drained in major land resource areas (MLRA) west of the Red River Valley; while the Red River Valley is dominantly drained, all polygons of most map units have had drainage manipulation by man. Refer to the map below, MLRA 56 is the Red River Valley. The areas where less than 20 percent of the poorly drained and wetter soils on less than or equal to 9 percent slopes are actually drained are in MLRAs 53A, 53B, 55A, 55B, and 102A east of the Mississippi River.
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The productivity index for a drained phase of a poorly drained soil is typically in the 80s, and for an undrained phase of the same soil the productivity index is much lower, in the 40s. What this means is that our initial correlation decision erroneously set the productivity index for poorly drained and wetter phases on less than or equal to 9 percent slopes, west of MLRA 56 about 40 percent too high for soils that had actually NEVER been drained. This is why the values were adjusted and there is such a significant change in the productivity index. These areas do not produce any more or less than they ever have in the past. Our data, however, now represents the landscape much more accurately as undrained phases in MLRAs west of MLRA 56.

Draining soil is a management decision. Producers that have chosen to invest in this type of “improvement” for the purpose of producing a commodity crop in areas west of MLRA 56, where the undrained phases are correlated, will be able to document the practice installation and the actual resulting yield enhancement above the undrained condition. Again, less than about 20 percent of these areas are actually drained.

A quick explanation of productivity indices. These values are ABSOLUTELY NOT STATIC! They are simply a method of arraying the soils for non-irrigated commodity crop production based on their inherent soil properties. For a given area, a basic productivity index takes the highest producing soil(s) and sets its value at 100; then all other soils are arrayed relative to that value based on the soil properties. Each time the boundary of the area being considered changes the set of soils within that area changes and thus the array values change depending on where in the array the new soils in the set are ranked. The ND productivity index is based on hard red spring wheat, assumes a high level of management by the producer, and supporting long term yield data were collected from NDSU Extension plots. Soil properties were used to array the soils relative to the known yield of the soils that were present on the plots. 

AS NRCS soil scientists continue to update the soil survey, there will continue to be some changes to the productivity indices in the short term. However, this will stabilize over time and the productivity indices will become more stable within the boundaries of the MLRAs depicted on the map above. The values will ALWAYS be arrayed relative to the highest potential producing soil in the area being considered, and be based on soil properties. PLEASE, do NOT expect the productivity index to remain static. Soil is a dynamic medium and thus any related interpretations are dynamic as well. As our scientists continue to study and understand soils more, the knowledge gained will be used to enhance the soil survey database and improve the soil interpretations provided from the data."
--
Wade D. Bott,
CPSS State Soil Scientist
Natural Resources Conservation Service
United States Department of Agriculture

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- soilareas.jpg 78.08 KB

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