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The Iowa Corn Suitability Rating (CSR) system was developed by Iowa State University in the early 1970's as a way to measure potential soil productivity based on soil profile, slope characteristics and weather conditions. CSR is an index ranging from 0 to 100 with values of 100 being the most productive. County assessors use the CSR to determine land values for property taxation. Appraisers, real estate agents and farmland investors use CSR to evaluate land values and analyze comparable sales.

Additionally, farmers use CSR for crop production, lease values, and conservation planning. One issue with this system is that it assumes that soils are 90% to 100% the same throughout a given soil series and researchers have since found this to be an error. Transparency was also an issue for CSR, because users could not tell how the numbers were being calculated. To combat these issues, Iowa State University has developed a new system called CSR2.

CSR2 is an index similar to CSR, but uses a clearly defined and easily calculable formula to determine a soil's productivity. By 2015, CSR2 will completely replace CSR. However for the coming years, CSR2 and CSR will both be listed, so as to ease people into the transition. Because of the ease in which CSR2 is calculated, the goal is to extend CSR2 ratings into different areas of the country, aside from just Iowa.

CSR2 is more consistent with the newer, more data-driven methods the Natural Resources Conservation Service uses compared to the original CSR, which was more experience and judgment based. The new system is more suitable to site-specific soil measurement and precision because instead of assuming that the soil is similar throughout, CSR2 uses all inclusions in the soil series and uses weighted averages of the inclusions to describe the series. Therefore, CSR2 values replace CSR, and are not comparable values.

The new CSR2 formula is: CSR2 = S - M - F - W - D +/- EJ

S - Taxonomic subgroup class of soil series. This is collected online. The USDA gathered data from thousands of different soil series and used statistical analysis to assign values to the soils. The best soil would begin with a 100, and less fertile soil, like extremely sandy soils, would start around 30 or so.

M - Family particle size can be found from the taxonomic group. It gives an explanation of how fine the soil particles are. The finest, silty particles would be 0, and the densest, loamy, particles would be a 12. Very sandy soil can be discounted up to 35 points. This number is then subtracted from the initial equation.

F - Field conditions for a particular soil map unit (SMU), which can be found under the soil map unit. A number such as "9B" may be listed, in which case that soil has a "B" rating. Soils with A ratings are given a 0, B ratings are 5, C ratings 10 and D ratings are 15. This number is then subtracted from the initial equation.

W - Water holding capacity of a particular soil. Field capacity is the amount of water a soil can hold against the force of gravity. This would be the maximum amount of water that a soil can hold before standing water appears on the surface. The wilting point is the point where there is no longer enough water for the plant to remain alive. This happens during droughts when the plants have absorbed all the water from the soil and can no longer survive. Available water hold capacity is the difference between field capacity and wilting point, it is the savings account of water for the soil.

Water holding capacity is measured by observing how many cubic inches of water can be retained in 60 in3 of soil. The more water a soil can retain, the better, with the highest ranking soils holding up to 24 inches of water. Any soil that can retain more than 9 in3 of water is given a 0. Soils that retain between 6 in3and 9 in3are given an 8, between 3 in3and 6 in3is given a 12, and under 3 in3is given 24. These scores are then subtracted from the equation.

D - Soil depth and tolerable rate of erosion. This shows how resilient the soil is to erosion. The unit for measuring this is the called the Tolerable Rate of Erosion which calculates how many tons of soil can be eroded per acre without the crop yield being affected. For example, a soil with a rating of 5T can withstand 5 tons of erosion per acre without losing productivity. 5T soil is the most resilient soil, and 1T would be the least. A 5T soil would equate to 0 in the CSR2 equation, 4T would be 10, 3T would be 20, 2T would be 30, and 1T would be 40. These scores are also subtracted from the equation.

EJ - Expert judgment. If the experts that created this system discover a trait about the soil that may affect the soil's production capacity, they may alter the CSR2 rating. For example, a soil may have a trait that is not listed in the CSR2 formula that still may alter the soil's ability to grow corn, such as contrasting textures in the soil. This is the "other" category that soil scientists use so that the formula does not get too complex.

One main difference between CSR and CSR2 is that the climate factor was removed from the formula. Over the past 3 decades, the climate in Iowa has changed in concerns to rainfall and temperatures. Because of these changes, there is no need to include climate in the CSR2 calculation, as it has not had any changes on the soil's productivity. Iowa State University professor Bob Wells believes that the need to change from CSR to CSR2 is long overdue. He sees the long-term benefits of having a transparent rating system because of the ease in which people from other states can calculate soil productivity.

Another difference is the expert judgment component. In the original CSR, most of the calculations were based on EJ, with little transparency into the calculations actually being used. In CSR2, EJ is considered the "other" category. A common criticism of CSR2 is in the EJ factor because some believe that it is too opaque and experts can choose to change the ratings without sound reasons. Dr. Lee Burras at Iowa State University, creator of CSR2, says that this is not the case. The reason the EJ factor was put into the equation was to keep the equation short and concise, instead of listing each possible soil trait into the formula.

There is an overall favorable response to CSR2 within the agriculture community. Just like any new system, people are still waiting to figure out how it works and what its benefits and disadvantages are. There are concerns on how this will affect individual property values, but in general, people are pleased with the transparency of the system. CSR2 can be used in any state to calculate soil values, and is already adopted in the southernmost 100 miles of Minnesota. Although, it is unlikely that other states will officially adopt CSR2 due to differing tax laws around the country, most will probably use it unofficially.

The need for CSR2 was evident, as most of the calculations in CSR were based on expert judgments, and most of the experts are no longer around. With the new transparency of CSR2, people can easily calculate their soil's value and use the number to universally compare it to other soils. The transition to CSR2 will be a gradual learning process, just like getting used to a new phone, and will offer a more accurate, clear, and simple reading of the land.

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Source: Understanding CSR2