What Is T?

“T” the Soil Loss Tolerance Factor

Our first featured topic is on “T”, a factor regularly used in many discussions on farm and soil management. It is used liberally, with the assumption that all understand what it is, how it is derived, and why it is significant. Frequently this is not true. Soil managers, agronomists, and soil scientists understand, but how many people outside these groups understand anything but that it is related to minimizing erosion. The following is a brief summary of T, what it implies, and technical references on how it is determined. The reference for this information is: Natural Resources Conservation Service, National Soil Survey Handbook, title 430-VI (Washington, D.C., U.S. Government Printing Office, September 1999). http://www.statlab.iastate.edu/soils/nssh/.

T is the soil loss tolerance factor. It is defined as the maximum amount of erosion at which the quality of a soil as a medium for plant growth can be maintained. This includes maintaining (1) the surface soil as a seedbed for plants, (2) the interface between the air and the soil that allows the entry of air and water into the soil and still protect the underlying soil from wind and water erosion, and (3) the total soil volume as a reservoir for water and plant nutrients, which is preserved by minimizing soil loss. Erosion losses are estimated by the Universal Soil Loss Equation and the Revised Universal Soil Loss Equation. The T factor is assigned to soils without respect to land use or cover. T factors are assigned to compare soils and do not directly relate to vegetation response. However, many of the factors used to define a T factor are important to vegetation response, but the T factor itself is not. The classes of T factor are 1, 2, 3, 4, and 5. The five classes range from 1 ton per acre per year for very shallow soil to 5 tons per acre per year for very deep soil that can more easily sustain productivity. The general guideline given in Exhibit 618-14 is used to assign T factors.

T factors commonly serve as objectives for conservation farm planning. These objectives assist in the identification of cropping sequences and management systems that will maximize production and sustain long-term productivity. T factors represent the goal for maximum annual soil loss, in the context of maintaining the long term sustainability goal.

Conservation objectives for soil loss tolerance are based on maintaining a suitable seedbed and nutrient supply in the surface soil, maintaining an adequate depth and quality of the rooting zone, and minimizing unfavorable changes in water availability throughout the soil. A single T factor is assigned to each map unit component (on a soil survey map).

For detail technical background on T factor soil type criteria please refer to the above reference and website. Additional information on soils can be found at http://www.statlab.iastate.edu/survey/SQI/pdf/surface.pdf and http://www.mo14.nc.nrcs.usda.gov/ssi2.htm . A very nice guide to understanding the soil survey is at http://www2.bae.ncsu.edu/courses/bae572/SpecialReports/kunickis/Soil_survey.html .