The “Tolerable” Erosion Myth

Conventional wisdom holds that there is some rate of soil erosion that can be tolerated before the productivity of the soil is damaged. This so-called “soil loss tolerance level” or “T value” is expressed as a number from one to five, representing an estimate of how many tons of soil can be lost per acre in a year without diminishing the land’s fertility. The rate of reduction in soil depth reflected by the T value supposedly matches the offsetting growth in soil depth through natural processes of soil formation.

The Natural Resources Conservation Service establishes T values based on information from soil surveys. T values are higher, meaning more soil erosion can be tolerated, on deeper soils. In Iowa, T values range from one to five tons per acre per year. About 70 percent of Iowa soils have a T value of 5 tons. Less than 1 percent is assigned T values of less than two tons per acre per year (Figure 11).

Figure 11: Most Iowa soil loss tolerance levels
(T values) set at 5 tons/acre/year.

Source: Personal communication with Douglas Oelmann, Soil Scientist, Iowa Natural Resources Conservation Service.

However, the very notion of tolerable rates of soil erosion has been seriously questioned for decades. In 1987, retired soil conservation specialist L. C. Johnson, formerly with the USDA’s Cooperative Extension Service, wrote in the Journal of Soil and Water Conservation: “The concept of tolerable soil loss, as now applied in soil conservation programs, does not serve the long-term interest of mankind in assuring the indefinite productive capability of cropland. Why? Because soil loss tolerances — T values — presently assigned to cropland soils are based on faulty premises concerning rates of topsoil development and mineral weathering processes.”31

How to put specific values on tolerable rates of soil erosion has been even more hotly debated, and today most scientists have concluded that current T values far exceed actual soil formation rates. In a 1982 paper published in an American Society of Agronomy publication, T. J. Logan estimated that most soil formation occurs at rates of less than 0.2 tons per acre per year.32 A T value of five tons per acre per year is 25 times greater than that.

Moreover, T values say nothing about the impact of soil erosion on water pollution and other environmental consequences. Five tons of soil would fall just short of filling the bed of a single axle dump truck. A 160-acre crop field losing soil at far less than five tons per acre can deliver large amounts of sediment — mud that smothers aquatic life — to streams, lakes and rivers. Attached to the mud particles are many of the chemicals commonly applied each year to crop fields. And T values tell us nothing at all about the large volumes of polluted water running off crop fields.

Despite these deficiencies, T values are the only commonly used standards available for soil erosion calculations, so EWG uses them in this report. We present most of the data in relation to the most common T value applied to Iowa soils — five tons per acre per year — despite convincing evidence that this standard does not protect the long-term health of soils or of lakes, streams and rivers.