A Values and T Values: What is That All About?

By Sjoerd Willem duiker and Jen Weld

While there are several types of erosion, T values and A values estimate sheet and rill erosion when soil erosion is calculated for your farm. How is soil erosion calculated for your farm, why are T values and A values important, and how do you know if your soil loss is too high?

In a previous article we reported that according to the latest USDA-NRCS estimates, as published in the last National Resource Inventory, soil erosion continues to be an area of concern in Pennsylvania. How is this reported soil erosion determined? How do I know if soil loss on my farm is too high?  Soil conservation specialists use the Revised Universal Soil Loss Equation (RUSLE2) model to calculate predicted average annual soil loss (A value) and compare that with published tolerable soil loss rates (T value) unique for each soil series. When developing a soil conservation plan, planners will calculate the average annual soil loss rate over the entire length of the crop rotation (A value) and compare that with the T value. The plan is developed so that the A value is less than or equal to the T value so that soil loss does not compromise long-term soil productivity. Soil conservation personnel use RUSLE2 to calculate the A value for a field based on:

• The rainfall erosion in your area based on total amount and common rainfall intensity
• The erodibility of the soil based on soil texture, soil organic matter content, and soil structure
• The slope length and steepness
• Crop production practices such as: tillage type, crops grown in the rotation, cover crops, row spacing, contour planting
• Other conservation practices such as terraces

You may wonder: but shouldn’t my soil loss be zero? In practicality, it is almost impossible to reduce soil loss to zero in crop production, and soil scientists have determined that new soil forms from parent materials all the time. Tolerable soil loss is a level of soil loss that does not exceed the rate of soil formation. While we have a poor understanding of soil formation rates, T values were established many years ago as a practical measure of an acceptable soil loss level that would not threaten long-term soil productivity. The result has been that deep soils have a higher T value than shallow soils because it takes a lot longer before the soils loss compromises the productivity of deep soils. Tolerable soil loss values for Pennsylvania soils vary from 1 T/A/yr to 5 T/A/yr.

While there are several types of erosion, T values and A values estimate sheet and rill erosion when soil erosion is calculated for your farm. Sheet erosion, also referred to as interrill erosion, results when rainfall impact disperses soil aggregates which are then lost through runoff. Rill erosion occurs when water concentrates in small rivulets that form running up and down the hill slope. Rills are different from gullies because gullies occur in concentrated-flow areas located in swales of the landscape. Other important forms of erosion that are not included when soil loss is calculated for your fields are ephemeral gully erosion, classical gully erosion, or streambank erosion. We challenge you to pull your conservation plan from the shelf and check the T values and A values on your farm.